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The Making of the Atomic Bomb
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table of contents
  1. The Making of the Atomic Bomb
    1. Introduction
    2. Table of Contents
    3. Topic 1: Early History. Atomic theory in the 1800’s. Dalton’s law of multiple proportions. Development of the molecular theory of gases. Loschmidt, and the first estimation of molecular sizes and masses.
      1. Galileo Galilei
      2. Antione Lavoisier
        1. Dalton
          1. Dalton's Law of Multiple Proportions
          2. Kinetic Theory of Gases.
      3. Developing of KTM: Gas Discharge Tubes
    4. Topic 2: Years 1895-1900: Experiment with gas discharge tubes and photographic emulsions. The discovery of X- rays, radioactivity, and discovery of electron. The nature and the effects of ionizing radiation.
      1. Discovery of X-Rays
      2. Discovery of Radioactivity
      3. Antoine Henri Becquerel
      4. Experiments by J.J. Thomson in 1897 Led to the discovery of a
      5. Fundamental Building Block of matter. Discovery of Electron
      6. Ionizing Radiation
    5. Topic 3: Radioactivity 1900-1910: Marie and Pierre Curie and the search for radioactive elements. Ernest Rutherford and classification of types of ionizing radiation. The discoveries of radioactive transmutation, half-lives and isotopes. Frederic Soddy and the first estimates of energy from radioactive decay vs. energy from chemical reactions.
      1. Ernest Rutherford: Alpha, Beta, and Half-Life
      2. Ernest Rutherford
      3. Isotopes
      4. Radioactive transmutation
      5. Units of Energy in Nuclear Physics and the Energy of Radioactive Decay
      6. First Estimation of Energy from Radioactive Decay vs Energy from Chemical Reactions
      7. More about Isotopes (Modern Terminology)
    6. Topic 4: The nuclear atom 1911-1920. Alpha scattering and the discovery of the nucleus. Nuclear sizes vs atomic sizes. Niels Bohr and the structure of the nuclear atom. Moseley’s work with x-rays and the significance of the atomic number.
      1. Alpha Particles and the Nuclear Atom
      2. Nuclear Size vs Atomic size
      3. Niels Bohr and Bohr’s Model of Atom
      4. Moseley’s Law
    7. Topic 5: Years 1920-1930: The invention of the mass-spectrometer. Atomic masses, the reinterpretation of isotopes, mass defects. E=Mc2 and nuclear binding energies.
      1. Mass-spectrometer
      2. Francis Aston
      3. Reinterpretation of Isotopes. Mass Defect
      4. Binding Energy
    8. Topic 6: Years 1920-1931continued: The discovery of nuclear reactions. The Coulomb barrier and limitations on nuclear studies and alpha particles. Accelerators.
      1. Reaction Notations, Q-Values
      2. Alpha Decay
      3. Beta Decay
      4. Artificial Transmutation
      5. The Coulomb Barrier and Particle Accelerators
    9. Topic 7: Years 1932-1934: The discovery of the neutron. Reinterpretation of nuclear structure. Leo Szilard and the concept of a nuclear chain reaction. Discoveries of the positron and artificial radioactivity.
      1. The Discovery of the Neutron
      2. Leo Szilard and Concept of Chain Reaction
      3. Artificially-Induced Radioactivity
    10. Topic 8: Years 1935-1938: Enrico Fermi’s discoveries in neutron activation and neutron moderation. Bohr’s development of the liquid drop model of the nucleus. The puzzle of the neutron bombardment of uranium.
      1. Enrico Fermi and his Research on Using Neutrons
      2. Discovery of U-235.
      3. Liquid Drop Model
      4. The Puzzle of the Neutron Bombardment of Uranium
    11. Topic 9: Years 1938-1939: Otto Hahn’s discovery of nuclear fission. Interpretation of fission by Lise Meitner and Otto Frisch; spread of the news to U.S. and initial reactions and experimental verifications. Bohr’s interpretation of the significance of U-235.
      1. Spread of the News to USA and Experimental Verification of Fission Reaction in US
    12. Topic 10: Years 1939-1942: The discovery of neutrons from fission. The awakening of the Germans to the potential consequences of fission. Einstein’s letter to FDR. The discoveries of neptunium and plutonium. Pearl Harbor. The entrance of the US into the war and its effect on fission research. The Chicago pile.
      1. The Awakening of the Germans to the Potential Consequences of Fission
        1. British Atomic Bomb Project
        2. Einstein’s Letter to the President of United States of America
      2. Discovery of Neptunium and Plutonium
        1. Neptunium
        2. Plutonium
        3. Pearl Harbor
      3.  Centrifuge
      4. Electromagnetic Separation
      5. Gaseous Diffusion
      6. Liquid Thermal Diffusion
      7. Manhattan Project Chronology
    13. Topic 11: Years 1942-1945: General Leslie Groves, Robert Oppenheimer, and the Manhattan Project. Oak Ridge, Hanford, and Los Alamos. The separation of U-235 and the production of plutonium. The development of the implosion lens.
      1. The Manhattan Project
        1. General Leslie Groves
        2. Robert Oppenheimer
        3. Manhattan Project Signature Facilities
        4. Oak Ridge
      2. Building Oak Ridge.
        1. Y-12 Plant
        2.  K-25 Plant
        3. S-50 Plant.
        4. Glen Seaborg and Plutonium Chemistry
        5. Hanford
      3. Los Alamos
        1. Recruiting the Staff
        2. Theory and the "Gadget"
      4. Bomb design
    14. Topic 12: Year 1945: The “Dragon” experiments on critical mass. The death of President Roosevelt. The Trinity test. Harry Truman and Potsdam. The decision to use the bomb. Hiroshima and Nagasaki. Final perspectives on war in the 20-th century, nuclear proliferation, and the challenge of nuclear terrorism
      1. From Roosevelt to Truman
      2. The Trinity Test
      3. Potsdam
      4. Reports on Trinity
        1. The Potsdam Proclamation
      5. What are Bomb Effects
        1. Blast
        2. Thermal radiation
        3. Initial radiation
        4. Residual radiation and fallout
      6. Hiroshima
      7. "Little Boy" Atomic Bomb
      8. Nagasaki
      9. Fat Man Atomic Bomb
        1. Surrender
        2. The Bomb Goes Public
      10. OTHER NUCLEAR PROGRAMS
      11. PROLIFERATION COUNTRY-BY-COUNTRY
    15. Appendices 1-10
    16. Appendix 1: Basics of Electricity
    17. Appendix 2: Range of the Electromagnetic Spectrum
    18. Appendix 3: Units of Energy
      1. More Units of Energy
      2. Multiplication Table of Units
      3. Multiplication of the Units of Power with Units of Time
    19. Appendix 4: Energy
    20. Appendix 5: Max Plank and the idea of quantum
      1. More Evidence for a Particle Theory of Energy
    21. Appendix 6: Periodic Table of the Elements
    22. Appendix 7: Metric (SI) Prefixes
      1. Metric Units of Measurement
    23. Appendix 8: Scientific method
      1. Step 1: Make observations
      2. Step 2: Formulate a hypothesis
      3. Step 3: Design and perform experiments
      4. Step 4: Accept or modify the hypothesis
      5. Step 5: Development into a law and/or theory
      6. Summary
    24. Appendix 9: International System of Units
    25. Appendix 10: Introduction to Wave Motion
      1. Young’s Double-Slit Experiment

The Making of the Atomic Bomb

By Professor Lyudmila Godenko of Brooklyn College 2019

E-Book Project of Brooklyn College OER Project

Attribution-NonCommercial-ShareAlike 4.0 International

C:\Users\Staff\AppData\Local\Microsoft\Windows\Temporary Internet Files\Content.IE5\8FLJ93A2\P_physics.svg[1].png The Making of the Atomic Bomb

Introduction

Welcome to the course “The Making of the Atomic Bomb”.

Spencer R. Weart , the former director of the Center for History of Physics of the American Institute of Physics (AIP) in his book “Scientists in Power”(1979) noticed that “We must be curious to learn how such a set of object’s- hundreds of power plants, thousands of bombs, ten of thousands of people massed in national establishments- can be traced back to a few people sitting at laboratory benches discussing the peculiar behavior of one type of atom”.

Despite the fact that a lot of efforts were made towards nuclear weapons non-proliferation, we still live in a world that could be easily destroyed by using nuclear weapons. It turns out that nowadays more countries try to have their own nuclear programs which goal is to make possible assembling of nuclear weapons. Discouraging countries from developing nuclear weapons while promoting the use of nuclear technology for peaceful purposes—such as energy generation and medical research—has been the main challenge world leaders have grappled with since the United States detonated the first nuclear weapons more than seventy years ago. With growing security threats from nuclear-capable countries, nuclear nonproliferation remains a critical issue today.

This course will discuss
the history of the development of the atomic bomb. Number of scientific breakthroughs in atomic and nuclear physics during 19-th and the first part of 20-th centuries led to possibility of the making of the atomic bomb. We also discuss the political context in which the bomb was developed, and personal stories of the leading scientist involved and corresponding moral issues arising from the development and use of the bomb. There is no development in modern history that has had more impact on man’s scientific, political, and moral consciousness than the making of the atomic bomb and its use against the Japanese at the end of WWII. It is a singularity of such power that its ultimate consequences for humanity are still beyond our perception. This course attempts to tell the story primarily from the point of view of the history of the science involved. Also the students will see the need for the integrated perspective in order to understand how science, political history, ethical values and personal motivations are interconnected in this story. To understand this story is to understand the complexities and responsibilities that have accompanied the emergence of modern society.

Objectives of this course include

  • the understanding of progression of scientific discoveries that clarified scientific picture of the atom and the atomic nucleus during the years from 1895 to 1945;

  • the understanding of how laboratory experiments interlay with the development of theoretical knowledge; how some experiments proceed by design to clarify specific issues, while other stumble on new and totally unexpected phenomena;

    • the understanding of how numbers, even of such abstract and esoteric quantities as cross sections, can change the direction of investigations

    • the understanding of fact that the incredibly small amount of matter inside the atomic nucleus can, through the multiplying effect of a chain reaction, produce the prodigious release of energy in a nuclear reactor or atomic bomb;

    • the understanding of the massive scale of the Manhattan Project and the enormous challenges involved in the creation of weapons-grade fissionable material; the story of the development of the atomic bomb in the historical context of the political history of the first half of the twentieth century;

    • the understanding of motivations and personalities of the scientists who pressed for US Government involvement leading up to Manhattan Project;

    • the understanding of the need for an integrated perspective of how science, political history, ethical values and personal motivations are interconnected;

    • the understanding of the role that nuclear weapon and nuclear power have played in

      popular culture since 1945.

The materials of the course include a variety of historical and contemporary books and articles, materials that were put on websites, historical texts, documentary films, feature films, power point presentations.
Because the course is usually taught for students that are not science major and often have very basic knowledge of physics, I consider that it will be useful to add so-called Appendixes where some pieces of scientific information are added. They serve mostly for inquiry.

Acknowledgments

Teaching of this course was started at Brooklyn College, CUNY by Prof. Albert Bond. He created the first syllabus of the course and installed ideas of how this course has to be taught. In my teaching I follow our discussions and findings.

The materials, the teaching of this course is based on, mostly are coming from excellent books. First of all, the ageless book “The Making of the Atomic Bomb” by Richard Rhodes (Simon & Shuster, 2012), which is the best source of knowledge and ideas that are taught in course. Then the relatively new books “The History and Science of the Manhattan Project’, by Bruce Cameron Reed ( Springer-Verlag, 2013) and “The Physics of the Manhattan Project”, by Bruce Cameron Reed ( Springer-Verlag,2015). These books are scientific ones and were used to discuss some questions in the course not only qualitatively but also quantitatively. And I also have to mention the book from National Security History Series “The Manhattan Project: Making the Atomic Bomb” by F. G. Gosling (DOE/MA-0001; Washington: History Division, Department of Energy ,December 2005).

This course.is rather popular at Brooklyn College, CUNY. Sometimes I have 3 sections (each of capacity of 50 students) running in the same semester. Usually students consider this course very interesting and informative. We have sometimes hot discussions, very well done presentations, and even some films and poetry devoted to events we consider in course. I would to thank my former and future students for sharing my passion to the course.

A lot of my friends and colleagues advised me to put together all materials about the course that I found out and make it available to the students. This is result of the work. I thank everybody and especially David Mashkevich for constant inspiration.


Table of Contents

The Making of the Atomic Bomb

Introduction

Table of Contents

Topic 1: Early History. Atomic theory in the 1800’s. Dalton’s law of multiple proportions. Development of the molecular theory of gases. Loschmidt, and the first estimation of molecular sizes and masses.

Topic 2: Years 1895-1900: Experiment with gas discharge tubes and photographic emulsions. The discovery of X- rays, radioactivity, and discovery of electron. The nature and the effects of ionizing radiation.

Topic 3: Radioactivity 1900-1910: Marie and Pierre Curie and the search for radioactive elements. Ernest Rutherford and classification of types of ionizing radiation. The discoveries of radioactive transmutation, half-lives and isotopes. Frederic Soddy and the first estimates of energy from radioactive decay vs. energy from chemical reactions.

Topic 4: The nuclear atom 1911-1920. Alpha scattering and the discovery of the nucleus. Nuclear sizes vs atomic sizes. Niels Bohr and the structure of the nuclear atom. Moseley’s work with x-rays and the significance of the atomic number.

Topic 5: Years 1920-1930: The invention of the mass-spectrometer. Atomic masses, the reinterpretation of isotopes, mass defects. E=Mc2 and nuclear binding energies.

Topic 6: Years 1920-1931continued: The discovery of nuclear reactions. The Coulomb barrier and limitations on nuclear studies and alpha particles. Accelerators.

Topic 7: Years 1932-1934: The discovery of the neutron. Reinterpretation of nuclear structure. Leo Szilard and the concept of a nuclear chain reaction. Discoveries of the positron and artificial radioactivity.

Topic 8: Years 1935-1938: Enrico Fermi’s discoveries in neutron activation and neutron moderation. Bohr’s development of the liquid drop model of the nucleus. The puzzle of the neutron bombardment of uranium.

Topic 9: Years 1938-1939: Otto Hahn’s discovery of nuclear fission. Interpretation of fission by Lise Meitner and Otto Frisch; spread of the news to U.S. and initial reactions and experimental verifications. Bohr’s interpretation of the significance of U-235.

Topic 10: Years 1939-1942: The discovery of neutrons from fission. The awakening of the Germans to the potential consequences of fission. Einstein’s letter to FDR. The discoveries of neptunium and plutonium. Pearl Harbor. The entrance of the US into the war and its effect on fission research. The Chicago pile.

Topic 11: Years 1942-1945: General Leslie Groves, Robert Oppenheimer, and the Manhattan Project. Oak Ridge, Hanford, and Los Alamos. The separation of U-235 and the production of plutonium. The development of the implosion lens.

Topic 12: Year 1945: The “Dragon” experiments on critical mass. The death of President Roosevelt. The Trinity test. Harry Truman and Potsdam. The decision to use the bomb. Hiroshima and Nagasaki. Final perspectives on war in the 20-th century, nuclear proliferation, and the challenge of nuclear terrorism

Appendices 1-10

Appendix 1: Basics of Electricity

Appendix 2: Range of the Electromagnetic Spectrum

Appendix 3: Units of Energy

Appendix 4: Energy

Appendix 5: Max Plank and the idea of quantum

Appendix 6: Periodic Table of the Elements

Appendix 7: Metric (SI) Prefixes

Appendix 8: Scientific method

Appendix 9: International System of Units

Appendix 10: Introduction to Wave Motion

The Making of the Atomic Bomb

Topic 1: Early History. Atomic theory in the 1800’s. Dalton’s law of multiple proportions. Development of the molecular theory of gases. Loschmidt, and the first estimation of molecular sizes and masses.

Greek Philosophers

Democritus

Democritus, “the laughing philosopher,” had ideas far in advance of his time

The first "atomic theorists" we have any record of were two fifth­century BC Greeks, Leucippus of Miletus (a town now in Turkey) and Democritus of Abdera. Their theories were naturally more philosophical than experimental in origin. The basic idea was that if you could look at matter on smaller and smaller scales (which they of course couldn't) ultimately you would see individual atoms objects that could not be divided further (that was the definition of atom). Everything was made up of these atoms, which moved around in a void (a vacuum). The different physical properties ­­ color, taste, and so on ­­ of materials came about because atoms in them had different shapes and/or arrangements and orientations with respect to each other. This was all pure conjecture, but the physical pictures they described sometimes seem uncannily accurate. These Greek philosophers believed that atoms were in constant motion, and always had been, at least in gases and liquids. Sometimes, however, as a result of their close­locking shapes, they joined in close­packed unions, forming materials such as rock or iron. Basically, Democritus and his followers had a very mechanical picture of the universe. They thought all natural phenomena could in principle be understood in terms of interacting, usually moving, atoms. This left no room for gods to intervene. Their atomic picture included the mind and even the soul, which therefore did not survive death. This was in fact a cheerful alternative to the popular religions of the day, in which the gods constantly intervened, often in unpleasant ways, and death was to be dreaded because punishments would surely follow. 

Aristotle

Little conceptual progress in atomic theory was made over the next two thousand years, in large part because Aristotle discredited it. 

Aristotle Altemps Inv8575.jpg

Roman copy in marble of a Greek bronze bust of Aristotle by Lysippus,  330 BC, with modern alabaster mantle.

In his” On Generation and Corruption”, Aristotle related each of the four elements, Earth, Water, Air, and Fire, to two of the four sensible qualities, hot, cold, wet, and dry. In the scheme, all matter was made of the four elements, in differing proportions. Aristotle's scheme added the heavenly Ether, the divine substance of the heavenly spheres, stars and planets.

Aristotle's elements
ElementHot/ColdWet/DryMotionModern state
of matter
EarthColdDryDownSolid
WaterColdWetDownLiquid
AirHotWetUpGas
FireHotDryUpPlasma
Ether(divine
substance)
—Circular
(in heavens)
—

Aristotle’s views held sway through the Middle Ages.

Galileo Galilei

Galileo Galilei

Things began to look up with the Renaissance. Galileo Galilei- Italian astronomer, physicist and engineer believed in atoms, although, like the early Greeks, he seemed to confuse the idea of physical indivisibility with that of having zero spatial extent, i.e. being a mathematical point. Nevertheless, his ideas in this area apparently got him into theological hot water. The Church felt that the doctrine of transubstantiation ­ the belief that the bread and wine literally became the body and blood of Christ ­ was difficult to believe if everything was made up of atoms. This was an echo of the tension between atoms and religion two thousand years earlier.  Galileo's theory of atoms 
was not very well developed. He gives the impression in some places they were infinitely small and in view of his excellent grasp of dimensional scaling arguments, he may have thought that vacuum suction between infinitesimally small surfaces would suffice to hold solids together, since smaller objects have proportionately more surface. Of course, this was on the wrong track. (Ironically, shortly after Galileo's death, his student Torricelli was the first to realize that suction forces were really a result of air pressure from the weight of the atmosphere.)

Pre‐Chemistry: Including Newton the Alchemist

http://www-groups.dcs.st-and.ac.uk/history/Thumbnails/Newton.jpg

Isaak Newton

Sir Isaak Newton was an English mathematician, physicist, astronomer; one of the most influential scientists of all time. His works laid the foundations of classical mechanics. He also made important contributions to optics and is inventor of calculus. Newton formulated the laws of motion and universal gravitation that formed for long tome scientific view point.
He also was interested in chemistry. Newton thought that part of chemistry (especially the physical part) could be explained in terms of the mechanics of corpuscles, but that there was something more important ­ a harder­to­pin­down vital spirit, which was the basis of life (and also somehow connected with mercury and other elements). He also felt this was the key to the way God ran the universe ­­ the merely mechanical interaction of corpuscles could not, in his opinion, generate the rich variety of life. . Newton probably spent more time studying alchemy than he did working on his laws, gravitation and calculus combined! In fact, Newton probed "the whole vast literature of the older alchemy as it has never been probed before or since" according to a recent historical study. He also used quite precise quantitative measures in many of his investigations. This did not provide the insight into mass conservation that Lavoisier's work did a century

 later, probably because Newton didn't count the various gases absorbed or emitted, these were still considered incidental and not really important to the reactions. Also, maybe they didn't smell too great ­­ a recipe for preparing phosphorus Newton copied from Boyle begins "Take of Urin one Barrel.” .  Not that this matters too much as far as developing the atomic concept is concerned. 

On the positive side, the alchemists, in their fruitless quest to turn lead into gold (and find the elixir of life, etc.) did get very skillful at managing a great variety of chemical reactions, and so learned the properties of many substances.  The alchemists' point of view was based on Aristotle's four elements, earth, air, fire and water, but they added what they called principles. For example, there was an active principle in air important in respiration and combustion. There was an acidic principle, and others. And then there was phlogiston. Looking at something in flames, it seems pretty clear that something is escaping the material. That they called phlogiston. After Boyle discovered that metals become heavier on combustion, it was decided that phlogiston had negative weight.

Antione Lavoisier

image

Antoine Lavoisier

The first major step towards modern quantitative chemistry was taken by Lavoisier towards the end of the eighteenth century .Lavoisier was French chemist who was central to the 18-th century chemical revolution He realized that combustion was a chemical reaction between the material being burned and a component of the air. He carried out reactions in closed vessels so that he could keep track of the amounts of the various reagents involved. One of his great discoveries was that in reactions, the total final weight of all the materials involved is exactly equal to the total initial weight. This was the first step on the road to thinking about chemistry in terms of atoms. He also established that pure water was not transmuted to earth by heating, as had long been believed ­ the residue left on boiling dry came from the container if the water itself was pure.  Lavoisier discovered oxygen. He was the first to realize that air has two (major) components, only one of which supports respiration, meaning life, and combustion. In 1783, working with the mathematician Laplace, and a guinea pig in a mask, he checked out quantitatively that the animal used breathed­in oxygen to form what we now term carbon dioxide (this is the origin of the "guinea pig" as experimental subject). Lavoisier tightened up the very loose terminology in use at that time: there were no generally agreed on definitions of elements, principles or atoms, although a century earlier Boyle had suggested that element be reserved for substances that could not be further separated chemically.  Lavoisier began the modern study of chemistry:
 he insisted on precise terminology and on precise measurement, and suggested as part of the agenda the classification of substances into elements and compounds. Once this program was truly underway, the atomic interpretation soon appeared.

Dalton

John Dalton by Charles Turner.jpg

John Dalton

John Dalton was English chemist, physicist, and meteorologist. He was born into a poor family near Manchester, England. He supported himself to some extent by teaching from the age of twelve, when he started his own small Quaker school. Dalton wrote A New System of Chemical Philosophy, from which the following quotes are taken: Matter, though divisible in an extreme degree, is nevertheless not infinitely divisible. That is, there must be some point beyond which we cannot go in the division of matter. The existence of these ultimate particles of matter can scarcely be doubted, though they are probably much too small ever to be exhibited by microscopic improvements. I have chosen the word atom to signify these ultimate particles …

He assumed that all atoms of an element were identical, and atoms of one element could not be changed into atoms of another element "by any power we can control.” He assumed further that compounds of elements had compound atoms: “I call an 
ultimate particle of carbonic acid a compound atom.” Now, though this atom may be divided, yet it ceases to be carbonic acid, being resolved by such division into charcoal and oxygen.  He also asserted that all compound atoms (molecules, as we would say) for a particular compound were identical, and, furthermore: "Chemical analysis and synthesis go no farther than to the separation of particles one from another, and to their reunion. No creation or destruction of matter is within reach of chemical agency.”  By Dalton's time it had become clear that when elements combine to form a particular compound, they always do in precisely the same ratio by weight. For example, when hydrogen burns in oxygen to form water, one gram of hydrogen combines with eight grams of oxygen. This constancy is to be expected in Dalton's theory, presumably the compound atom, or molecule, of water has a fixed number of hydrogen atoms and a fixed number of oxygen atoms. Of course, the weight ratio doesn't tell us the numbers, since we don't know the relative weights of the hydrogen atom and the oxygen atom. To make any progress, some assumptions are necessary. Dalton suggested a rule of greatest simplicity: if two elements form only one compound, assume the compound atom has only one atom of each element. Since H2O2 had not been discovered, he assumed water was HO. (He actually used symbols to represent the elements, H was a circle with a dot in the center. However, just as we do, he used strings of such symbols to represent an actual molecule, not a macroscopic mixture.) On putting together data on many different reactions, it became apparent to Dalton that the rule of greatest simplicity wasn't necessarily correct, by 1810 he was suggesting that the water molecule perhaps contained three atoms.
The main points of Dalton’s atomic theory are:

  1. Everything is composed of atoms, which are the indivisible building blocks of matter and cannot be destroyed. All atoms of an element are identical.

  2. The atoms of different elements vary in size and mass.

  3. Compounds are produced through different whole-number combinations of atoms.

  4. A chemical reaction results in the rearrangement of atoms in the reactant and product compounds.

Atomic theory has been revised over the years to incorporate the existence of atomic isotopes and the interconversion of mass and energy. In addition, the discovery of subatomic particles has shown that atoms can be divided into smaller parts. However, Dalton’s importance in the development of modern atomic theory has been recognized by the designation of the atomic mass unit as a Dalton.

Dalton's Law of Multiple Proportions

One of the strongest arguments for Dalton's atomic theory was the Law of Multiple Proportions).John Dalton developed the law of multiple proportions (first presented in 1803) by studying and expanding upon the works of Antoine Lavoisier and Joseph Proust.
Proust had studied tin oxides and found that their masses were either 88.1% tin and 11.9% oxygen or 78.7% tin and 21.3% oxygen (these were tin(II) oxide and tin dioxide respectively). Dalton noted from these percentages that 100g of tin will combine either with 13.5g or 27g of oxygen; 13.5 and 27 form a ratio of 1:2. Dalton found an atomic theory of matter could elegantly explain this common pattern in chemistry – in the case of Proust’s tin oxides, one tin atom will combine with either one or two oxygen atoms.
Law of multiple proportions is the statement that when two elements combine with each other to form more than one compound the weights of one element that combine with a fixed weight of the other are in a ratio of small whole numbers. For example, there are five distinct oxides of nitrogen and the weights of oxygen in combination with 14 grams of nitrogen are, in increasing order, 8, 16, 24, 32, and 40 grams, or in a ratio of 1, 2, 3, 4, 5.
Dalton also believed atomic theory could explain why water absorbed different gases in different proportions: for example, he found that water absorbed carbon dioxide far better than it absorbed nitrogen. Dalton hypothesized this was due to the differences in the mass and complexity of the gases’ respective particles. Indeed, carbon dioxide molecules (CO2) are heavier and larger than nitrogen molecules (N2).
Dalton proposed that each chemical element is composed of atoms of a single, unique type, and though they cannot be altered or destroyed by chemical means, they can combine to form more complex structures (chemical compounds). Since Dalton reached his conclusions by experimentation and examination of the results in an empirical fashion, this marked the first truly scientific theory of the atom.

Kinetic Theory of Gases.

The idea of the atom was first applied to discuss the properties of gas. The theory that describes gases as collection of atoms is called Kinetic Molecular Theory (KMT). The kinetic theory describes gas as a large number of submicroscopic particles (atoms or molecules), all of which arein constant rapid motion that has randomness arising from their many collisions with each other and with the walls of the container.

Kinetic theory explains macroscopic properties of gases, such as pressure, temperature, viscosity, thermal conductivity, and volume, by considering their molecular composition and motion. The theory posits that gas pressure is due to the impacts, on the walls of a container, of molecules or atoms moving at different velocities. Kinetic theory defines temperature.  Under a microscope, the molecules making up a liquid
are too small to be visible, but the jittery motion of pollen grains or dust particles can be seen. Known as Brownian motion, it results directly from collisions between the grains or particles and liquid molecules. As analyzed by Albert Einstein in 1905, this experimental evidence for kinetic theory is generally seen as having confirmed the concrete material existence of atoms and molecules.

The theory for ideal gases makes the following assumptions:

1. The gas consists of very small particles known as molecules. This smallness of their size is
  such that total volume of the individual gas molecules added up is negligible compared to
 the volume of the smallest ball containing all the molecules.
This is equivalent to stating that the average distance separating the
gas particles is large compared to their size.

2. These particles have the same mass.

3. The number of molecules is so large that statistical treatment can be applied.
4. These molecules are in constant, random, and rapid motion.
5. The rapidly moving particles constantly collide among themselves and with the walls
of container. All these collision are perfectly elastic.

6. Except during collisions, the interactions among molecules are negligible. 

First Estimation of Molecular Sizes and Masses

When atoms and molecules were still quite hypothetical, Jan Josef Loschmidt used kinetic theory to get the first reasonable estimate of molecular size. Josef Loschmidt, a pioneer of 19th-century physics and chemistry, In 1865, Loschmidt was the first to estimate the size of the molecules that make up the air: his result was only twice the true size, a remarkable feat given the approximations he had to make. His method allowed the size of any gas molecules to be related to measurable phenomena, and hence to determine how many molecules are present in a given volume of gas. This latter quantity is now known as the Loschmidt constant in his honor, and its modern value is 2.69×1019 molecules per cubic centimeter at standard temperature and pressure (STP).His estimation of the size of molecular of air came to

S =9.69x10-10m, or roughly 1.0x 10-9 m= 1.0 nm.

Today’s value of the size of air molecular, depending on type of gas, is approximately 0.3 nm.

Developing of KTM: Gas Discharge Tubes

Many scientists have modified and elaborated on Dalton’ atomic theory. The first major advances were possible with the development of gas discharge tubes. Sir William Crookes was the leader in experiments with gas discharged tubes.
In 1838, Michael Faraday passed a current through a rarefied air-filled glass tube and noticed a strange light arc with its beginning at the cathode (negative electrode) and its end almost at the anode (positive electrode).
In the 1870s, British physicist William Crookes and others were able to evacuate rarefied tubes to a pressure below 10−6 atm. These were called Crookes tubes. Faraday had been the first to notice a dark space just in front of the cathode, where there was no luminescence. This came to be called the cathode dark space, Faraday dark space, or Crookes dark space. Crookes found that as he pumped more air out of the tubes, the Faraday dark space spread down the tube from the cathode toward the anode, until the tube was totally dark. But at the anode (positive) end of the tube, the glass of the tube itself began to glow. What was happening was that as more air was pumped from the tubes, the electrons could travel farther, on average, before they struck a gas atom. By the time the tube was dark, most of the electrons could travel in straight lines from the cathode to the anode end of the tube without a collision. With no obstructions, these low mass particles were accelerated to high velocities by the voltage between the electrodes. When rays reached the anode end of the tube, they were traveling so fast that, although they were attracted to it, they often flew past the anode and struck the back wall of the tube. When they struck atoms in the glass wall, they excited their orbital electrons to higher energy levels, causing them to fluoresce.
Later researchers painted the inside back wall with fluorescent chemicals such as zinc sulfide, to make the glow more visible. Rays themselves are invisible, but this accidental fluorescence allowed researchers to notice that objects in the tube in front of the cathode, such as the anode, cast sharp-edged shadows on the glowing back wall. In 1869, German physicist Johann Hittorf was first to realize that something must be traveling in straight lines from the cathode to cast the shadows. Eugene Goldstein named them cathode rays.

The Making of the Atomic Bomb

Topic 2: Years 1895-1900: Experiment with gas discharge tubes and photographic emulsions. The discovery of X- rays, radioactivity, and discovery of electron. The nature and the effects of ionizing radiation.

Discovery of X-Rays

Wilhelm Conrad Roentgen

Wilhelm Rontgen

Developments to 1932 the era of ‘‘modern’’ physics is usually considered to have begun in late 1895, when Wilhelm Conrad Rontgen, working in Germany, accidentally discovered X-rays. Rontgen discovered that not only could his mysterious rays pass through objects such as his hand, but they also ionized air when they passed through it; this was the first known example of what we now call ‘‘ionizing radiation’’.   Rontgen experimented at Würzburg University focused on light phenomena and other emissions generated by discharging electrical current in so-called "Crookes tubes," glass bulbs with positive and negative electrodes, evacuated of air, which display a fluorescent glow when a high voltage current is passed through it. He was particularly interested in cathode rays and in assessing their range outside of charged tubes.

On November 8, 1895, Roentgen noticed that when he shielded the tube with heavy black cardboard, the green fluorescent light caused a platinobarium screen nine feet at away to glow - too far away to be reacting to the cathode rays as he understood them. He determined the fluorescence was caused by invisible rays originating from the Crookes tube he was using to study cathode rays (later recognized as electrons), which penetrated the opaque black paper wrapped around the tube. Further experiments revealed that this new type of ray was capable of passing through most substances, including the soft tissues of the body, but left bones and metals visible. One of his earliest photographic plates from his experiments was a film of his wife Bertha's hand, with her wedding ring clearly visible.

To test his observations and enhance his scientific data, Roentgen plunged into seven weeks of meticulous planned and executed experiments. On December 28, he submitted his first "provisional" communication, "On a New Kind of Rays," in the Proceedings of the Wurzburg Physio-Medical Society. In January 1896 he made his first public presentation before the same society, following his lecture with a demonstration: he made a plate of the hand of an attending anatomist, who proposed the new discovery be named "Roentgen's Rays."

The news spread rapidly throughout the world. Thomas Edison was among those eager to perfect Roentgen's discovery, developing a handheld fluoroscope, although he failed to make a commercial "X-ray lamp" for domestic use. The apparatus for producing X-rays was soon widely available, and studios opened to take "bone portraits," further fueling public interest and imagination. Poems about X-rays appeared in popular journals, and the metaphorical use of the rays popped up in political cartoons, short stories, and advertising. Detectives touted the use of Roentgen devices in following unfaithful spouses, and lead underwear was manufactured to foil attempts at peeking with "X-ray glasses."

As frivolous as such reactions may seem, the medical community quickly recognized the importance of Roentgen's discovery. By F By February 1896, X-rays were finding their first clinical use in the US in Dartmouth, MA, when Edwin Brant Frost produced a plate of a patient's Colles fracture for his brother, a local doctor. Soon attempts were made to insert metal rods or inject radio-opaque substances to give clear pictures of organs and vessels, with mixed results. The first angiography, moving-picture X-rays, and military radiology, were performed in early 1896.

In addition to the diagnostic powers of X-rays, some experimentalists began applying the rays to treating disease. Since the early 19th century, electrotherapy had proved popular for the temporary relief of real and imagined pains. The same apparatus could generate X-rays. In January 1896, only a few days after the announcement of Roentgen's work, a Chicago electrotherapist named Emil Grubbe irradiated a woman with a recurrent cancer of the breast, and by the end of the year, several researchers had noted the palliative effects of the rays on cancers. Others found remarkable results in the treatment of surface lesions and skin problems while others investigated the possible bacterial action of the rays. X-rays even found cosmetic uses in depilatory clinics set up in the US and France.
Roentgen was awarded the first Nobel Prize in physics in 1901 for his discovery.C:\Users\LYUDMILA\Desktop\Capture 2.PNG

Discovery of Radioactivity

Henri Becquerel
Antoine Henri Becquerel


A part of Rontgen’s discovery involved X-rays illuminating a phosphorescent screen, a fact which caught the attention of Antoine Henri Becquerel, who lived in France. Becquerel was an expert in the phenomenon of phosphorescence, where a material emits light in response to illumination by light of another color. Becquerel wondered if phosphorescent materials such as uranium salts might be induced to emit X-rays if they were exposed to sunlight. While this supposition was wrong, investigating it led Becquerel, in February 1896, to the accidental discovery- one of the most well-known accidental discoveries in the history of physics. On an overcast day in March 1896, French physicist Henri Becquerel opened a drawer and discovered spontaneous radioactivity. 
Becquerel first heard about Roentgen’s discovery in January 1896 at a meeting of the French Academy of Sciences. After learning about Roentgen’s finding, Becquerel began looking for a connection between the phosphorescence he had already been investigating and the newly discovered x-rays. Becquerel thought that the phosphorescent uranium salts he had been studying might absorb sunlight and reemit it as x-rays. 
To test this idea (which turned out to be wrong), Becquerel wrapped photographic plates in black paper so that sunlight could not reach them. He then placed the crystals of uranium salt on top of the wrapped plates, and put the whole setup outside in the
sun. When he developed the plates, he saw an outline of the crystals. He also placed objects such as coins or cut out metal shapes between the crystals and the photographic plate, and found that he could produce outlines of those shapes on the photographic plates penetrating radiation similar to x-rays. He reported this result at the French Academy of Science meeting on February 24, 1896.
Seeking further confirmation of what he had found, he planned to continue his experiments. But the weather in Paris did not cooperate; it became overcast for the next several days in late February. Thinking he couldn’t do any research without bright sunlight, Becquerel put his uranium crystals and photographic plates away in a drawer.

On March 1, he opened the drawer and developed the plates, expecting to see only a very weak image. Instead, the image was amazingly clear.
The next day, March 2, Becquerel reported at the Academy of Sciences that the uranium salts emitted radiation without any stimulation from sunlight.
Many people have wondered why Becquerel developed the plates at all on that cloudy March 1, since he didn’t expect to see anything. Possibly he was motivated by simple scientific curiosity. Perhaps he was under pressure to have something to report at the next day’s meeting. Or maybe he was simply impatient.
Whatever his reason for developing the plates, Becquerel realized he had observed something significant. He did further tests to confirm that sunlight was indeed unnecessary, that the uranium salts emitted the radiation on their own. 
At first he thought the effect was due to particularly long-lasting phosphorescence, but he soon discovered that non-phosphorescent uranium compounds exhibited the same effect. In May he announced that the element uranium was indeed what was emitting the radiation. 
Becquerel initially believed his rays were similar to x-rays, but his further experiments showed that unlike x-rays, which are neutral, his rays could be deflected by electric or magnetic fields.

Experiments by J.J. Thomson in 1897 Led to the discovery of a

Fundamental Building Block of matter. Discovery of Electron

The British physicist J.J. Thompson was the first one who started studding the interior of the atom. At the Cavendish Laboratory at Cambridge University, Thomson was experimenting with currents of electricity inside empty glass tubes. He was investigating a long-standing puzzle known as “cathode rays”. His experiments prompted him to make a bold proposal: these mysterious rays are streams of particles much smaller than atoms, they are in fact minuscule pieces of atoms. He called these particles "corpuscles," and suggested that they might make up all of the matter in atoms. It was startling to imagine a particle residing inside the atom--most people thought that the atom was indivisible, the most
fundamental unit of matter

. .https://www.aip.org/history/exhibits/electron/images/thomb1.jpg

J.J. Thomson in his office

Thomson’s speculation was not unambiguously supported by his experiments. It took more experimental work by Thomson and others to sort out the confusion. The atom is now known to contain other particles as well. Yet Thomson's bold suggestion that cathode rays were material constituents of atoms turned out to be correct. The rays are made up of electrons: very small, negatively charged particles that are indeed fundamental parts of every atom.

"Could anything at first sight seem more impractical than a body which is so small that its mass is an insignificant fraction of the mass of an atom of hydrogen?"
-- J.J. Thomson.

Modern ideas and technologies based on the electron, leading to television and the computer and much else, evolved through many difficult steps. Thomson's careful experiments and adventurous hypotheses were followed by crucial experimental and theoretical work by many others in the United Kingdom, Germany, France and elsewhere. These physicists opened for us a new perspective--a view from inside the atom.
Science lecturers who traveled from town to town in the mid nineteenth century delighted audiences by showing them the ancestor of the neon sign. They took a glass tube with wires embedded in opposite ends... put a high voltage across... pumped out most of the air... and the interior of the tube would glow in lovely patterns. In 1859 a German physicist sucked out still more air with an improved pump and saw that where this light from the cathode reached the glass it produced a fluorescent glow. Evidently some kind of ray was emitted by the cathode and lighting up the glass.

What could these rays be? One possibility was that they were waves traveling in a hypothetical invisible fluid called the "ether." At that time, many physicists thought that this ether was needed to carry light waves through apparently empty space. Maybe cathode rays were similar to light waves? Another possibility was that cathode rays were some kind of material particle. Yet many physicists, including J.J. Thomson, thought that all material particles themselves might be some kind of structures built out of ether, so these views were not so far apart.

Experiments were needed to resolve the uncertainties. When physicists moved a magnet near the glass, they found they could push the rays about. But when the German physicist Heinrich Hertz passed the rays through an electric field created by metal plates inside a cathode ray tube, the rays were not deflected in the way that would be expected of electrically charged particles. Hertz and his student Philipp Lenard also placed a thin metal foil in the path of the rays and saw that the glass still glowed, as though the rays slipped through the foil. Didn't that prove that cathode rays were some kind of waves?

Other experiments cast doubt on the idea that these were ordinary particles of matter, for example gas molecules as some suggested. In France, Jean Perrin had found that cathode rays carried a negative charge. In Germany, in January 1897 Emil Wiechert made a puzzling measurement indicating that the ratio of their mass to their charge was over a thousand times smaller than the ratio for the smallest charged atom. When Lenard passed cathode rays through a metal foil and measured how far they traveled through various gases, he concluded that if these were particles, they had to be very small.

Drawing on work by his colleagues J.J. Thompson refined some previous experiments, designed some new ones, carefully gathered data, and then... made a bold speculative leap. Cathode rays are not only material particles, he suggested, but in fact the building blocks of the atom: they are the long-sought basic unit of all matter in the universe.

Do atoms have parts? J.J. Thompson suggested that they do. He advanced the idea that cathode rays are really streams of very small pieces of atoms. Three experiments led him to this

First, in a variation of an 1895 experiment by Jean Perrin, Thomson built a cathode ray tube ending in a pair of metal cylinders with a slit in them. These cylinders were in turn connected to an electrometer, a device for catching and measuring electrical charge. Perrin had found that cathode rays deposited an electric charge. Thomson wanted to see if, by bending the rays with a magnet, he could separate the charge from the rays. He found that when the rays entered the slit in the cylinders, the electrometer measured a large amount of negative charge. The electrometer did not register much electric charge if the rays were bent so they would not enter the slit. As Thomson saw it, the negative charge and the cathode rays must somehow be stuck together: you cannot separate the charge from the rays.

All attempts had failed when physicists tried to bend cathode rays with an electric field. Now Thomson thought of a new approach. A charged particle will normally curve as it moves through an electric field, but not if it is surrounded by a conductor (a sheath of copper, for example). Thomson suspected that the traces of gas remaining in the tube were being turned into an electrical conductor by the cathode rays themselves. To test this idea, he took great pains to extract nearly all of the gas from a tube, and found that now the cathode rays did bend in an electric field after all.

Thomson concluded from these two experiments, "I can see no escape from the conclusion that [cathode rays] are charges of negative electricity carried by particles of matter." But, he continued, "What are these particles? Are they atoms, or molecules, or matter in a still finer state of subdivision?"

Thompson’s third experiment sought to determine the basic properties of the particles. Although he couldn't measure directly the mass or the electric charge of such a particle, he could measure how much the rays were bent by a magnetic field, and how much energy they carried. From this data he could calculate the ratio of the mass of a particle to its electric charge (m/e). He collected data using a variety of tubes and using different gases. Thomson presented three hypotheses about cathode rays based on his 1897 experiments:

  1. Cathode rays are charged particles (which he called call "corpuscles").

  2. These corpuscles are constituents of the atom.

  1. These corpuscles are the only constituents of the atom.
    Thompson then announced  that "we have in the cathode rays matter in a new state, a state in which the subdivision of matter is carried very much further than in the ordinary gaseous state: a state in which all matter... is of one and the same kind; this matter being the substance from which all the chemical elements are built up."

Thompson Model of Atom

J. J. Thomson, who discovered the electron in 1897, proposed the plum pudding model of the atom in 1904 before the discovery of the atomic nucleus in order to include the electron in the atomic model. In Thomson’s model, the atom is composed of electrons (which Thomson still called “corpuscles,” though G. J. Stoney had proposed that atoms of electricity be called electrons in 1894) surrounded by a soup of positive charge to balance the electrons’ negative charges, like negatively charged “plums” surrounded by positively charged “pudding”. The electrons (as we know them today) were thought to be positioned throughout the atom in rotating rings. In this model the atom was also sometimes described to have a “cloud” of positive charge.
With this model, Thomson abandoned his earlier “nebular atom” hypothesis, in which the atom was composed of immaterial vortices. Now, at least part of the atom was to be composed of Thomson’s particulate negative corpuscles, although the rest of the positively charged part of the atom remained somewhat nebulous and ill-defined.

The 1904 Thomson model was disproved by the 1909 gold foil experiment performed by Hans Geiger and Ernest Marsden. This gold foil experiment was interpreted by Ernest Rutherford in 1911 to suggest that there is a very small nucleus of the atom that contains a very high positive charge (in the case of gold, enough to balance the collective negative charge of about 100 electrons). His conclusions led him to propose the Rutherford model of the atom.

image

Plum pudding model of the atom:

A schematic presentation of the plum pudding model of the atom; in Thomson’s mathematical model the “corpuscles” (in modern language, electrons) were arranged non-randomly, in rotating rings.

Ionizing Radiation

After the experiments of J.J. Thompson it became clear that the understanding of electricity originates inside the atom itself. The positive and negative charge in the atom have exactly the same magnitude, but their signs are opposite. It makes an atom neutral. But sometimes an atom may lose one or more of its electrons, or may gain extra electrons in which case an atom acquires a net positive or negative charge and is called an ion. The minimum energy required to remove electron from the ground state of an atom is called the ionization energy. This energy can be supplied to atom through radiation, more precisely ionizing radiation. So ionizing radiation is radiation that carries enough energy to free electrons from atoms, thereby ionizing them. Ionizing radiation is made up of energetic subatomic particles, ions or atoms, moving at high speeds (usually greater than 1% of the speed of light) and electromagnetic waves with very small wavelength. Gamma rays, x-rays are carriers of ionizing radiation. Typical ionizing subatomic particles from radioactivity include alpha particles, beta particles and neutrons. Almost all [products of radioactive decay are ionizing because the energy of radioactive decay is typically far higher than that required to ionize.

The Making of the Atomic Bomb



Topic 3: Radioactivity 1900-1910: Marie and Pierre Curie and the search for radioactive elements. Ernest Rutherford and classification of types of ionizing radiation. The discoveries of radioactive transmutation, half-lives and isotopes. Frederic Soddy and the first estimates of energy from radioactive decay vs. energy from chemical reactions.

Pier and Marie Curie and the Search for Radioactive Elements

Becquerel’s work came to the attention of Marie Sklodowski, a native of Poland who had graduated from the Sorbonne (part of the University of Paris) with a degree in physical science in 1893; the following year she would add a degree in mathematics. In 1895 she married Pierre Curie, a physicist at the Paris School of Physics and Chemistry Seeking a subject for a doctoral thesis, Marie turned to Becquerel’s work, a subject about which

Marie Curie. Pierre Curie.

Marie Curie Pierre Curie

not a great deal had been published. She set up a laboratory in her husband’s School, and began work in late 1897. Becquerel had reported that the energetic ‘‘rays’’ emitted by uranium could ionize air as they passed through it; in modern parlance the rays collide with molecules in the air and cause them to lose electrons. Pierre Curie and his brother had developed a device known as an electrometer for detecting minute electrical currents. Making use of this device, Marie found that the amount of electricity generated was directly proportional to the amount of uranium in a sample. Testing other materials, she found that the heavy element thorium also emitted Becquerel rays (a fact discovered independently by Gerhard Schmidt in Germany), although not as many per gram per second as did uranium. Further work, however, revealed that samples of pitchblende ore, a blackish material rich in uranium oxides, emitted more Becquerel rays than could be accounted for solely by the quantity of uranium that they contained. Drawing the conclusion that there must be some other ‘‘active element’’ present in pitchblende, Curie began the laborious task of chemically isolating it from the tons of ore she had available. By this time, Pierre had abandoned his own research on the properties of crystals in order to join Marie in her work. Spectroscopic analysis of the active substance proved that it was a new, previously unknown element. Christening their find ‘‘polonium’’ (Po) in honor of native country, they published their discovery in July, 1898, in the weekly proceedings of the French Academy of Sciences. That paper introduced two new words to the scientific community: ‘‘radioactivity’’ to designate whatever process deep within atoms was giving rise to Becquerel’s ionizing rays, and ‘‘radioelement’’ to any element that possessed the property of doing so. The term ‘‘radioisotope’’ is now more commonly used in place of ‘‘radioelement,’’ as not all of the individual isotopes of elements that exhibit radioactivity are themselves radioactive. In December, 1898, the Curies announced that they had found a second radioactive substance, which they dubbed ‘‘radium’’ (Ra). By the spring of 1902, after starting with ten tons of pitchblende ore, they had isolated a mere tenth of a gram of radium, which was enough for definite spectroscopic confirmation of its status as a new element. In the summer of 1903 Marie defended her thesis, ‘‘Researches on Radioactive Substances,’’ and received her doctorate from the Sorbonne. In the fall of that year the Curies would be awarded half of the 1903 Nobel Prize for Physics; Henri Becquerel received the other half.

Ernest Rutherford: Alpha, Beta, and Half-Life

Ernest Rutherford

Ernest Rutherford

In the fall of 1895, Ernest Rutherford a New Zealand native, arrived at the Cavendish Laboratory of Cambridge University in England on a postgraduate scholarship. The Director of the Laboratory was Joseph John ‘‘J. J.’’ Thomson, who in the fall of 1897 was credited with discovering the electron, the fundamental, negatively-charged particles of matter which account for the volumes of atoms. It is rearrangements of the outermost electrons of atoms which cause the chemical reactions by which, for example, we digest meals to provide the energy we use to do useful work such as the preparation of book manuscripts. Rutherford’s intrinsic intelligence, capacity for sheer hard work, and unparalleled physical insight combined with propitious timing to set him on a path to become one of history’s great nuclear pioneers. Soon after Rutherford arrived in Cambridge, Rontgen discovered X-rays. As a student, Rutherford had developed considerable experience with electrical devices, and the Cavendish Laboratory was well-equipped with Thomson’s ‘‘cathode ray tubes,’’ the core apparatus for generating X-rays. Rutherford soon began studying their ionizing properties.

When the discovery of radioactivity was announced, it was natural for him to turn his attention to this new ionizing phenomenon. Rutherford discovered that he could attenuate some of the uranium activity by wrapping the samples in thin aluminum foils; adding more layers of foil decreased the activity. Rutherford deduced that there appeared to be two types of radiation present, which he termed ‘‘alpha’’ and ‘‘beta.’’ Alpha-rays could be stopped easily by a thin layer of foil or a few sheets of paper, but beta-rays were more penetrating. Henri Becquerel later showed that both types could be deflected by a magnetic field, but in opposite directions and by differing amounts. This meant that the rays must be electrically charged; alphas proved to be positive, and betas negative. Becquerel also later proved that beta rays were identical to electrons. Alpha-rays were much less affected by a magnet, which meant that they must be much more massive than electrons .In the fall of 1898, Rutherford completed his studies at Cambridge, and moved to McGill University in Montreal, Canada, where he had been appointed as the McDonald Professor of Physics. In 1903 Rutherford found that alpha rays were deflected slightly in the opposite direction, showing that they are massive, positively charged particles. Much later Rutherford proved that alpha rays are nuclei of helium atoms by collecting the rays in an evacuated tube and detecting the buildup of helium gas over several days. Over the next three decades he continued his radioactivity research, both at McGill and later back in England.

A third kind of radiation was identified by French chemist Paul Villard in 1900. Designated as the gamma ray, it is not deflected by magnets and is much more penetrating than alpha particles. Gamma rays were later shown to be a form of electromagnetic radiation, similar to light or X-rays, but with much shorter wavelengths. Because of these shorter wavelengths, gamma rays have higher frequencies and are even more penetrating than X-rays wavelengths but with much shorter wavelength. Because of these shorter wavelengths, gamma rays have higher frequencies and are even more penetrating than X-rays.

Soon Ernest Rutherford separated the new rays into alpha, beta radiation, and in 1902 Rutherford and Frederick Soddy explained radioactivity as a spontaneous transmutation of elements. 

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Rutherford’s first major discovery at McGill occurred in 1900, when he found that, upon emitting its radiation, thorium simultaneously emitted a product which he termed ‘‘emanation.’’ The emanation was also radioactive, and it happen to be a new noble gas, an isotope of radon (Rn). With Frederic Soddy they created “disintegration theory” of radioactivity which regards radioactive phenomena as atomic-not molecular- processes. The theory was supported by a large amount of experimental evidence, a number of new radioactive substances were discovered and their positions in the series of transformations were fixed.

When isolated, radioactivity was observed to decline in a geometrical progression with time. Specifically, the activity decreased by a factor of one-half for every minute of time that elapsed. Rutherford and Soddy had discovered the property of radioactive half-life, the quintessential natural exponential decay process. As an example, suppose that at ‘‘time zero’’ you have 1,000 atoms of some isotope that has a half-life of 10 days. You can then state that 500 of them will have decayed after 10 days. You cannot predict which of the 500 will have decayed, however. Over the following 10 days a further 250 of the original remaining atoms will decay, and so on. Remarkably, the probability that a given atom will decay in some specified interval of time is completely independent of how long it has managed to avoid decaying; in the subatomic world, age is not a factor in the probability of continued longevity. Related image

The Nobel Prize in Chemistry 1908 was awarded to Ernest Rutherford "for his investigations into the disintegration of the elements, and the chemistry of radioactive substances".

Isotopes

The concept of isotopy first arose from evidence gathered in studies of natural radioactive decay chains .Substances that appeared in different decay chains through different modes of decay often seemed to have similar properties, but could not be separated from each other by chemical means. The term ‘‘isotope’’ was introduced in 1913 by Frederick Soddy, who had taken a position at the University of Glasgow. Soddy argued that the decay-chain evidence suggested that ‘‘the net positive charge of the nucleus is the number of the place which the element occupies in the periodic table’’. Basing his hypothesis on the then-current idea that the electrically neutral mass in nuclei was a combination of protons and electrons, Soddy went on to state that the ‘‘algebraic sum of the positive and negative charges in the nucleus, when the arithmetical sum is different, gives what I call ‘‘isotopes’’ or ‘‘isotopic elements,’’ because they occupy the same place in the periodic table.’’ The root ‘‘iso’’ comes from the Greek word ‘‘isos,’’ meaning ‘‘equal,’’ and the p in tope serves as a reminder that it is the number of protons which is the same in all isotopes of a given element. In the same paper, Soddy also developed an ingenious argument to show that the electrons emitted in beta-decay had to be coming from within the nucleus, not from the “orbital” electron

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Frederick Soddy

Radioactive transmutation

Any change made to the nucleus of an atom instantly affects the structure of the atom itself. As soon as the nucleus has emitted an alpha particle, say, or an electron or positron, the delicate balance of electric charges that exists in the atom is broken, with the nucleus. In order to regain stability, the atom is forced to either release some of the orbiting electrons, or attract in more from outside.

Once these exchanges have finished, the atom's basic chemical nature has changed. This phenomenon was first noticed by Ernest Rutherford in 1900. Rutherford's experiment was to observe the emanation of a radioactive gas, later known as radon, in the disintegration of metallic radium.
In 1901, along with Frederick Soddy, he was able to show that both alpha- and beta- emissions are indicative of a fundamental change in the nature of the nuclei and atoms involved. As well as providing evidence for the controversial 'solar system' model of the atom by proving the existence of a nucleus, he was thus able to conclusively disprove one of science longest-held dogmas: the indivisibility of matter. The age-old belief that 'the atom could not be split'. Soddy later recalled that when, in the sheer exultation of the moment, he triumphantly claimed to have witnessed 'transmutation', and Rutherford warned him: "For Christ's sake, Soddy, don't call it transmutation. They'll have our heads off as alchemists."
Today, such 'transmutation' is basic currency for all physicists, and can take place on relatively large scales. At the heart of nuclear reactors, where uranium is transformed into plutonium, a one Gigawatt reactor can produce 200 kilograms a year; admittedly, a modest result compared to the amount of energy involved.

Units of Energy in Nuclear Physics and the Energy of Radioactive Decay

In the circumstances when people consider the quantities of energy that they consume or produce, the unit of measure involved will likely be something such as the kilowatt-hours that appear on an electric bill or the food-calories on a nutrition label. Science students will be familiar with units such as Joules and physical calories (1 cal = 4.187 J). The food calorie appearing on nutrition labels is equivalent to 1,000 physical calories, a so-called kilocalorie. The food calorie was introduced because the physical calorie used by physicists and chemists is inconveniently small for everyday use.

The words energy and power are often confused in common usage. Power is the rate at when energy is created or used. For physicists, the standard unit of power is the Watt, which is equivalent to producing (or consuming) one Joule of energy per second.
A kilowatt (kW) is 1,000 W, or 1,000 J/s.
A kilowatt-hour (kWh) is 1,000 W times one hour, that is, 1,000 J/s times 3,600 s, or 3.6 million Joules
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A 60-W bulb left on for one hour will consume (60 J/s)x(3,600 s) = 216,000 J, or 0.06 kWh.
If electricity costs 10 cents per kWh, your bill for that hour will be six tenths of one cent.. When dealing with processes that happen at the level of individual atoms, however, calories, Joules, and Watts are all far too large to be convenient; one would be dealing with exceedingly tiny fractions of them in even very energetic reactions.

To address this, physicists who study atomic processes developed a handier unit of energy: the so-called electron-Volt.
One electron-Volt is equivalent to a mere 1.602 x 10-19 J.
This oddly-named quantity, abbreviated eV, and actually has a very sound basis in fundamental physics. You can skip this sentence if you are unfamiliar with electrical units, but for those in the know, an eV is technically defined as the kinetic energy acquired by a single electron when it is accelerated through a potential difference of one Volt. As an everyday example, the electrons supplied by a 1.5-V battery each emerge with 1.5 eV of kinetic energy. A common 9-V battery consists of six 1.5-V batteries connected in series, so their electrons emerge with 9 eV of energy. On an atom-by-atom basis, chemical reactions involve energies of a few eV. For example, when dynamite is detonated, the energy released is equivalent to 9.9 eV per molecule.

Nuclear reactions are much more energetic than chemical ones, typically involving energies of millions of electron-volts (MeV). If a nuclear reaction liberates 1 MeV per atom involved (nucleus, really) while a chemical reaction liberates 10 eV per atom involved, the ratio of the nuclear to chemical energy releases will be 100,000. This begins to give you a hint as to the compelling power of nuclear weapons.

An ‘‘ordinary’’ bomb that contains 1,000 pounds of chemical explosive could be replaced with a nuclear bomb that utilizes only 1/100 of a pound of a nuclear explosive, presuming that the weapons detonate with equal efficiency. Thousands of tons of conventional explosive can be replaced with a few tens of kilograms of nuclear explosive. Nuclear fission weapons like those used at Hiroshima and Nagasaki involved reactions which liberated about 200 MeV per reaction, so a nuclear explosion in which even only a small amount of the ‘‘explosive’’ actually reacts (e.g., one kilogram) can be incredibly devastating. It did not take physicists long to appreciate that natural radioactivity was accompanied with substantial energy releases.

First Estimation of Energy from Radioactive Decay vs Energy from Chemical Reactions

In 1903, Pierre Curie and a collaborator, A. Laborde, found that just one gram of radium released on the order of 100 physical calories of heat energy per hour. Rutherford and Soddy were also on the same track. In a May, 1903, paper titled ‘‘Radioactive Change,’’ they wrote that (expressed in modern units) ‘‘the total energy of radiation during the disintegration of one gram of radium cannot be less than 108 calories and may be between 109 and 1010 calories. The union of hydrogen and oxygen liberates approximately 4 x 103 calories per gram of water produced, and this reaction sets free more energy for a given weight than any other chemical change known. The energy of radioactive change must therefore be at least 20,000 times, and may be a million times, as great as the energy of any molecular change.’’

Another statistic Rutherford was fond of quoting was that a single gram of radium emitted enough energy during its life to raise a 500-ton weight a mile high. The moral of these numbers is that nuclear reactions liberate vastly more energy per reaction than any chemical reaction. As Rutherford and Soddy wrote: ‘‘All these considerations point to the conclusion that the energy latent in the atom must be enormous compared with that rendered free in ordinary chemical change.’’ That enormity would have profound consequences.

More about Isotopes (Modern Terminology)

In modern terminology, an element’s location in the periodic table is dictated by the number of protons in the nuclei of its atoms. This is known as the atomic number, and is designated by the letter Z. Atoms are usually electrically neutral, so the atomic number also specifies an atom’s normal complement of electrons. Chemical reactions involve exchanges of so-called valence electrons, which are the outermost electrons of atoms. Quantum physics shows us that the number of electrons in an atom, and hence the number of protons in its nucleus, accounts for its chemical properties. The periodic table as it is published in chemistry texts is deliberately arranged so that elements with similar chemical properties (identical numbers of valence electrons) appear in the same column of the table. Elements with the same number of protons but different number of neutrons are called isotopes.

The number of neutrons in a nucleus is designated by the letter N, and the total number of neutrons plus protons is designated by the letter A: A = N + Z. A is known as the mass number, and also as the nucleon number; the term nucleon means either a proton or a neutron. By specifying Z and A, we specify a given isotope. Be careful: A is also used to designate the atomic weight of an element (or isotope) in grams per mole. The atomic weight and nucleon number of an isotope are always close, but the difference between them is important. The nucleon number is always an integer, but the atomic weight will have decimals. For example, the nucleon number of uranium-235 is 235, but the atomic weight of that species is 235.0439 g/mol.

The term nuclide is also sometimes encountered, and is completely synonymous with isotope. The general form for isotope notation is A Z X. In this expression, X is the symbol for the element involved. The subscript is always the atomic number, and the superscript is always the mass number.

Example, the oxygen that you are breathing while reading this passage consists of three stable isotopes: 16 8 O, 17 8 O, and 18 8 O. All oxygen atoms have eight protons in their nuclei, but either eight, nine, or ten neutrons. These nuclides are also referred to as ogexyn-16 (O-16), oxygen-17 (O-17), and oxygen-18 (O-18). By far the most common isotope of oxygen is the first one: 99.757 % of naturally-occurring oxygen is O-16, with only 0.038 and 0.205 % being O-17 and O-18, respectively. Three isotopes that will prove very important in the story of the Manhattan Project are uranium-235, uranium-238, and plutonium-239: 235 92 U, 238 92 U, and 239 94 Pu:

The concepts of atomic number and isotopy developed over many years. The foundations of modern atomic theory can be traced back to 1803, when English chemist John Dalton put forth a hypothesis that all atoms of a given element are identical to each other and equal in weight. An important development in Dalton’s time came about when chemical evidence indicated that the masses of atoms of various elements seemed to be very nearly equal to integer multiples of the mass of hydrogen atoms. This notion was formally hypothesized about 1815 by English physician and chemist William Proust, who postulated that all heavier elements are aggregates of hydrogen atoms. He called the hydrogen atom a ‘‘protyle,’’ a forerunner of Ernest Rutherford’s ‘‘proton.’’ Parts of both Dalton’s and Proust’s hypotheses would be verified, but other aspects required modification. In particular, something looked suspicious about Proust’s idea from the outset, as some elements had atomic weights that were not close to integer multiples of that of hydrogen. For example, chlorine atoms seemed to weigh 35.5 times as much as hydrogen atoms.

The Making of the Atomic Bomb

Topic 4: The nuclear atom 1911-1920. Alpha scattering and the discovery of the nucleus. Nuclear sizes vs atomic sizes. Niels Bohr and the structure of the nuclear atom. Moseley’s work with x-rays and the significance of the atomic number.

Alpha Particles and the Nuclear Atom

In the spring of 1907, Rutherford returned to England to take a position at Manchester University. When he arrived there, he made a list of promising research projects, one of which was to pin down the precise nature of alpha particles.

Based on experiments where the number of alphas emitted by a sample of radium had been counted and the charge carried by each had been determined, he had begun to suspect that they were ionized helium nuclei. However, he needed to trap a sample of alphas for confirming spectroscopic analysis. Working with student Thomas Royds, Rutherford accomplished this with one of his typically elegant experiments. In the Rutherford-Royds experiment, a sample of radon gas was trapped in a very thin-walled glass tube, which was itself surrounded by a thicker-walled tube. The space between the two tubes was evacuated, and the radon was allowed to decay for a week. The energetic radon alphas could easily penetrate through the 1/100-mm thick wall of the inner tube. During their flights they would pick up electrons, become neutralized, and then become trapped in the space between the tubes. The neutralized alphas were then drawn off for analysis, and clearly showed a helium spectrum. Rutherford and Royds published their finding in 1909. In the notation described in the preceding section, alpha particles are identical to helium4 nuclei: 4 2He:

The discovery for which Rutherford is most famous is that atoms have nuclei; this also had its beginnings in 1909. One of the projects on Rutherford’s to-do list was to investigate how alpha particles ‘‘scattered’’ from atoms when they (the alphas) were directed through a thin metal foil. At the time, the prevailing notion of the structure of an atom was of a cloud of positive electrical material within which were embedded negatively-charged electrons. Thomson had determined that electrons weighed about 1/1,800 as much as a hydrogen atom; since hydrogen was the lightest element, it seemed logical to presume that electrons were small in comparison to their host atoms. This picture has been likened to a pudding, with electrons playing the role of raisins inside the body of the pudding. Another line of atomic structure evidence came from the chemistry community. From the bulk densities of elements and their atomic weights, it could be estimated that individual atoms behaved as if they were a few Angstroms in diameter (1 Å = 10-10 m). The few Angstroms presumably represented the size of the overall cloud of positive material. Rutherford had been experimenting with the passage of alpha-particles through metal foils since his earliest days of radioactivity research, and all of his experience indicated that the vast majority of alphas were deflected by only a very few degrees from straight-line paths as they barreled their way through a layer of foil. This observation was in line with theoretical expectations. Thomson had calculated that the combination of the size of a positively-charged atomic sphere and the kinetic energy of an incoming alpha (itself also presumably a few Angstroms in size) would be such that the alpha would typically suffer only a small deflection from its initial trajectory. Deflections of a few degrees would be rare, and a deflection of 900 was expected to be so improbable as to never have any reasonable chance of being observed. C:\Users\LYUDMILA\Desktop\res.PNG

In the Thomson atomic model, a collision between an alpha and an atom should not be imagined as like that between two billiard balls, but rather more like two diffuse clouds of positive electricity passing through each other. The alphas would presumably strike a number of electrons during the collision, but the effect of the electrons’ attractive force on the alphas would be negligible due to the vast difference in their masses, a factor of nearly 8,000. Electrons played no part in Rutherford’s work. Rutherford was working with Hans Geiger) of Geiger counter fame, who was looking for a project to occupy an undergraduate student, Ernest Marsden, another New Zealand native. Rutherford suggested that Geiger and Marsden check to see if they could observe any large-angle deflections of alphas when they passed through a thin gold foil (this why this experiment usually is called gold-foil experiment), fully expecting a negative result. Gold was used because it could be pressed into a thin foil only about a thousand atoms thick. To Geiger and Marsden’s surprise, a few alphas, about one in every 8,000, were bounced backward toward the direction from which they came. C:\Users\LYUDMILA\Desktop\res.2.PNG

The number of such reflections was small, but was orders of magnitude more than what was expected on the basis of Thomson’s model. Rutherford was later quoted as saying that the result was ‘‘almost as incredible as if you had fired a 15-in. shell at a piece of tissue paper and it came back and hit you.’’

Geiger and Marsden published their anomalous result in July, 1909. The work of detecting the scattered alpha-particles was very difficult. A Geiger counter could have been used to detect the alphas, but they had to be seen to get detailed information on their direction of travel. This was done by having the scattered alphas strike a phosphorescent screen; a small flash of light
(a‘‘scintillation’’) would be emitted, and could be counted by an observer working in a darkened room. Geiger and Marsden counted thousands of such scintillations
. So unexpected was Geiger and Marsden’s result that it took Rutherford the better part of 18 months to infer what it meant. The conclusion he came to was that the positive electrical material within atoms must be confined to much smaller volumes than had been thought to be
the case. The alpha-particles (themselves also nuclei) had to be similarly minute; only in this way could the electrical force experienced by an incoming alpha be intense enough to achieve the necessary repulsion to turn it back if it should by chance strike a target nucleus head-on; the vast majority of alpha nuclei sailed through the foil, missing gold nuclei by wide margins. The compaction of the positive charge required to explain the scattering experiments was stunning: down to a size of about 1/100,000 of an Angstrom. But, atoms as a whole still behaved in bulk as if they were a few Angstroms in diameter
. Both numbers were experimentally secure and had to be accommodated. His, then, was the origin of our picture of atoms as miniature solar systems: very small, positively-charged ‘‘nuclei’’ surrounded by orbiting electrons at distances out to a few Angstroms. This configuration ow known as the ‘‘Rutherford atom.’’ C:\Users\LYUDMILA\Desktop\re.3.PNG

A sense of the scale of Rutherford’s atom can be had by thinking of the lone proton that forms the nucleus of an ordinary hydrogen atom as scaled up to being two millimeters in diameter, about the size of an uncooked grain of rice. If this enlarged proton is placed at the center of a football field, the diameter of the lowest-energy electron orbit (that which comes closest to the nucleus) would reach to about the goal lines.

In giving us nuclei and being credited with the discovery of the positively-charged protons that they contain, Rutherford bequeathed us atoms that are largely empty space. The first public announcement of this new model of atomic structure seems to have been made on March 7, 1911, when Rutherford addressed the Manchester Literary and Philosophical Society; this date is often cited as the birthdate of the nuclear atom. The formal scientific publication came in July, and directly influenced Niels Bohr’s famous atomic model which was published two years later.

Rutherford’s nucleus paper is a masterpiece of fusion of experimental evidence and theoretical reasoning. After showing that the Thomson model could not possibly generate the observed angular distribution of alpha scatterings, he demonstrated that the nuclear ‘‘point-mass’’ model gave predictions in accord with the data. Rutherford did not use the term ‘‘nucleus’’ in his paper; that nomenclature seems to have been introduced by Cambridge astronomer John Nicholson in a paper published in November, 1911. The term ‘‘proton’’ was not introduced until June, 1920, but was coined by Rutherford himself.

With the understanding that scattering events were the results of such nuclear collisions, Rutherford’s analysis could be applied to other elements in the sense of using an observed scattering distribution to infer how many fundamental ‘‘protonic’’ charges the element possessed; this helped to place elements in their proper locations in the periodic table. Elements had theretofore been defined by their atomic weights (A), but it was the work of researchers such as Rutherford, Soddy, Geiger, and Marsden which showed that it is an element’s atomic number (Z) that dictates its chemical identity. The atomic weights of elements were still important, however, and very much the seat of a mystery. Together, chemical and scattering evidence indicated that the atomic weights of atoms seemed to be proportional to their number of protonic charges. Specifically, atoms of all elements weighed about twice as much or more as could be accounted for on the basis of their numbers of protons. For some time, this extra mass was thought to be due to additional protons in the nucleus which for some reason contained electrons within themselves, an electrically neutral combination. This would give net-charge nuclei consistent with the scattering experiments, while explaining measured atomic weights.

By the mid-1920s, however, this proposal was becoming untenable: the Uncertainty Principle of quantum mechanics ruled against the possibility of containing electrons within so small a volume as a single proton, or even a whole nucleus. For many years before its discovery, Rutherford speculated that there existed a third fundamental constituent of atoms, the neutron. He would live to see his suspicion proven by one of his own students. That atoms are built of electrons orbiting nuclei comprised of protons and neutrons is due very much to Rutherford and his collaborators and students.

Difficulties of Rutherford Model of Atom

Two basic difficulties exist with Rutherford’s planetary model of atom.
First, it was already known that an atom emits (and absorbs) certain characteristic frequencies of electromagnetic radiation and no others, but Rutherford model cannot explain this phenomenon.
A second difficulty is that Rutherford’s electron is described by the particle in uniform circular motion model: that have a centripetal acceleration. According to Maxwell’s theory of electromagnetism, centripetally accelerated charges revolving with frequency f should radiate electromagnetic waves with frequency f. This leads to ultimate collapse of an atom, because the radius of electron orbit decreases when the energy leaves the system.

Nuclear Size vs Atomic size


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Data for Scale Model of Atom

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Various types of scattering experiments suggest that nuclei are roughly
spherical and appear to have essentially the same density. The data are summarized in the expression called the Fermi model:
r= r0A1/3 where r0 = 1.2x10-15m = 1.2 fm,,
where r is the radius of the nucleus of mass number A.
The assumption of constant density of the nucleus leads to nuclear density
ρ = 2.3x1017 kg/m3..

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Niels Bohr and Bohr’s Model of Atom

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Niels Bohr


The motion of the electrons in the Rutherford model was unstable because, according to classical mechanics and electromagnetic theory, any charged particle moving on a curved path emits electromagnetic radiation; thus, the electrons would lose energy and spiral into the nucleus. To remedy the stability problem, Niels Bohr modified the Rutherford model by requiring that the electrons move in orbits of fixed size and energy. The energy of an electron depends on the size of the orbit and is lower for smaller orbits. Radiation can occur only when the electron jumps from one orbit to another. The atom will be completely stable in the state with the smallest orbit, since there is no orbit of lower energy into electron can jump.

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Bohr's starting point was to realize that classical mechanics by itself could never explain the atom's stability. A stable atom has a certain size so that any equation describing it must contain some fundamental constant or combination of constants with a dimension of length. The classical fundamental constants--namely, the charges and the masses of the electron and the nucleus--cannot be combined to make a length. Bohr noticed, however, that the quantum constant formulated by the German physicist Max Planck has dimensions which, when combined with the mass and charge of the electron, produce a measure of length. Numerically, the measure is close to the known size of atoms. This encouraged Bohr to use Planck's constant in searching for a theory of the atom.

Planck had introduced his constant in 1900 in a formula explaining the light radiation emitted from heated bodies. According to classical theory, comparable amounts of light energy should be produced at all frequencies. This is not only contrary to observation but also implies the absurd result that the total energy radiated by a heated body should be infinite. Planck postulated that energy can only be emitted or absorbed in discrete amounts, which he called quanta (the Latin word for "how much"). The energy quantum is related to the frequency of the light by a new fundamental constant, h. When a body is heated, its radiant energy in a particular frequency range is, according to classical theory, proportional to the temperature of the body. With Planck's hypothesis, however, the radiation can occur only in quantum amounts of energy. If the radiant energy is less than the quantum of energy, the amount of light in that frequency range will be reduced. Planck's formula correctly describes radiation from heated bodies. Planck's constant has the dimensions of action, which may be expressed as units of energy multiplied by time, units of momentum multiplied by length, or units of angular momentum. For example, Planck's constant can be written as h = 6.6x10-34 joule x seconds.

Using Planck's constant, Bohr obtained an accurate formula for the energy levels of the hydrogen atom. He postulated that the angular momentum of the electron is quantized--i.e., it can have only discrete values. He assumed that otherwise electrons obey the laws of classical mechanics by traveling around the nucleus in circular orbits. Because of the quantization, the electron orbits have fixed sizes and energies. The orbits are labeled by an integer, the quantum number n.

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Moseley’s Law

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Henry Gwyn Jeffrey Moseley was an English physicist and a graduate of Trinity College Oxford. His main contributions to science were the quantitative justification of the previously empirical concept of atomic number, and Moseley's law. This law advanced chemistry by immediately sorting the elements of the periodic table in a more logical order. Moseley could predict the existence of several then unknown elements. Moseley's law also advanced basic physics by providing independent support for the Bohr model of the Rutherford atom containing positive nuclear charge equal to atomic number. As Niels Bohr once said in 1962, "You see actually the Rutherford work [the nuclear atom] was not taken seriously. We cannot understand today, but it was not taken seriously at all. There was no mention of it any place. The great change came from Moseley."

In 1913 Henry Moseley found an empirical relationship between the strongest X-ray line emitted by atoms under electron bombardment (then known as the K-alfa line), and their atomic number Z. Moseley's empiric formula was found to be derivable Bohr's formula with two additional assumptions that [1] this X-ray line came from a transition between energy levels with quantum numbers 1 and 2, and [2], that the atomic number Z when used in the formula for atoms heavier than hydrogen, should be diminished by 1, to (Z−1)2.

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In brief, the law states that the square root of the frequency of the emitted x-ray is proportional to the atomic number.
Moseley's formula, by Bohr's later account, not only established atomic number as a measurable experimental quantity, but gave it a physical meaning as the positive charge on the atomic nucleus (number of protons). Because of Moseley's x-ray work, elements could be ordered in the periodic system in order of atomic number rather than atomic weight. This reversed the ordering of nickel (Z = 28, 58.7 u) and cobalt (Z = 27, 58.9 u). The fact that Bohr's model of the energies in the atom could be made to calculate X-ray spectral lines from aluminum to gold in the periodic table, and that these depended reliably and quantitatively on atomic number, did a great deal for the acceptance of the Rutherford/Bohr view of the structure of the atom.

The Making of the Atomic Bomb

Topic 5: Years 1920-1930: The invention of the mass-spectrometer. Atomic masses, the reinterpretation of isotopes, mass defects. E=Mc2 and nuclear binding energies.

Mass-spectrometer

True understanding of the nature and consequences of isotopy came with the invention of mass spectroscopy, an instrumental technique for making extremely precise measurements of atomic masses. In his 1897 work, J. J. Thomson measured the ratio of the electrical charge carried by electrons to their mass by using electric and magnetic fields to deflect them and track their trajectories. In 1907, Thomson modified his apparatus to investigate the properties of positively-charged (ionized) atoms, and so developed the first ‘‘mass spectrometer.’’ In this device, electric and magnetic fields were configured to force ionized atoms to travel along separate, unique parabolic-shaped trajectories which depended on the ions’ masses. The separate trajectories could be recorded on a photographic film for later analysis.

In 1909, Thomson acquired an assistant, Francis Aston, a gifted instrument maker. Aston improved Thomson’s instrument, and, in November, 1912, obtained evidence for the presence of two isotopes of neon, of mass numbers 20 and 22 (taking hydrogen to be of mass unity). The atomic weight of neon was known to be 20.2. Aston reasoned that this number could be explained if the two isotopes were present in a ratio of 9:1, as is now known to be the case. (There is a third isotope of neon, of mass 21, but it comprises only 0.3 % of natural neon.) Aston tried to separate the two neon isotopes using a technique known as diffusion. This refers to the passage of atoms through a porous membrane. Aston passed neon through clay tobacco pipes, and did achieve a small degree of enrichment. C:\Users\LYUDMILA\Desktop\Capture.PNG

Francis Aston

Following a position involving aircraft research during World War I, Aston returned to Cambridge, and in 1919 he built his own mass spectrometer which incorporated some improvements over Thomson’s design. In a series of papers published from late that year through the spring of 1920, he presented his first results obtained with the new instrument. These included a verification of the two previously-detected neon isotopes, and a demonstration that chlorine comprised a mixture of isotopes of masses 35 and 37 in an abundance ratio of about 3:1. In later years (1927 and 1937), Aston developed improved instruments, his so-called second and third mass spectrometers.

The principle of Aston’s mass spectrometer is as following. Inside a vacuum chamber, the sample to be investigated is heated in a small oven. The heating will ionize the atoms, some of which will escape through a narrow slit. The ionized atoms are then accelerated by an electric field, and directed into a region of space where a magnetic field of strength B is present. The magnetic field is arranged to be perpendicular to the plane of travel of the positively-charged ions. The magnetic field gives rise to an effect known as the Lorentz Force Law, which causes the ions to move in circular trajectories; an ion of mass m and net charge q that is moving with speed v will enter into a circular orbit of radius r = mv/qB. If all ions are ionized to the same degree and have the same speed, heavier ones will be deflected somewhat less than lighter ones; that is, they will have larger radius orbits. There will be one stream for each mass-species present. The streams will be maximally separated after one-half of an orbit, where they can be collected on a film. Present day models incorporate electronic detectors which can feed data to a computer for immediate analysis.

During his career, Aston discovered over 200 naturally-occurring isotopes, including uranium-238. Surprisingly, he does not have an element named after him, but he did receive the 1922 Nobel Prize for Chemistry.

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Reinterpretation of Isotopes. Mass Defect

Aston’s work showed that John Dalton’s 1803 conjecture had been partially correct: atoms of the same element behave identically as far as their chemistry is concerned, but the presence of isotopy means that not all atoms of the same element have the same weight. Similarly, Aston found that Proust’s conjecture that the masses of all atoms were integer multiples of that of hydrogen, if one substitutes ‘‘isotopes’’ for ‘‘atoms,’’ was also very nearly true. But that very nearly proved to involve some very important physics. What is meant by very nearly here?

As an example, consider the common form of iron, Fe-56, nuclei of which contain 26 protons and 30 neutrons. Had Proust been correct, the mass of an iron-56 atom should be 56 ‘‘mass units,’’ if one neglects the very tiny contribution of the electrons. (A technical aside: 56 electrons would weigh about 1.4 % of the mass of a proton. We are also assuming, for sake of simplicity, that protons and neutrons each weigh one ‘‘mass unit’’; neutrons are about 0.1 % heavier than protons.)

Mass spectroscopy can measure the masses of atoms to remarkable precision; the actual weight of an iron-56 atom is 55.934937 atomic mass units. The discrepancy of 55.934937 - 56 =-0.065063 mass units, what Aston called the ‘‘mass defect,’’ is significant, amounting to about 6.5 % of the mass of a proton. This mass defect effect proved to be systemic across the periodic table: all stable atoms are less massive than one would predict on the basis of Proust’s whole number hypothesis. Iron has a fairly large mass defect, but by no means the largest known .The mass-defect is not an artifact of protons and neutrons having slightly different masses; if one laboriously adds up the masses of all of the constituents of atoms, the defects are still present.

The unavoidable conclusion is that when protons and neutrons assemble themselves into nuclei, they give up some of their mass in doing so. Physicists now quote mass defects in terms of equivalent energy in MeVs, thanks to E = mc2. One mass unit is equivalent to 931.4 MeV, so the iron-56 mass defect amounts to just over 60 MeV. Because this is a mass defect, it is formally cited as a negative number, -60.6 MeV. The capital Greek letter delta (as in ‘‘Defect’’) is now used to designate such quantities: D = -60.6 MeV.

Binding Energy

Where does the mass go when nature assembles nuclei? Empirically, nuclei somehow have to hold themselves together against the immense mutual repulsive Coulomb forces of their constituent protons; some sort of nuclear ‘‘glue’’ must be present. To physicists, this ‘‘glue’’ is known synonymously as the ‘‘strong force’’ or as ‘‘binding energy,’’ and is presumed to be the ‘‘missing’’ mass transformed into some sort of attractive energy. The greater the magnitude of the mass defect, the more stable will be the nucleus involved. Figure below shows a plot of binding energy per nucleon as a function of mass number A for various stable nuclei. Image result for binding energy

And next figure shows a graph of the mass defects of 350 nuclides that are stable or have half-lives greater than 100 years, as a function of mass number A.

Image result for mass defect

The deep valley centered at A * 120 attests to the great stability of elements in the middle part of the periodic table; negative values of D connote intrinsic stability. The gap between A * 210 and 230 is due to the fact that there are no long-lived isotopes of elements between bismuth (Z = 83) and thorium (Z = 90). Isotopes with A [230 could be said to have a ‘‘mass surplus.’’ Consistent with the idea that negative D-values connote stability, all such positive D-valued nuclei eventually decay.

Strictly, these are separate (but related) quantities. At a qualitative level, the details of the technical distinctions between them do not really add to the central concept that ‘‘lost mass’’ transforms to ‘‘binding energy.’’

The above figure can be used to estimate the energy released in hypothetical nuclear reactions. The essence is straightforward: Add up the D-values of all of the input reactants (be careful with negative signs!), and then subtract from that result the sum of the D-values of the output products.

In late 1938 it was discovered that reactions like this are very real possibilities indeed. There exist 266 apparently permanently stable, naturally occurring isotopes of the various elements, and about a hundred more ‘‘quasi-stable’’ ones with half-lives of a hundred years or greater. A compact way of representing all these nuclides is to plot each one as a point on a graph where the x-axis represents the number of neutrons, and the y-axis the number of protons. All isotopes of a given element will then lie on a horizontal line, since the number of protons in all nuclei of a given element is the same. Clearly, stable nuclei follow a very well-defined Z(N) trend. Nature provides nuclei with neutrons to hold them together against the mutual repulsion of their protons, but she is economical in doing so. Mass represents energy (E = mc2), and Nature is evidently unwilling to invest more mass-energy to stabilize nuclei than is strictly necessary. Note also that the points in the graph curve off to the right; this indicates that the vast majority of nuclei, except for a very few at the bottom-left of the graph, contain more neutrons than protons; this effect is known as the neutron excess.


The Making of the Atomic Bomb


Topic 6: Years 1920-1931continued: The discovery of nuclear reactions. The Coulomb barrier and limitations on nuclear studies and alpha particles. Accelerators.

Reaction Notations, Q-Values

In nuclear physics and nuclear chemistry, a nuclear reaction is considered to be the process in which two nuclei, or else a nucleus of an atom and a subatomic particle from outside the atom, collide to produce one or more nuclides that are different from the nuclide that began the process. 

The notation for writing a nuclear reaction is very similar to used that for describing a chemical reaction. Reactants or input nuclides are written on the left side of a rightward-pointing arrow, and products or output nuclides are placed on the other side, like this:

Reactants --> products

Decades of experimental evidence indicate that there are two rules that are always obeyed in nuclear reactions: (1) The total number of input nucleons must equal the total number of output nucleons. The numbers of protons and neutrons may (and usually do) change, but their sum must be conserved. (2) Total electric charge must be conserved. Protons count as one unit of positive charge.

Beta decays involve nuclei which create within themselves and then eject either an electron or a positively-charged particle with the same mass as an electron, a so-called positron. The charges of these ejected particles must be taken into account in ensuring charge conservation (negative or positive one unit), but they are not considered to be nucleons and so are not counted when applying rule (1). Positrons are also known as beta-positive (β+) particles, while ordinary electrons are also known as beta-negative particles (β–).

As an example of a typical reaction alpha-bombardment of nitrogen to produce hydrogen and oxygen:

24He + 714N -> 11H +817O.

Verification that both rules are followed can be seen in that (1) 4 + 14 = 1 + 17, and (2) 2 + 7 = 1 + 8. In this type of bombardment reaction, the notational convention is to write the lighter incoming reactant first on the left side, followed by the target nucleus. Note that a hydrogen nucleus, 1 1H; is simply a proton. A proton is sometimes written as just ‘‘p’’.

In any reaction where the input and input reactants are different, experiments show that mass is not conserved. That is, the sum of the input masses will be different from the sum of the output masses. Mass can either be created or lost; what happens depends on the nuclides involved. The physical interpretation of this relates to Einstein’s famous E = mc2 equation. If mass is lost (sum of output masses<sum of input masses), the lost mass will appear as kinetic energy of the output products. If mass is gained (sum of output masses> sum of input masses), energy must be drawn from somewhere to create the mass gained, and the only source available is the kinetic energy of the ‘‘bombarding’’ input reactant.

Nuclear physicists always express the mass gain or loss in units of energy equivalent, almost always in MeV. Such energy gains or losses are termed Q-values. If Q>0, kinetic energy is created by consuming input-reactant mass, whereas if Q<0, input-particle kinetic energy has been consumed to create additional output mass. The technical definition of Q is

Q=( sum of input masses) –(sum of product masses )

quoted in units of equivalent energy (one atomic mass unit = 931.4 MeV).

Returning for a moment to mass spectroscopy, the development of means to measure precise masses for isotopes was a crucial step forward in the progress of nuclear physics. With precise masses and knowledge of Einstein’s E = mc2 equivalence, the energy liberated or consumed in reactions could be predicted. Measurements of the kinetic energies of reaction products would then serve as checks on the mass values. Conversely, for a reaction where the mass or identity of some of the particles involved was not clear, measurements of the kinetic energies could be used to infer what was happening.

On reflecting on these connections, you might wonder how Rutherford measured such kinetic energies; after all, tracking a nucleus is obviously not the same as using a radar gun to measure the speed of a car or a baseball. Experimenters had to rely on proxy measurements such as how far a particle traveled through air or a stack of thin metal foils before being brought to a stop. If precise mass defects are known from mass spectroscopy, the energy liberated (or consumed) in a reaction can be computed, and the numbers can be used to calibrate a range-versus-energy relationship. This combination of theory, experimental technique, and instrumental development is an excellent example of scientific cross-fertilization.


Alpha Decay

Ernest Rutherford decoded alpha-decay as a nucleus spontaneously transmuting itself to a more stable mass-energy configuration by ejecting a helium nucleus. In doing so, the original nucleus loses two protons and two neutrons, which means that it ends up two places down in atomic number on the periodic table and has four fewer nucleons in total. Alpha-emission is a common decay mechanism in heavy elements, and can be written in the arrowed notation as

AZX -> Z-2 A-4Y + 24He.

Here, X designates the element corresponding to the original nucleus, and Y that of the ‘‘daughter product’’ nucleus. Sometimes the half-life is written below the arrow; for example, the alpha-decay of uranium-235 can be written as

92 235 U -> 90 231Th +24He.

As always, electrical charge and nucleon number are conserved. In such decays, the total mass of the output products is always less than that of the input particles: Nature spontaneously seeks a lower mass-energy configuration (Q>0). The energy release in alpha-decays is typically Q = 5 - 10 MeV, the majority of which appears as kinetic energy of the alpha-particle itself. Helium nuclei tend to be ubiquitous in nuclear reactions as they have a large mass defect and so are very stable. As a tool to induce nuclear reactions, the Curies and Rutherford often utilized alpha particles emitted in radium decay.\

Beta Decay

Two types of beta decay occur naturally. Suppose that a nucleus is too neutron-rich for its number of protons. Purely empirically, it has been found that Nature deals with this by having a neutron spontaneously decay into a proton. But this, by itself, would represent a net creation of electric charge, and hence a violation of charge conservation. So, negative electron is created in the bargain to render no net charge created. Nucleon number is conserved; remember that electrons do not count as nucleons. The electron is also known as a β- particle, and the reaction can be symbolized as n-> p +e, The number of neutrons drops by one while the number of protons grows by one, so the number of nucleons is unchanged. The overall effect is

ZAX -> Z+1 A Y + -1 0 e.

Note that a ‘‘nucleon-like’’ notation has been appended to the electron to help keep track of the charge and nucleon numbers. The result of β- decay is to move a nucleus up one place in the periodic table. It was Henri Becquerel who showed, in 1900, that the negatively-charged beta-rays being observed in such decays were identical in their properties to Thomson’s electrons. If a nucleus is neutron-poor, a proton will spontaneously decay into a neutron. But this would represent a loss of one unit of charge, so Nature creates a positron—an anti-electron—to maintain..

Artificial Transmutation

Rutherford’s last great discovery came in 1919. This was his realization that it was possible to set up experimental situations wherein atoms of a given element could be transmuted into those of another, when bombarded by nuclei of yet a third. The idea of elemental transmutation was not new; after all, this is precisely what happens in natural alpha and beta-decays. What was new was the realization that transmutations could be induced by human intervention. The work that led to this discovery began around 1915, and was carried out by Ernest Marsden. As part of an experimental program involving measurements of reaction energies, Marsden bombarded hydrogen atoms with alpha-particles produced by the decay of samples of radon gas contained in a small glass vials. A hydrogen nucleus would receive a significant kick from a collision with an alpha-particle and be set into motion at high speed. These experiments were done by sealing the alpha source and hydrogen gas inside a small chamber.

Image result for artificial transmutation

At one end of the chamber was a small scintillation screen which could be viewed through a microscope, as had been done in the alpha-scattering experiments. By placing thin metal foils just behind the screen, Marsden could determine the ranges, and hence the energies, of the struck protons. So far, there is nothing unusual here; these experiments were routine work that involved the use of known laws of conservation of energy and momentum to cross-check and interpret measurements. Breakthroughs favor an attentive and experienced mind, and Marsden’s was ready. His crucial observation was to notice that when the experimental chamber was evacuated, the radon source itself seemed to give rise to scintillations like those from hydrogen, even though there was no hydrogen in the chamber. The implication seemed to be that hydrogen was arising in radioactive decay, an occurrence that had never before been observed.

Marsden returned to New Zealand in 1915, and Rutherford, heavily occupied with research for the British Admiralty, could manage only occasional experiments until World War I came to an end in late 1918. In 1919, he turned to investigating Marsden’s unexpected radium/hydrogen observation, and was rewarded with yet another pivotal discovery.

Rutherford placed a source of alpha particles within a small brass chamber which could be evacuated and then filled with a gas with which he wished to experiment. As Rutherford reported in his June, 1919, discovery paper, he set out to investigate the phenomenon that ‘‘a metal source, coated with a deposit of radium-C [bismuth-214], always gives rise to a number of scintillations on a zinc sulphide screen far beyond the range of the a particles. The swift atoms causing these scintillations carry a positive charge and are deflected by a magnetic field, and have about the same range and energy as the swift H atoms produced by the passage of a particles through hydrogen. These ‘natural’ scintillations are believed to be due mainly to swift H atoms from the radioactive source, but it is difficult to decide whether they are expelled from the radioactive source itself or are due to the action of a particles on occluded hydrogen.’’

Rutherford proceeded by investigating various possibilities as to the origin of the hydrogen scintillations. No vacuum pump is ever perfect; some residual air would always remain in the chamber no matter how thoroughly it had been pumped down. While hydrogen is normally a very minute component of air (about half a part per million), more could be present if the air contained water vapor. Suspecting that the alpha particles might be striking residual hydrogen-bearing water molecules, Rutherford began by introducing dried oxygen and carbon dioxide into the chamber, observing, as he expected, that the number of scintillations decreased. Surprisingly, however, when he admitted dry air into the chamber, the number of hydrogen-like scintillations increased. This suggested that hydrogen was arising not from the radium-C itself, but from some interaction of the alpha particles with air. The major constituents of air are nitrogen and oxygen; having eliminated oxygen, Rutherford inferred that nitrogen might be involved. On admitting pure nitrogen into the chamber, the number of scintillations increased yet again. As a final test that hydrogen was not somehow arising from the radioactive source itself, he found that on placing thin metal foils close to the radioactive source, the scintillations persisted, but their range was reduced in accordance with what would be expected if the alpha particles were traveling through the foils before striking nitrogen atoms; the scintillations were evidently arising from within the volume of the chamber.

As Rutherford wrote, ‘‘it is difficult to avoid the conclusion that the long-range atoms arising from collision of a particles with nitrogen are … probably atoms of hydrogen …. If this be the case, we must conclude that the nitrogen atom is disintegrated under the intense forces developed in a close collision with α-particle.

The Coulomb Barrier and Particle Accelerators

Consider again Rutherford’s alpha-bombardment of nitrogen, the first artificial transmutation of an element (neglecting the gamma-ray):

Neglecting the fact that this reaction has a negative Q-value, a simple interpretation of this equation is that if you were to mix helium and nitrogen, say at room-temperature conditions, hydrogen and oxygen would result spontaneously. But even if Q were positive, this would not happen because of an effect that is not accounted for in merely writing down the reaction or in computing the Q-value: the so-called ‘‘Coulomb barrier’’ problem.

Electrical charges of the same sign repel each other. This effect is known as ‘‘the Coulomb force’’ after French physicist Charles-Augustin de Coulomb, who performed some of the first quantitative experiments with electrical forces in the late 1700s.

Because of the Coulomb force, nitrogen nuclei will repel incoming alpha-particles; only if an alpha has sufficiently great kinetic energy will it be able to closely approach a nitrogen nucleus. Essentially, the two have to collide before stronger but shorter-range ‘‘nuclear forces’’ between nucleons that effect transmutations can come into play. The requisite amount of kinetic energy that the incoming nucleus must possess to achieve a collision is called the ‘‘Coulomb barrier.’’ For an alpha-particle striking a nitrogen nucleus, the barrier amounts to about 4.2 MeV, a fairly substantial amount of energy. An atom or molecule at room temperature will typically possess only a fraction of an eV of kinetic energy (about 0.025 eV on average), not nearly enough to initiate the reaction. Rutherford was able to induce the nitrogen transmutation because his radium-C alphas possessed over 5 MeV of kinetic energy.

Consider a reaction when alpha-particles strike nuclei of uranium-235. The experiment would be hopeless if you are using an alpha whose kinetic energy is of the typical 5–10 MeV decay energy. According to calculations, the Coulomb barrier in this reaction is about 25.5 MeV

If one is using alphas created in natural decays, it is practical to carry out bombardment experiments with target elements only up to Z about 20. By the mid1920s this was becoming a serious problem: researchers were literally running out of elements to experiment with. The curiosity-driven desire to bombard heavier elements thus generated a technological challenge: Was there any way that the alpha (or other) particles could be accelerated once they had been emitted by their parent nuclei? It was this challenge that gave birth to the first generation of particle accelerators. The first practical particle acceleration scheme was published by Norwegian native Rolf Wideröe in a German electrical engineering journal in 1928.

Image result for Rolf Wideroe

The essence of Wideröe’s proposal is pictured here.

Two hollow metal cylinders are placed end-to-end and connected to a source of variable polarity voltage. This means that the cylinders can be made positively or negatively charged, and the charges can be switched as desired. A stream of protons (say) is directed into the leftmost cylinder, which is initially negatively charged. This will attract the protons, which will speed up as they pass through the cylinder. Just as the bunch of protons emerges from the first cylinder, the voltage polarity is switched, making the left cylinder positive and the right one negative. The protons then get a push from the first cylinder while being pulled into the second one, which further accelerates them. By placing a number of such units back-to-back, substantial accelerations can be achieved; this is the principle of a linear accelerator. Obviously, many of the incoming particles will be lost by crashing into the side of a cylinder or because their speed does not match the frequency of the polarity shifts of the voltage supplies; only a small number will emerge from the last cylinder. But the point here is not necessarily efficiency; it is to generate some high-speed particles which could surmount the Coulomb barriers of heavy target nuclei. The longest linear accelerator in the world is now the Stanford Linear Accelerator in California, which can accelerate electrons to 50 billion electronvolts of kinetic energy over a distance of 3.2 km (2 miles).

Wideröe’s work came to the attention of Ernest Orlando Lawrence, C:\Users\LYUDMILA\AppData\Local\Microsoft\Windows\INetCache\Content.Word\w3.png

an experimental physicist at the University of California at Berkeley. Lawrence and collaborator David Sloan built a Wideröe device, which by late 1930 they had used to accelerate mercury ions to kinetic energies of 90,000 eV. While experimenting with the Wideröe design, however, Lawrence had an inspiration that was to have profound consequences. He desired to achieve higher energies, but was daunted by the idea of building an accelerator that would be meters in length. How could the device be made more compact?

Lawrence’s new device, which he called a cyclotron, made use of Lorentz force , but in a way that simultaneously incorporated Wideröe’s alternating voltage acceleration scheme. Lawrence’s cyclotron is sketched in Figure, which is taken from his application for a patent on the device. C:\Users\LYUDMILA\Desktop\w2.PNG

Here the voltage supply is connected to two D-shaped metal tanks placed back-to back; they are known to cyclotron engineers as ‘‘Dees.’’ The entire assembly must be placed within a surrounding vacuum tank to avoid deflective effects of collisions of the accelerated particles with air molecules. The source of the ions (usually positive) is placed between the Dees. In the diagram, the ions are initially directed toward the upper Dee, which is set to carry a negative charge to attract them. If the voltage polarity is not changed and there is nothing to otherwise deflect the ions, they would crash into the edge of the Dee. But Lawrence knew from Aston’s work that if the assembly were placed between the poles of a magnet (with the magnetic field again emerging from the page), the Lorentz force would try to make the ions move in circular paths. The net result of the combination of the ions’ acceleration toward the charged Dee and the Lorentz force is that the ions move in outward-spiraling trajectories. If the magnetic field is strong, the spiral pattern will be ‘‘tight’’, and the ions will get nowhere near the edge of the Dee in their first orbit. As ions leave the upper Dee, the polarity is switched in order to attract them to the lower Dee. Switching and acceleration continues (for microseconds only) until the ions strike a target at the periphery of one of the Dees.

Particle accelerators allowed experimenters to surmount the Coulomb barrier and so open up a broad range of energies and targets to experimentation. Lawrence’s ingenuity earned him the 1939 Nobel Prize for Physics, and a variant of his cyclotron concept would play a significant role in the Manhattan Project. Today’s giant accelerators at the Fermi National Accelerator Laboratory (Fermi lab) and the European Organization for Nuclear Research (CERN) are the descendants of Wideröe’s and Lawrence’s pioneering efforts, and still use electric and magnetic fields to accelerate and direct particles.

The Making of the Atomic Bomb

Topic 7: Years 1932-1934: The discovery of the neutron. Reinterpretation of nuclear structure. Leo Szilard and the concept of a nuclear chain reaction. Discoveries of the positron and artificial radioactivity.

The Discovery of the Neutron

The discovery of the neutron in early 1932 by Ernest Rutherford’s protégé, James Chadwick, was a critical turning point in the history of nuclear physics. Within two years, Enrico Fermi would generate artificially-induced radioactivity by neutron bombardment, and five years after that, Otto Hahn, Fritz Strassman, and Lise Meitner would discover neutron-induced uranium fission. The latter would lead directly to the Little Boy uranium-fission bomb, while Fermi’s work would lead to reactors to produce plutonium for the Trinity and Fat Man bombs.

The experiments which led to the discovery of the neutron were first reported in 1930 by Walther Bothe and his student, Herbert Becker, who were working in Germany. Their research involved studying the gamma radiation which is produced when light elements such as magnesium and aluminum are bombarded by energetic alpha-particles. In such reactions, the alpha particles often interact with a target nucleus to yield a proton and a gamma-ray, as Ernest Rutherford had found when he first achieved an artificially-induced nuclear transmutation:

24He + 714N -> 11H + 817O + γ.

The mystery began when Bothe and Becker found that boron, lithium, and particularly beryllium gave evidence of gamma emission under alpha bombardment, but with no accompanying protons being emitted. A key point here is that they were certain that some sort of energetic but electrically neutral ‘‘penetrating radiation’’ was being emitted; this radiation could penetrate foils of metal but could not be deflected by a magnetic field as charged particles would be. Gamma rays were the only electrically neutral form of penetrating radiation known at the time, so it was natural for them to interpret their results as evidence of gamma-ray emission despite the anomalous lack of protons.

Bothe and Becker’s beryllium result was picked up by the Paris-based husband and-wife team of Frederic Joliot and Irene Curie (the daughter of Pierre and Marie), hereafter referred to as the Joliot-Curies. In January, 1932, they reported that the presumed gamma-ray ‘‘beryllium radiation’’ was capable of knocking protons out of a layer of paraffin wax that had been put in its path. The situation is shown schematically in Fig. where the supposed gamma-rays are labeled as ‘‘mystery radiation.’’

At Cambridge, this interpretation struck Chadwick as untenable. He had searched for neutrons for many years with no success, and suspected that Bothe and Becker and the Joliot-Curies had stumbled upon them. He immediately set about to reproduce, re-analyze, and extend their work. In his recreation of the Joliot-Curies’ work, Chadwick’s experimental setup involved polonium (the alpha source deposited on a silver disk 1 cm in diameter placed close to a disk of pure beryllium 2 cm in diameter, with both enclosed in a small vessel which could be evacuated. In comparison to the gargantuan particle accelerators of today, these experiments were literally table-top nuclear physics. Let us first assume that Bothe and Becker and the Joliot-Curies were correct in their interpretation that a-bombardment of beryllium creates gamma-rays. To account for the lack of protons created in the bombardment, the Joliot-Curies hypothesized that the reaction was

24He +49Be -> 613C + γ

The Q-value of this reaction is 10.65 MeV. Polonium decay yields alpha particles with kinetic energies of about 5.3 MeV, so the emergent c-ray can have at most an energy of about 16 MeV. A more detailed analysis which accounts for the energy and momentum transmitted to the carbon atom shows that the energy of the gamma ray comes out to be about 14.6 MeV. The 14.6-MeV gamma-rays then strike protons in the paraffin, setting them into motion. Upon reproducing the experiment, Chadwick found that the struck protons would emerge with maximum kinetic energies of about 5.7 MeV. The problem, Chadwick realized, was that if a proton was to be accelerated to this amount of energy by being struck by a gamma-ray, conservation of energy and momentum demanded that the gamma-ray would have to possess about 54 MeV of energy, nearly four times what it could have!

This strikingly high energy demand is a consequence of the fact that photons do not possess mass. Relativity theory shows that massless particles do carry momentum, but much less than a ‘‘material’’ particle of the same kinetic energy; only an extremely energetic gamma-ray can kick a proton to a kinetic energy of several MeV. Analyzing a collision between a photon and a material particle involves relativistic mass-energy and momentum conservation. The results of such an analysis show that if a target nucleus of rest-energy Et (that is, mc2 equivalent energy) is to be accelerated to kinetic energy K by being struck head-on by a photon of energy Ec which then recoils backwards (this transfers maximum momentum to the struck nucleus), then the energy of the photon must be for a proton, Et = 938 MeV; with Kt =5.7 MeV, the value of Ec works out to about 54 MeV, as claimed above. Remarkably, the Joliot-Curies had realized that this discrepancy was a weak point in their interpretation, but attributed it to the difficulty of accurately measuring the energy of their ‘‘gamma rays.’’

Another clue that led Chadwick to suspect a material particle as opposed to a high-energy photon was that the ‘‘beryllium radiation’’ was more intense in the forward direction than in the backward direction; if the radiation was photonic, it should have been of equal intensity in all directions. Before invoking a mechanism involving a (hypothetical) neutron, Chadwick devised a further test to investigate the remote possibility that 54-MeV gamma rays could be being created in the α-Be collision. In addition to having the ‘‘beryllium radiation’’ strike protons, he also arranged for it to strike a sample of nitrogen gas. If struck by such a photon, a nucleus of nitrogen should acquire a kinetic energy of about 450 keV. A nitrogen nucleus has a rest energy of about 13,000 MeV; From prior experience, Chadwick knew that when an energetic particle travels through air it produces ions, with about 35 eV required to produce a single ionization, which yields one pair of ions. A 450 keV nitrogen nucleus should thus generate some (450 keV/35 eV) = 13,000 ion pairs. Upon performing the experiment, however, he found that some 30,000–40,000 ion pairs would typically be produced, which implied kinetic energies of about 1.1–1.4 MeV for the recoiling nitrogen nuclei. Such numbers would in turn require the nitrogen nuclei to have been struck by gamma-rays of energy up to 90 MeV, a value completely inconsistent with the 854 MeV indicated by the proton experiment. Upon letting the supposed gamma rays strike heavier and heavier target nuclei, Chadwick found that ‘‘if the recoil atoms are to be explained by collision with a quantum, we must assume a larger and larger energy for the quantum as the mass of the struck atom increases.’’ The absurdity of this situation led him to write that, ‘‘It is evident that we must either relinquish the application of conservation of energy and momentum in these collisions or adopt another hypothesis about the nature of the radiation.’’

After refuting the Joliot-Curies’ interpretation, Chadwick provided a more physically realistic one. This was that if the protons in the paraffin were in reality being struck by neutral material particles of mass equal or closely similar to that of a proton, then the kinetic energy of the striking particles need only be on the order of the kinetic energy that the protons acquired in the collision. As an everyday example, think of a head-on collision between two equal-mass billiard balls: the incoming one stops, and the struck one is set into motion with the speed that the incoming one had. This is the point at which the neutron makes it debut. Chadwick hypothesized that instead of the Joliot-Curie reaction, the a-Be collision leads to the production of carbon and a neutron via the reaction

24He + 49Be -> 612C + o1n.

01n denotes a neutron: it carries no electric charge but it does count as one nucleon. In this interpretation, a C-12 atom is produced as opposed to the Joliot Curies’ proposed C-13. Since the ‘‘beryllium radiation’’ was known to be electrically neutral, Chadwick could not invoke a charged particle such as a proton or electron to explain the reaction. Fig. below:  Left: Bothe's experiment. Be when bombarded by α-particles (in red) of Po emits a radiation (in green) of great penetrating power, which Bothe assumed to be γ-rays. Middle: Joliot-Curie's experiment. H, when hit by the radiation marked in green, emits protons of high energy. Right: Chadwick's experiment. He analyzed the conservation of energy and momentum in these nuclear reactions.

Fig. 1

Hypothesizing that the neutron’s mass was similar to that of a proton (he was thinking of neutrons as being electrically neutral combinations of single protons and single electrons), Chadwick was able to show that the kinetic energy of the ejected neutron would be about 10.9 MeV. A subsequent neutron/proton collision will be like a billiard-ball collision, so it is entirely plausible that a neutron which begins with about 11 MeV of kinetic energy would be sufficiently energetic to accelerate a proton to a kinetic energy of 5.7 MeV, even after the neutron battered its way out of the beryllium target and through the window of the vacuum vessel on its way to the paraffin. As a check on his hypothesis, Chadwick calculated that a neutron of kinetic energy 5.7 MeV striking a nitrogen nucleus should set the latter into motion with a kinetic energy of about 1.4 MeV, which was precisely what he had measured in the ion-pair experiment! Further experiments with other target substances showed similarly consistent results. Chadwick estimated the mass of the neutron as between 1.005 and 1.008 atomic mass units; the modern figure is 1.00866. The accuracy he obtained with equipment which would now be regarded as primitive is nothing short of awe-inspiring.
Chadwick reported his discovery in two papers. The first, titled ‘‘Possible Existence of a Neutron,’’ was dated February 17, 1932, and was published in the February 27 edition of Nature. An extensive follow-up analysis dated May 10 was published in the June 1 edition of the Proceedings of the Royal Society of London. Chadwick was awarded the 1935 Nobel Prize in Physics for his discovery. While later experiments showed that the neutron is a fundamental particle in its own right (as opposed to being a proton/electron composite), that development does not affect the above analysis.

Why the discovery of the neutron is regarded as such a pivotal event in the history of nuclear physics? The reason is that neutrons do not experience any electrical forces, so they experience no Coulomb barrier. With neutrons, experimenters now had a way of producing particles that could be used to bombard nuclei without being repelled by them, no matter what the kinetic energy of the neutron or the atomic number of the target nucleus. It was not long before such experiments were taken up. Neutrons would prove to be the gateway to reactors and bombs, but, at the time, Chadwick anticipated neither development. In the February 29, 1932, edition of the New York Times, he is quoted as stating that, ‘‘I am afraid neutrons will not be of any use to any one.’’

Leo Szilard and Concept of Chain Reaction

About 18 months after Chadwick’s dismissal of the value of neutrons, an idea did arise as to a possible application for them: As links in the progression of a nuclear chain reaction. This notion seems to have occurred inspirationally to a Hungarian-born engineer, physicist, and inventor named Leo Szilard, a personal friend and sometimes collaborator of Albert Einstein.

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Leo Szilárd

Szilard was living in London in the fall of 1933, and happened to read a description of a meeting of the British Association for the Advancement of Science published in the September 12 edition of the London Times. In an article describing an address to the meeting by Rutherford on the prospects for reactions that might be induced by accelerated protons, the Times quoted Rutherford as stating that, ‘‘We might in these processes obtain very much more energy than the proton supplied, but on the average we could not expect to obtain energy in this way. It was a very poor and inefficient way of producing energy, and anyone who looked for a source of power in the transformation of the atoms was talking moonshine.’’ Historian of science John Jenkin has pointed out that Rutherford’s private thoughts on the matter may have been very different, however. Some years before World War II, Rutherford evidently advised a high government official that he had a hunch that nuclear energy might one day have a decisive effect on war. Szilard reflected on Rutherford’s remarks while later strolling the streets of London. From a 1963 interview:

Pronouncements of experts to the effect that something cannot be done have always irritated me. That day as I was walking down Southampton Row and was stopped for a traffic light, I was pondering whether Lord Rutherford might not prove to be wrong. As the light changed to green and I crossed the street, it suddenly occurred to me that if we could find an element which is split by neutrons and which would emit two neutrons when it absorbed one neutron, such an element, if assembled in sufficiently large mass, could sustain a nuclear chain reaction, liberate energy on an industrial scale, and construct atomic bombs. The thought that this might be possible became an obsession with me. It led me to go into nuclear physics, a field in which I had not worked before, and the thought stayed with me.
It did not take Szilard long to get up to speed in his new area. Envisioning a chain reaction as a source of power and possibly as an explosive, he filed for patents on the idea in the spring and summer of 1934. His British patent number 630,726, ‘‘Improvements in or relating to the Transmutation of Chemical Elements,’’ was issued on July 4, 1934 (curiously, the date of Marie Curie’s death), and referred specifically to being able to produce an explosion given a sufficient mass of material. To keep the idea secret, Szilard assigned the patent to the British Admiralty in February, 1936. The patent was reassigned to him after the war, and finally published in 1949.

Artificially-Induced Radioactivity

Irene and Frederic Joliot-Curie must have been deeply disappointed at their failure to detect the neutron in early 1932, but scored a success almost exactly two years later when they discovered that normally stable nuclei could be induced to become radioactive upon alpha-particle bombardment. In early 1934, the Joliot-Curies were performing some follow-up experiments involving bombarding thin foils of aluminum with alpha-particles emitted by decay of polonium, the same source of alphas used in the neutron-discovery reaction. To their surprise, their Geiger counter continued to register a signal after the source of the alpha particles was removed. The signal decayed with a half-life of about 3 min. Performing the experiment in a magnetic field led them to conclude that positrons were being emitted, that is, that β+ decays were occurring. They proposed a two-stage reaction to explain their observations. First was formation of phosphorous-30 by alpha-capture and neutron emission:

24He + 1327Al -> 01n +1530P.

The phosphorous-30 nucleus subsequently undergoes positron decay to silicon; the modern value for the half-life is 2.5 min (the emitted beta-particle is omitted here; it is the decay product that is important):

1530P ->1430Si + β+

To be certain of their interpretation, the Joliot-Curies dissolved the bombarded aluminum in acid; the small amount of phosphorous created could be separated and chemically identified as such. That the radioactivity ‘‘carried with’’ the separated phosphorous and not the aluminum verified their suspicion. Bombardment of boron and magnesium showed similar effects. They first observed the effect on January 11, 1934, and reported it in the January 15 edition of the journal of the French Academy of Sciences; an English version appeared in the February 10 edition of Nature.

The discovery of artificially-induced radioactivity opened up the whole field of synthesizing short-lived isotopes for medical treatments. Emilio Segre, one of Enrico Fermi’s students, described this development as one of the most important discoveries of the century. Induced radioactivity had almost been discovered in California, where Ernest Lawrence’s cyclotron operators often noticed that their detectors kept registering a signal after the cyclotron had been shut down following bombardment experiments. Thinking that the detectors were misbehaving, they arranged circuitry to shut them down simultaneously with the cyclotron. The history of nuclear physics, particularly events surrounding the discovery of fission, is replete with such missed chances.

Discovery of Positron

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In 1928, British physicist Paul Dirac showed that Einstein's relativity implied that every particle in the universe has a corresponding antiparticle, each with the same mass as its twin, but with the opposite electrical charge. There was a big challenge to find experimental verification of this hypothesis; a Caltech postdoc named Carl D. Anderson would win the race.
Anderson spent most of his career at Caltech. His early research was on X-rays, but then Victor Hess discovered cosmic rays in 1930. At the advice of his mentor, Robert A. Millikan, Anderson turned his attention to studying those high energy particles. Most scientists were doing this by using cloud chambers: a short cylinder with glass end plates containing a gas saturated with water vapor. If an ionizing particle passes through the chamber, it leaves a trail of water droplets, which can be photographed. By measuring the density of the droplets, scientists can deduce how much ionization is produced—indicating the kind of particle that passed through.
When starting working on a problem to discuss the content of cosmic rays, Anderson
built his own, improved version of a cloud chamber. He surrounded his chamber with a large electromagnet that caused the paths of ionizing particle to bend into circular
path. By measuring the curvature of those tracks, he could determine the particle sign.
The resulting photographs surprised Anderson by revealing that cosmic rays produced showers of both positively and negatively charged particles and the positive charges could not be protons, as one might expect, because the track radius would specify a proton stopping distance much shorter than the length of the track.
In August 1932, Anderson recorded the historic photograph (above) of a positively charged electron (now known as a positron) passing through the lead plate in the cloud chamber. It was definitely a positively charged particle, and it was traveling upwards.
For this discovery Anderson was awarded by Nobel prize in 1936.

The Making of the Atomic Bomb

Topic 8: Years 1935-1938: Enrico Fermi’s discoveries in neutron activation and neutron moderation. Bohr’s development of the liquid drop model of the nucleus. The puzzle of the neutron bombardment of uranium.

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Enrico Fermi and his Research on Using Neutrons

Surprisingly, neither the Joliot-Curies nor James Chadwick particularly experimented with using neutrons as bombarding particles. Norman Feather, one of Chadwick’s collaborators, did carry out some experiments with light elements, and found that neutrons would disintegrate nitrogen nuclei to produce an alpha-particle and a boron nucleus:
01n = 714N -> 24He + 511B.

The same type of reaction also occurs with elements such as oxygen, fluorine, and neon, but apparently neither British nor French researchers carried out experiments with heavy-element targets. The idea of systematically using neutrons as bombarding particles did occur to a physicist at the University of Rome, Enrico Fermi.

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Enrico Fermi

Fermi had established himself as a first-rate theoretical physicist at a young age, publishing his first paper while still a student. As a postdoctoral student with quantum mechanist Max Born, Fermi had prepared an important review article on relativity theory while in his early twenties, and a few years later made seminal contributions to statistical mechanics. At the young age of 26 he was appointed to a full professorship at the University of Rome, and in late 1933 he developed a quantum–mechanically-based theory of beta decay. He was to prove equally gifted as a nuclear experimentalist.

The reticence of Chadwick and the Joliot-Curies to carry out neutron bombardment experiments may seem strange, but was understandable in view of the low yields expected. Chadwick estimated that he produced only about 30 neutrons for every million alpha-particles emitted by his sample of polonium. If the neutrons interacted with target nuclei with similarly low yields, virtually nothing could be expected to result. Otto Frisch, one of the co-interpreters of fission, later remarked that, ‘‘I remember that my reaction and probably that of many others was that Fermi’s was a silly experiment because neutrons were so
much fewer than alpha particles.’’ But this overlooked the fact that neutrons would not experience a Coulomb barrier. Fermi desired to break into nuclear experimentation, and saw his opening in this under-exploited possibility.

Fermi’s first challenge was to secure a strong neutron source. In this sense he was fortunate in that his laboratory was located in the same building as the Physical Laboratory of the Institute of Public Health, which was charged with controlling radioactive substances in Italy. The Laboratory held many radium sources that had been used for cancer treatments, and Fermi used them as a source of radon gas. When mixed with powdered beryllium, the radon gave rise to a copious supply of neutrons. Radon is produced in the decay of radium,

226 88 Ra -------α---- 222 86 Rn + 4 2He.
1,599year

The radon daughter product has a very short half-life, which means a correspondingly great flux of alpha-particles from the decay

222 86 Rn ----α---- 218 84 Po +4 2He.
3.82days

After being harvested from the decaying radium, the radon gas was captured in inch-long glass vials which contained powdered beryllium. The radon-produced alphas in (2.40) then gave rise to neutrons via the same reaction that Bothe and Becker, the Joliot-Curies, and Chadwick had experimented with:

4 2He + 9 4Be---- 12 6 C + 1 0n.

This series of reactions yields neutrons with energies of up to about 10 MeV, more than energetic enough to escape through the thin walls of the glass vials and so bombard a sample of a target element. Fermi estimated that his sources yielded about 100,000 neutrons per second.

Because the neutrons generated by his radon -beryllium sources tended to be emitted in all directions, Fermi usually formed samples of the target elements to be investigated into cylinders which could be placed around the sources in order to achieve maximum exposure. The cylinders were made large enough so that after being irradiated they could be slipped around a small handmade Geiger counter. Fermi’s goal was to see if he could induce artificial radioactivity with neutron bombardment.

Possibly anxious to see if he could induce heavy-element radioactivity, his first target was the heavy element platinum (atomic number 78). Fifteen minutes of irradiation gave no discernible signal.

Perhaps inspired by the Joliot-Curies’ experience, he then turned to aluminum. Here he did succeed, and found a different half-life than they had. The reaction involved ejection of a proton from the bombarded aluminum, leaving behind magnesium,

1 0n + 27 13Al--- 1 1H + 27 12Mg .

The magnesium beta-decays back to aluminum with a half-life of about 10 min:

27 12Mg ---β---- 27 13Al.
9.5min

After aluminum, Fermi tried lead, but with negative results. His next attempt was with fluorine, irradiation of which produced a ver11s y short-lived heavier isotope of that element:

1 0n +19 9 F -- 20 9 F --β- 20 10Ne
11s

Fermi announced his discovery in the official journal of the Italian National Research Council, and an English-language report dated April 10 appeared in the May 19 edition of Nature.

By late April, the Rome group had performed experiments on about 30 elements, 22 of which yielded positive results, including the four medium-weight elements antimony (Z = 51), iodine (Z=56), barium (Z=53) and lanthanum (Z=57).

Fermi and his co-workers found that, as a rule, light elements exhibited three reaction channels: a proton or an alpha could be ejected, or the element might simply capture the neutron to become a heavier isotope of itself and then subsequently decay. In all three cases, the products would undergo b- decay. Aluminum is typical in this regard:

1 1H + 27 12Mg ----β- 27 13 Al
9.5min

1 0n + 27 13Al - 4 2He + 24 11Na ---β- 24 12Mg
15h

28 13Al ----β- 28 14Si
2.25min

With a heavy-element target, the result is typically the latter of the above channels. Gold is characteristic in this regard:

1 0n + 197 79 Au-> 198 79 Au ------β --- 198 80 Hg
2:69 days

By the early summer of 1934, Fermi had prepared improved sources, which he estimated were yielding about a million neutrons per second. Based on work with these new sources, he published a stunning result in the June 16, 1934, edition of Nature: that his group was producing transuranic elements, that is, ones with atomic numbers greater than that of uranium.
Since uranium was the heaviest known element, this meant that they believed that they were synthesizing new elements. If true, this would be a remarkable development. Fermi’s radical assertion was based on the fact that uranium could be activated to produce beta-decay upon neutron bombardment. The results were complex, however, with evidence for half-lives of 10 s, 40 s, 13 min, and at least two more half-lives of up to one day. Whether this was a chain of decays or some sort of parallel sequence was unknown. Whatever sequence was occurring, however, the initial step was presumably the formation of a heavy isotope of uranium, followed by a beta-decay as in the gold reaction above:

1 0n + 238 92 U-- 239 92 U+β, -239 93 X
where X denotes a new, transuranic element.

The 13-min decay was convenient to work with, and the Rome group managed to separate chemically its decay product from the bombarded uranium. Analysis showed that the decay product did not appear to be any of the elements between lead (Z = 82) and uranium.

Since no natural or artificial transmutation had ever been observed to change the identity of a target element by more than one or two places in the periodic table, it would have seemed perfectly plausible to assume that a new element was being created.
To isolate the product of the 13-min activity, Fermi and his group began with manganese dioxide as a chemical carrier. The rationale for this was that if element 93 were actually being created, it was expected that it would fall in the same column of the periodic table as manganese (Z = 25), and so the two should have similar chemistry.
In any case, by the summer of 1934, Fermi’s group had developed an improved rhenium-based chemical analysis of the 13-min uranium activation which appeared to strengthen the transuranic interpretation.

Fermi’s next discovery would prove pivotal to the eventual development of plutonium-based nuclear weapons. In the fall of 1934, his group decided that they needed to more precisely quantify their assessments of activities induced in various elements; previously they had assigned only qualitative ‘‘strong-medium-weak’’ designations. As a standard of activity, they settled on a 2.4-min half-life induced in silver:

1 0n + 107 47 Ag ---- 108 47 Ag -------β---108 48 Cd
2:39min

However, they soon ran into a problem: the activity induced in silver seemed to depend on where in the laboratory the sample was irradiated. In particular, silver irradiated on a wooden table became much more active then when irradiated on a marble-topped one.
To try to understand what was happening, a series of calibration experiments was undertaken, some of which involved investigating the effects of ‘‘filtering’’ neutrons by interposing layers of lead between the neutron source and the target sample. Fermi made the key breakthrough on October 22, 1934: ‘‘One day, as I came into the laboratory, it occurred to me that I should examine the effect of placing a piece of lead before the incident neutrons. Instead of my usual custom, I took great pains to have the piece of lead precisely machined. I was clearly dissatisfied with something; I tried every excuse to postpone putting the piece of lead in its place. When finally, with some reluctance, I was going to put it in its place, I said to myself: ‘‘No, I don’t want this piece of lead here; what I want is a piece of paraffin.’’ It was just like that with no advance warning, no conscious prior reasoning. I immediately took some odd piece of paraffin and placed it where the piece of lead was to have been.’’
To Fermi’s surprise, the presence of the paraffin caused the level of induced radioactivity to increase. Further experimentation showed that the effect was characteristic of filtering materials which contained hydrogen; paraffin and water were most effective. Within a few hours of the discovery, Fermi developed a working hypothesis: That by being slowed by collisions with hydrogen nuclei, the neutrons would have more time in the vicinity of target nuclei to induce a reaction. Neutrons and protons have essentially identical masses, and, as with a billiard-ball collision, a head-on strike would essentially bring a neutron to a stop. Since atoms always have random motions due to being at a temperature that is above absolute zero, the incoming neutrons will never be brought to dead stops, but in practice only a few centimeters of paraffin or water are needed to bring them to an average speed characteristic of the temperature of the slowing medium. This process is now called ‘‘thermalization.’’ Nuclear physicists define ‘‘thermal’’ neutrons as having kinetic energy equivalent to a temperature of 298 K, or 77 0F—not much warmer than the average daily temperature in Rome in October. The speed of a thermal neutron is about 2,200 m/s, and the corresponding kinetic energy is about 0.025 eV, much less than the 10 MeV of Fermi’s radon-beryllium neutrons. Thermal neutrons are also known as ‘‘slow’’ neutrons; those of MeV-scale kinetic energies are, for obvious reasons, termed ‘‘fast.’’
The water or paraffin is now known as a ‘‘moderator’’; graphite (crystallized carbon) also works well in this respect. Fermi’s wooden lab bench, by virtue of its water content, was a more effective moderator than was his marble-topped one.

Be sure to understand what is meant by ‘‘fast’’ and ‘‘slow’’ neutrons. When uranium is bombarded by neutrons, what happens depends very critically on the kinetic energies of the neutrons. Fast and slow neutrons lie at the heart of why nuclear reactors and bombs function differently, and why a bomb requires ‘‘enriched’’ uranium to function. This is a complex topic with a number of interconnecting aspects.
Following Fermi’s serendipitous discovery, his group began re-investigating all elements which they had previously subjected to fast (energetic) neutron bombardment. Extensive results were reported in a paper published in the spring of 1935. For some target elements, the effect was dramatic: activity in vanadium and silver were increased by factors of 40 and 30, respectively, unmoderated over that achieved by neutrons. Uranium also showed increased activation, by a factor of about 1.6.

Fermi’s hypothesis that slower neutrons have a greater chance of inducing a reaction is now quantified in the concept of a reaction cross-section. This is a measure of the cross-sectional area that a target nucleus effectively presents to a bombarding particle that results in a given reaction. Because of a quantum– mechanical effect known as the de Broglie wavelength, a target nucleus will appear larger to a slower bombarding particle than to a faster one, sometimes by factors of hundreds. Each possible reaction channel for a target nucleus will have its own characteristic run of cross-section as a function of bombarding-particle energy.

Cross-sections are designated with the Greek letter sigma, equivalent to the English letter ‘‘s,’’ which serves as a reminder that they have units of surface area. The fundamental unit of cross-section is the ‘‘barn’’;
1 bn = 10-28 m2. This miniscule number is characteristic of the geometric cross-sectional area of nuclei, which is given approximately in terms of the mass number A by the empirical relationship: Geometric cross-section =0.0452 A2/3 barns.

Fermi was awarded the 1938 Nobel Prize for Physics for his demonstration of the existence of new radioactive elements produced by neutron irradiation. Fermi’s wife and children were Jewish, and he and his family used the excuse of the trip to Stockholm to escape the rapidly deteriorating fascist political situation in Italy by subsequently immigrating to America, where he had arranged for a position at Columbia University. The American branch of the Fermi family was established on January 2, 1939.

Discovery of U-235.

Before proceeding to the story of the discovery of nuclear fission, a brief but important intervening discovery needs to be mentioned. This is that uranium possesses a second, much less abundant isotope than the U-238 that Fermi had assumed was the sole form of that element. In 1931, Francis Aston had run uranium hexafluoride through his mass spectrometer and concluded that only an isotope of mass number 238 was present.

In the summer of 1935, Arthur Dempster of the University of Chicago

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discovered evidence for a lighter isotope of mass number 235. Dempster estimated U-235 to be present to an extent of less than one percent of the abundance of its sister isotope of mass 238. Within a few years that one percent would prove very important indeed.

Liquid Drop Model

The liquid drop model in nuclear physics treats the nucleus as a drop of incompressible nuclear fluid of very high density. It was first proposed by George Gamow and then developed by Niels Bohr and John Archibald Wheeler.
The nucleus is made of nucleons (protons and neutrons), which are held together by the nuclear force (a residual effect of the strong force).This is very similar to the structure of a spherical liquid drop made of microscopic molecules. This is a crude model that does not explain all the properties of the nucleus, but does explain the spherical shape of most nuclei. It also helps to predict the nuclear binding energy and to assess how much is available for consumption.
The liquid drop model of the nucleus takes into account the fact that the nuclear forces on the nucleons on the surface are different from those on nucleons in the interior of the nucleus. The interior nucleons are completely surrounded by other attracting nucleons.

Here is the analogy with the forces that form a drop of liquid.
In the ground state the nucleus is spherical. If the sufficient kinetic or binding energy is added, this spherical nucleus may be distorted into a dumbbell shape and then may be split into two fragments. Since these fragments are a more stable configuration, the splitting of such heavy nuclei must be accompanied by energy release. This model does not explain all the properties of the atomic nucleus, but does explain the predicted nuclear binding energies

Image result for image for nuclear liquid drop model

The Puzzle of the Neutron Bombardment of Uranium

As well known, the history of splitting uranium nucleus began in 1938 with articles by Otto Hahn and Fritz Strassman in Berlin. However, the prehistory already started in Rome in March 1934 when Fermi's coworkers “bombarded uranium (U) and thorium (Th) with neutrons. They got complicated results, the interpretation of which was difficult. Nowadays it is obvious that they resulted from the fission of the nucleus, i.e. the division of the heavy nucleus into two fragments of different masses, rather than the formation of transuranium elements, i.e. with an atomic number larger than 92… Fermi, although very cautiously, announced the discovery of transuranium elements”. In line with the logics of the preceding research with light nuclei, irradiation of heavy nuclei with neutrons was expected to lead to the formation of heavy transuranium nuclei.

Fermi's article, however, was criticized by a German chemist, Ida Noddack, the discoverer of rhenium (Re). “It is also conceivable,” she wrote in  “that when heavy nuclei are bombarded with neutrons these nuclei could break down into several fairly large fragments, which are certainly isotopes of known elements, but not neighbors of the irradiated elements.” However, Frau Noddack did not try to check her hypothesis, she did not propose a theoretical basis for this process, and so her suggestion was forgotten until 1938.

Lise Meitner, Otto Hahn, Fritz Strassman in Germany, and Irène Curie in France performed similar experiments, with the same purpose of producing and investigating new transuranium nuclei. The study of the radioelements resulting from the irradiation of U and Th with slow and fast neutrons was particularly complicated because of many nuclei/isotopes/isomers produced as well as the similarity of chemical properties of series of corresponding elements in the periodic table. With the accumulation of new data, internal contradictions of the interpretation were becoming more evident, without clear indication, however, of the origin of the puzzle. In particular, chemical properties of some produced nuclei were different from the properties of any nuclei with an atomic number close to that of U or Th, also the number of isomers to assume for explaining the data was too large.

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Things changed when Hahn and Strassman identified barium (Ba)  in the reaction products. Does so large a departure from the initial mass of U (Th) mean a nuclear rupture? The authors were unsure about the explanation of their results, as it looked strange that a tiny neutron can break in parts a heavy nucleus. Moreover, prominent scientists around were all discussing the opposite process: the formation of transuranium elements. Hahn was a nuclear chemist and needed the help of a physicist. In several letters, Hahn exposed his latest results and his concerns to his colleague Lise Meitner: “Actually there is something about the ‘radium isotopes’ that is so remarkable that for now we are telling only you. … Our Ra isotopes act like Ba. […] Perhaps you can come up with some sort of fantastic explanation. We do know that it can't actually burst apart into Ba.” This situation was known as “barium fantasy.”

The Making of the Atomic Bomb

Topic 9: Years 1938-1939: Otto Hahn’s discovery of nuclear fission. Interpretation of fission by Lise Meitner and Otto Frisch; spread of the news to U.S. and initial reactions and experimental verifications. Bohr’s interpretation of the significance of U-235.

Otto Hahn’s Discovery of Nuclear Fission
Nuclear fission is the process in which a large nucleus splits into two smaller nuclei with the release of energy. In other words, fission the process in which a nucleus is divided into two or more fragments, and neutrons and energy are released.
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After Enrico Fermi started bombarding the heavy elements on order to create transuranic elements other scientific groups began performing similar experiments. Otto Hahn, Lise Meitner, and Fritz Strassman began performing r experiments in Berlin. Meitner, an Austrian Jew, lost her citizenship with the "Anschluss", the occupation and annexation of Austria into Nazi Germany in March 1938, but she fled in July 1938 to Sweden and started a correspondence by mail with Hahn in Berlin. By coincidence, her nephew Otto Robert Frisch, also a refugee, was also in Sweden when Meitner received a letter from Hahn dated 19 December describing his chemical proof that some of the product of the bombardment of uranium with neutrons was barium. Hahn suggested a bursting of the nucleus, but he was unsure of what the physical basis for the results were. Barium had an atomic mass 40% less than uranium, and no previously known methods of radioactive decay could account for such a large difference in the mass of the nucleus. Frisch was skeptical, but Meitner trusted Hahn's ability as a chemist. Marie Curie had been separating barium from radium for many years, and the techniques were well-known.

Lise Meitner and Otto Robert Frisch and Interpretation of Reaction of Fission

Lise Meitner and her nephew Otto Robert Frisch promptly found the correct explanation: nuclear fission named by Frisch in analogy to biological fission of living cells. They assumed a nuclear reaction like

U-92+ 10n→ Ba-56 + Kr-36+⋯

Where the dots (…) represent gamma photons and a number of neutrons that depends on the mass numbers of the three nuclei. Inspired by an article by Niels Bohr, they imagined the process of fission in terms of the nuclear liquid-drop model. “On account of their close packing and strong energy exchange, the particles in a heavy nucleus would be expected to move in a collective way which has some resemblance to the movement of a liquid drop,” Meitner and Frisch wrote. When absorbing a neutron, the drop constituted by the U nucleus elongates (Fig. below). This motion is opposed by the surface tension, but, according to Meitner and Frisch, “the surface tension of a charged droplet is diminished by its charge, and a rough estimate shows that the surface tension of nuclei, decreasing with increasing nuclear charge, may become zero for atomic numbers of the order of 100.” Thus, the drop can split into two smaller drops, which move apart due to their electric repulsion and gain a kinetic energy of some 200 MeV. The released nuclear binding energy is also some 200 MeV and the mass of the fragments is smaller than the initial U mass, as follows from Einstein's formula E=mc2. Otto Frisch, and shortly later others observed experimentally the great ionizing power of the nuclear fission fragments.

Fig. 2

Fig. Nuclear fission in the liquid drop model. When hit by a neutron (blue ball) the nucleus elongates and splits into two parts, creating more neutrons.

The articles by Hahn and Strassman, and by Meitner and Frisch, were submitted rather hastily and published very quickly. For instance, it is not quite true to say that the “surface tension is diminished by the charge.” Surface tension and Coulomb repulsion are rather antagonistic effects, and the latter wins in heavy nuclei. Also, the kinetic energy might be significantly lower than 200 MeV (say 180 MeV) because of the excitation energy of fission fragments.

The research on fission exploded all over the world. New researchers entered the field; others working on production of transuranium nuclei had to rethink their preceding results. Irène Curie, together with her coauthor Paul Savitch, explained her hesitations : “We had considered the possibility of a rupture of the uranium atom, but we had rejected this idea […],” in fact because Hahn, Meitner and Strassman had, in earlier papers, misinterpreted their experimental results and announced the formation of transuranium elements – the same mistake as Fermi's in 1934  In fact, transuranium elements were discovered later – neptunium was the first, in 1939, discovered by Edwin McMillan and Philip Abelson in Berkeley

Frederic Joliot clearly proved the explosive character of fission and separated from U the new nuclei. Due to its kinetic energy, a fragment can escape from the U sample surface and traverse an air gap. A complex mixture of radioactive fragments collected at a distance from the U target appeared to be the same as the radioactive products accumulated in the target. An absorption of fragments in thin screens inserted between the U sample and the surface corresponded to kinetic energies estimated from the energy balance in the fission process. Measurements with Th showed the same result.

Already in this article, Frederic Joliot mentions the production of a few neutrons per fission. Hahn and Strassman note this possibility in their second publication on fission. As the excess of the number of neutrons over the number of protons in a nucleus increases as a function of atomic weight, it is natural that sufficiently heavy nuclei, like U, can emit not one, but a few neutrons. However, this means that their number can multiply at every step or stay constant if one controls the reaction by means of absorbing extra neutrons or letting them escape out of the reaction zone. Thus, fission can run “in chain”! Immediately, neutrons from fission were observed experimentally. “Recent experiments have shown that neutrons are liberated in the nuclear fission of uranium induced by slow neutron bombardment: secondary neutrons have been observed which show spatial, energetic or temporal properties different from those which primary neutrons possess or may acquire.” The complexity of all these features related to neutron diffusion in matter, their thermalization in matter, both as a function of neutron energy, a huge variety of nuclear reactions involved in the process, prompt and delayed neutrons, nuclear isomers and so on and so on, was recognized or at least suspected already at that time.

Frederic Joliot already discussed the possibility of a nuclear chain reaction with important release of energy in his Nobel Prize lecture in 1935. With his colleagues, Hans von Halban, Lev Kowarski, and Francis Perrin, he demonstrated it explicitly in 1939. They placed a source of neutrons in the center of a copper (Cu) sphere immerged into a water bath (which slowed down the neutrons and scattered them, thus increasing the number of fission events because of their back-scattering). They filled the sphere with uranium oxide (U3O8), or water, or a mixture of both, and measured the neutron fluxes in the various cases. They observed in this so-called subcritical assembly a large increase in the number of neutrons emitted from the initial neutron source in the middle of the sphere. They even estimated, with a limited knowledge of all relevant cross-sections at that time, the mean number of neutrons produced per fission to be 3.5±0.7. This evaluation took into account the emission of neutrons by fission products. The precise value is 2.41. Without the initial neutron source in the middle of the sphere, the reaction would stop. The authors concluded that they were observing a chain of nuclear reactions.

If the size of the reaction zone had been larger and, therefore, a smaller fraction of produced neutrons would have escaped from the sphere surface, the neutron multiplication factor would have increased. For a certain finite size, it would have exploded.

Francis Perrin estimated accordingly a so-called critical mass of the active zone without a reflector around it. .

In these experiments, some external neutron flux provided initial fission events, which resulted in the nuclear chain reaction. Can fission occur in other ways than induced by an external neutron source? Bohr and Wheeler's theory of the fission process based on the “liquid drop” model, predicted that an external “kick” can be given by a γ-quantum. Indeed, photo-fission of uranium and thorium were promptly observed in the Westinghouse Research Laboratories, in Pennsylvania. Moreover, Georgy Flyorov and Konstantin Petrzhak in Leningrad (Russia) announced that a heavy enough nucleus can fission spontaneously, without any initial “kick,” thus releasing extra neutrons. Thus, the fission chain reaction will develop itself as soon as the conditions corresponding to the critical mass are met.

The main lines of further developments were clear for those involved in the fission research. Low absorbing neutron moderators and reflectors, like heavy water (D2O) or pure graphite, if placed around the reaction zone, would increase the efficiency of the reaction. Neutron absorbers, like cadmium (Cd), would decrease the efficiency, and thus allow controlling the reaction at some chosen intensity. Provided in such a way, a controlled steady chain fission reaction would not need any more an external neutron source to keep it running. Such a so-called critical assembly provides an “infinite” multiplication factor for the initial neutron intensity. Due to such multiplication, it would dramatically improve the intensity of usually weak neutron sources, thus providing useful tools for research. Due to larger neutron fluxes, it would allow producing much larger amounts of artificial radioactive elements, which make them a real instrument in medicine, biology, and other domains. Due to 200 MeV of energy released per one fission event, it would produce a quantity of energy that one can hardly imagine in advance. The list of thought applications was increasing. However, one still had to do significant steps towards the practical realization of these ideas.

Until 1939, nuclear science had been the object of a tight international competition in several languages (English, German, French, Russian, occasionally Italian and others). It was rather friendly, since young researchers occasionally moved from one laboratory to another. For instance, Ettore Majorana spent some time in Germany with Heisenberg, Bruno Pontecorvo in France with Joliot-Curie. Two events were going to change the situation drastically. On the one hand, the extreme right became very powerful in continental Europe, and was victorious in the beginning of the Second World War, so that many major scientists from Italy, Germany, Austria, Denmark, and France fled to the United States of America (as Fermi did) or to the United Kingdom (as Otto Frisch did, while his aunt Lise Meitner, in July 1938, fled to Sweden). On the other hand, nuclear fission and chain reactions gave nuclear physics a military importance.

In 1939, Francis Perrin could still discuss the critical mass of a radioactive material (a very new topic!) in the Comptes rendus de l'Académie des sciences. In the first days of May 1939, Joliot, Halban, and Kowarski filed patents on power production from a nuclear chain reaction as well as on nuclear explosion. The next year, this subject had become highly confidential. It was treated in detail and quantitatively by the German Rudolf Peierls and the Austrian Otto Frisch in a memorandum written in English for the use of the British Government.

Spread of the News to USA and Experimental Verification of Fission Reaction in US

News spread quickly of the new discovery, which was correctly seen as an entirely novel physical effect with great scientific—and potentially practical—possibilities. Meitner's and Frisch's interpretation of the discovery of Hahn and Strassman crossed the Atlantic Ocean with Niels Bohr, who was to lecture at Princeton University. I.I. Rabi and Willis Lamb, two Columbia University physicists working at Princeton, heard the news and carried it back to Columbia. Rabi said he told Enrico Fermi; Fermi gave credit to Lamb. Bohr soon thereafter went from Princeton to Columbia to see Fermi. Not finding Fermi in his office, Bohr went down to the cyclotron area and found Herbert L. Anderson. Bohr grabbed him by the shoulder and said: “Young man, let me explain to you about something new and exciting in physics.” It was clear to a number of scientists at Columbia that they should try to detect the energy released in the nuclear fission of uranium from neutron bombardment.

On 25 January 1939, a Columbia University team conducted the first nuclear fission experiment in the United States, which was done in the basement of Pupin Hall; the members of the team were Herbert L. Anderson, Eugene T. Booth, John R. Dunning, Enrico Fermi, G. Norris Glasoe, and Francis G. Slack. The experiment involved placing uranium oxide inside of an ionization chamber and irradiating it with neutrons, and measuring the energy thus released. The results confirmed that fission was occurring and hinted strongly that it was the isotope uranium- 235 in particular that was fissioning. The next day, the Fifth Washington Conference on Theoretical Physics began in Washington, D.C. under the joint auspices of the George Washington University and the Carnegie Institution of Washington. There, the news on nuclear fission was spread even further, which fostered many more experimental demonstrations.

During this period the Hungarian physicist Leó Szilárd, who was residing in the United States at the time, realized that the neutron-driven fission of heavy atoms could be used to create a nuclear chain reaction. Such a reaction using neutrons was an idea he had first formulated in 1933, upon reading Rutherford's disparaging remarks about generating power from his team's 1932 experiment using protons to split lithium. However, Szilárd had not been able to achieve a neutron-driven chain reaction with neutron-rich light atoms. In theory, if in a neutron-driven chain reaction the number of secondary neutrons produced was greater than one, then each such reaction could trigger multiple additional reactions, producing an exponentially increasing number of reactions. It was thus a possibility that the fission of uranium could yield vast amounts of energy for civilian or military purposes (i.e., electric power generation or atomic bombs).

Szilard now urged Fermi (in New York) and Frederic Joliot-Curie (in Paris) to refrain from publishing on the possibility of a chain reaction, lest the Nazi government become aware of the possibilities on the eve of what would later be known as World War II. With some hesitation Fermi agreed to self-censor. But Joliot-Curie did not, and in April 1939 his team in Paris, including Hans von Halban and Lew Kowarski, reported in the journal Nature that the number of neutrons emitted with nuclear fission of 235U was then reported at 3.5 per fission. (They later corrected this to 2.6 per fission.) Simultaneous work by Szilard and Walter Zinn confirmed these results. The results suggested the possibility of building nuclear reactors (first called "neutronic reactors" by Szilard and Fermi) and even nuclear bombs. However, much was still unknown about fission and chain reaction systems.

Bohr’s Interpretation of the Significance of U-235

After fission reaction was introduced Otto Frisch started working on various aspects of the problem. He found that the probability of uranium fission was enhanced if he used slow bombarding neutrons rather than fast ones, and found that thorium fissioned only under fast‐neutron bombardment. This latter observation would prove a key clue for Bohr a few weeks later as he developed a theory of the fission process.
Later Bohr developed arguments to show that it was likely the rare isotope U‐235 that was responsible for slow‐neutron fission and why, in contrast, one would not expect thorium to fission by the same process. This behavior involves what can be termed as the “parity” of nuclei in the sense of the evenness or oddness of the number of protons (Z) and neutrons (N) that they possess; this is always quoted in the order Z/N. For reasons relating to the behavior of inter‐nucleon pairing forces, target nuclei such as U‐235 which are even/odd (Z/N = 92/143) liberate more mass‐energy upon capturing a neutron into their internal potential wells than do those which are even/even, such as U‐238 (Z/N = 92/146) and Th‐232 (Z/N = 90/142). In the even/odd case the liberated energy can be sufficient to cause the nucleus to fission no matter what the energy of the bombarding neutrons, whereas to fission an even/even nucleus requires bombarding neutrons more energetic, on average, than those emitted in fissions. Bohr reasoned that since U‐238 is even/even, then the only candidate for slow‐neutron fission in uranium must be U‐235. Further, since Th‐232 (even/even) is the only naturally‐occurring isotope of that element, one would not expect thorium to suffer slow‐neutron fission, as Otto Frisch and others had observed experimentally.

Bohr also offered an important hypothesis concerning fast‐neutron fission, which is utilized in nuclear weapons. This argument is paraphrased here. Quantum‐mechanically, one would expect that the fission cross‐section for any nuclide should decrease as the energy of bombarding neutrons increases: The de Broglie wavelength decreases with increasing energy. Since the slow‐neutron fission cross‐section of U‐235 was apparently very large, it was conceivable that this isotope might have a chance of sustaining fast‐neutron fissions if its fission cross‐section remained sufficiently large for such neutrons despite the expected decrease with energy. While Bohr left unstated what might happen if U‐235 could be separated from U‐238 and bombarded with fast neutrons, his argument was interpreted at the time to mean that a nuclear weapon was likely not practical given the anticipated difficulty of separating the two isotopes.
In 1940 the experimental proof of Bohr’s theory was obtained. At University of Minnesota mass spectroscopist Alfred Nier succeeded in separating a nanogram‐mass sample of U‐235, which was subjected to slow‐neutron bombardment in the cyclotron at Columbia University. In accordance with Bohr and Wheeler's theory, U‐235 showed clear evidence for slow‐neutron fission, while U‐238 showed none at all. Unfortunately, Nier's sample of U‐235 was too small to test for fast‐neutron fission.

The Making of the Atomic Bomb


Topic 10: Years 1939-1942: The discovery of neutrons from fission. The awakening of the Germans to the potential consequences of fission. Einstein’s letter to FDR. The discoveries of neptunium and plutonium. Pearl Harbor. The entrance of the US into the war and its effect on fission research. The Chicago pile.

The Discovery of Neutrons from Fission

 Many questions had to be answered before atomic power and bombs might be realized: Were secondary neutrons emitted in fission, and could they sustain a chain reaction against various neutron‐loss effects? What elements/isotopes undergo fission? Would critical masses be small enough to be practical? Might an effect such as spontaneous fission render the idea of making bombs infeasible from the outset?
To sustain a chain reaction, a fissioning nucleus must eject, on average, at least between 1 and 2 free neutrons. Groups at Columbia involving Enrico Fermi and Leo Szilard were particularly active, confirming Frisch's fast/slow and U/Th asymmetries and presented, the measurements of reaction cross‐sections and numbers of neutrons liberated per fission. These results pointed to an average of about 2 neutrons per fission; the currently‐accepted value is about 2.4. The presence of secondary neutrons attracted considerable attention in view of the possibility of using them to set up a chain reaction. In separate experiments Frederic Joliot-Curie, Lew Kowarski, and Hans von Halban (Paris)  measured also appropriately high numbers. 

The Awakening of the Germans to the Potential Consequences of Fission

In the late 1930s, the most famous physicist in Germany (Einstein having left Germany for New Jersey) was Werner Heisenberg. Heisenberg was internationally renowned for his work in quantum mechanics and the Uncertainty Principle that usually bore his name. He was a brilliant theorist and mathematician and prided himself on his practical abilities as a physicist. For a time he was Germany’s youngest full professor.
While not a card-carrying Nazi, Heisenberg was a loyal and patriotic German. Like many German academics and professional soldiers of his time, he considered himself above politics, and so was willing to serve whatever government ruled Germany, even Hitler’s. He was the logical choice to lead the country’s atomic weapons program.

When Germany invaded Poland in 1939, Werner Heisenberg was already drafted into a reserve mountain infantry unit. With the outbreak of war he and other physicists received military orders, not to the front, but to the Army Weapons Bureau (Heereswaffenamt) in Berlin. Here they were asked to explore the prospects for the practical utilization of a new discovery: nuclear fission. Nuclear fission involved the splitting of nuclei with the release of enormous amounts of energy. Under the right circumstances, the fission process in uranium can be controlled, leading to a heat-producing reactor that can be harnessed to the production of electricity. In other circumstances, if the reaction is uncontrolled, the energy is released extremely rapidly, producing an enormous explosion--an atomic bomb.
Heisenberg seized the initiative in German fission research, sending the Weapons Bureau within three months a secret two-part survey on its prospects.

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First page of Heisenberg's secret report to the Army Weapons Bureau "On the Possibility of Technical Energy Production from Uranium Splitting. II," 20 February 1940.

While Germany began state-sponsored atomic research several years before the Allies, its efforts did not go unnoticed. Because so many physicists were driven from the Reich, Allied governments were quickly able to form a relatively clear picture of German efforts. America’s program was sparked in part by Einstein’s warning to President Franklin D. Roosevelt concerning possible German successes.
"For the present I believe that the war will be over long before the first atom bomb is built.’--Heisenberg, recollection of a statement in 1939

British Atomic Bomb Project

The United Kingdom was in many ways the birthplace of atomic imagination and scientific research. One of the first steps toward the development of atomic energy was the research of Ernest Rutherford, a New Zealand-born British physicist and recipient of the 1908 Nobel Prize in Chemistry. In 1917, Rutherford became the first person to split the atom, leading to further speculation about the existence and structure of the neutron. Neutrons would be discovered by Rutherford’s colleague and former student James Chadwick in 1932. Rutherford’s work would influence other nuclear physicists. Many of them would go on to work on the British atomic project and the Manhattan Project, including James Chadwick, Niels Bohr, and Mark Oliphant.

The outbreak of World War II was a catalyst for the development of the atomic bomb. Alarmed by Hitler’s mention of a “secret weapon,” the British government began taking the bomb project much more seriously. Mark Oliphant reorganized research at Birmingham University, where he would bring together Otto Frisch and Rudolf Peierls. Together, the two physicists calculated that an atomic bomb could in fact be built and wrote their findings in a memorandum, “On the Construction of a ‘Super-Bomb’” .

Birmingham UniversityI

Birmingham University

In May 1940, Winston Churchill became Prime Minister. In direct response to the Frisch-Peierls memorandum, one of his first acts was the creation of a uranium subcommittee to advise his government on how to proceed. It would eventually be known as the MAUD Committee, an inconspicuous name actually referring to the former caretaker of the Bohrs’ children. (When the Germans invaded Denmark, Bohr had asked that news of his safety be sent to Frisch and to “Maud Ray Kent.” The confused scientists at first thought the message contained coded instructions. They later realized that it in fact referred to a person, Maud Ray, who lived in Kent.)

The MAUD Committee proceeded in absolute secrecy. They were not allowed to recruit anyone the government classified as an “illegal alien,” including Frisch and Peierls The MAUD Committee Report of 1941 concluded that a bomb could be built and recommended collaboration with the United States to do so.

After examining the report, the British Scientific and Advisory Committee judged the bomb to be high priority. They recommended that a pilot plant for the separation of U-235 be built in the U.K. followed by a full scale plant in Canada. Churchill asserted, “Although personally I am quite content with the existing explosives, I feel we must not stand in the path of improvement, and I therefore think that action should be taken…” He thus created within the Department of Scientific and Industrial Research (DSID) the organization responsible for all atomic resources, the Directorate of Tube Alloys. Tube Alloys would be the code name for the British atomic project for the duration of the war.

Einstein’s Letter to the President of United States of America

In the summer of 1939, six months after the discovery of uranium fission, American newspapers and magazines openly discussed the prospect of atomic energy. Most American physicists, however, doubted that atomic energy or atomic bombs were realistic possibilities. No official U.S. atomic energy project existed. 

Leo Szilard, 1946 photo

Leo Szilard was profoundly disturbed by the lack of American action. He had conceived the idea of a nuclear chain reaction in 1933, and kept his ideas from publication with a secret British patent. He had warned colleagues about the danger for years. Yet even the discovery of uranium fission, followed by proof that it released neutrons, had not lessened their disbelief.

If atomic bombs were possible, as Szilard believed they might be, Nazi Germany could gain an unbeatable lead in developing them. It was especially troubling that Germany had stopped the sale of uranium ore from occupied Czechoslovakia.

Unable to find official support, and unable to convince Enrico Fermi of the need to continue their experiments at Columbia University, Szilard turned to his old friend Albert Einstein. He had collaborated with Einstein in Berlin in the 1920s on the invention of novel refrigerators without moving parts. Now, he sought Einstein’s help about a very different matter.

Einstein was enjoying a sailing vacation in Peconic on the northern tip of Long Island, New York. On or about July 12, Szilard and fellow Hungarian physicist Eugene Wigner made the short drive from Manhattan in Wigner’s car. Szilard explained the state of international research on uranium and the evidence that a bomb might be possible. Given the seriousness of the situation, Szilard’s request was quite modest. He asked if Einstein would warn the Belgian Queen Mother, whom he knew, to prevent the large stockpile of uranium ore in the Belgian Congo from falling into Nazi hands. Einstein agreed to the idea, but he preferred to write to another friend, the Belgian ambassador. Einstein dictated a letter in German, which Wigner took down.

Einstein and Szilard re-enact discussion of letter to FDR for 1946 documentary Atomic Power
Einstein and Szilard re-enact their discussion for 1946 documentary

Within days, however, the plan became much more far-reaching when Szilard discussed the matter with economist Alexander Sachs. Sachs, who was an unofficial adviser to President Franklin Roosevelt, urged that Einstein should write directly to the President. If Einstein wrote such a letter, Sachs promised to deliver it to the President personally.

If they could gain the ear of the President, the Belgian uranium ore became a minor issue. Szilard produced a four-page draft letter, which he mailed to Einstein on July 19. By telephone, Einstein asked to discuss it with Szilard in person.

In the last days of July, Szilard returned to Einstein’s vacation cabin. Because Wigner was out of town, Hungarian physicist Edward Teller acted as Szilard’s chauffeur.

Einstein was willing to write to the President. As a life-long pacifist, he opposed the making of weapons. He had been forced to conclude, however, that pacifism would not succeed against the Nazis, who viewed violence as an end in itself. He could not, he decided, let his inaction give Germany sole possession of such destructive power. His only objection was that Szilard’s letter was long and somewhat awkward. He preferred a shorter message stressing the main points. Einstein dictated a short draft in German, which Szilard took down.

Over the next few days, Szilard translated Einstein’s dictation, going through draft after draft. In the end, he prepared both a short and a long version. On August 2, he mailed them to Einstein. Einstein returned both versions signed, but he expressed a preference for the longer version. This was the version, dated August 2, that Szilard gave to Sachs for delivery to the President.

Image of Einstein letter to FDR, August 2, 1939 - page 1

Image of Einstein letter to FDR, August 2, 1939 - page 2

Einstein’s letter did not reach the President quickly, however, nor did it have much effect. On September 1, 1939, Hitler invaded Poland and World War II began. Sachs finally met with the President on October 11 and presented Einstein’s letter. The President appointed a “Uranium Committee,” but it approved only $6,000 to buy graphite and uranium for experiments Szilard proposed. Even that small sum was not provided promptly.

For years, as Hitler conquered Europe, official skepticism continued to stall American progress. A large-scale U.S. atomic project, still limited only to research, did not begin until December 6, 1941, one day before the bombing of Pearl Harbor. It became the “Manhattan Project” in August 1942.

Several months later, thanks to Szilard’s efforts, construction of the first nuclear reactor was completed as soon as sufficient uranium and graphite were made available. Its design was essentially the same as he had proposed in July 1939. The reactor, named CP-1, operated successfully at the University of Chicago on December 2, 1942.

Discovery of Neptunium and Plutonium


Neptunium

Named for the planet Neptune (named after the Roman god of the sea), the next planet out from the Sun after Uranus. There were many early false reports of the discovery of neptunium. The most significant was by Enrico Fermi who believed that bombarding uranium with neutrons followed by beta decay would lead to the formation of element 93. In 1934, he bombarded uranium atoms with neutrons and reported that he had produced elements 93 . As it turned out, Fermi had actually fissioned or split uranium atoms into many fragment radioisotopes.

Element 93 was accepted as an existing element in 1940 at the University of California, Berkeley. Professor Edwin McMillan and graduate student Philip Abelson used a technique similar to Fermi, but with one important difference: they used slow-moving neutrons. McMillan used a machine called a cyclotron to slow the neutrons and then directed them at a uranium-238 target. This time, the neutrons actually worked to create element 93 by fusing with the uranium atoms instead of breaking them apart. Abelson analyzed the resulting sample, and noted unusual beta radiation that showed a new isotope (later named Np-289) was present. McMillan and Abelson decided to call the element neptunium because Neptune is the next planet beyond Uranus in the solar system. The discovery was the first transuranium element to be synthesized in a lab and earned McMillan a Nobel Prize in 1951:

23892U + 10n → 23992U → 23993Np + β-

Isotope of neptunium has a beta-decay half-life of 2.36 days, which forms daughter product plutonium-239 with a half-life of 24,000 years.

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The cyclotron at the University of California, Berkeley (Courtesy of the University of California, Berkeley)

Plutonium

The transuranium element, plutonium, was the first synthetic element to be produced on a large scale. In addition to being fissionable, it has interesting and unusual chemical and metallurgical properties. The story of its discovery and isolation is among the most fascinating in the history of science.

https://chem.libretexts.org/@api/deki/files/106174/McMillian.jpg?revision=1&size=bestfit&width=180&height=264   https://chem.libretexts.org/@api/deki/files/106175/Seaborg.jpg?revision=1&size=bestfit&width=180&height=264

(left) E. M. McMillan  June 8, 1940 and (right) Glenn Seaborg, 1941.Courtesy of the University of California, Berkeley)

In the summer of 1940, Glen Seaborg, Arthur Wahl, and Joseph Kennedy, a group of chemists at Berkeley, began a search for the next transuranium element, 94, which they thought to be a decay product (daughter) of Np-239.

Continuing the search for element 94 in the winter of 1941, they bombarded uranium oxide with 16 Mev deuterons from the Berkeley cyclotron. They chemically identified another isotope of neptunium, Np-238, which decayed by beta emission to an isotope of element 94 (plutonium) that then emitted alpha particles.

Seaborg remarked, "During this time, a great deal was learned about the chemistry of plutonium. It was established that plutonium in its higher oxidation state was not carried by lanthanum fluoride or cerium fluoride, in contrast to plutonium in the lower state, which was quantitatively coprecipitated with these compounds. The lower state could be oxidized to the higher state with oxidizing agents such as persulfate, dichromate, permanganate, or periodate ions, and then reduced by treatment with sulfur dioxide or bromide ion to the lower oxidation state."

Later in the spring of 1941, another more important isotope of plutonium, Pu-239, was produced using neutrons from the Berkeley cyclotron to target a uranium compound surrounded by paraffin. As Fermi’s group had discovered, the paraffin acted as a moderator to slow the neutrons and thus increase the chances of interaction with the target. This new Pu isotope, an alpha emitter with a half-life of about 24,000 years, was separated from other reaction products using the same chemistry as that used to isolate Pu-238. However, the longer half-life of Pu-239 reduced its activity, making it more difficult to detect than Pu-238.

In March 1941, Seaborg’s group irradiated a sample estimated to contain 0.25 mg of Pu-239 surrounded by paraffin with neutrons produced in the cyclotron. Under these conditions, this isotope appeared to undergo fission. When the Pu was replaced with a sample containing approximately 0.5 mg U-235, the other known fissionable material, neutron-induced fission was also observed, but at a rate approximately half that of Pu-239. This discovery raised the possibility of using a controlled chain reaction to produce quantities of Pu-239 sufficient for nuclear weapons. The product Pu-239 would have to be separated from the unreacted uranium and fission products by chemical means. It now became important to investigate the chemistry of plutonium to develop large-scale separation procedures.

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 Twenty micrograms of plutonium hydroxide in a capillary tube, September 1942 (Courtesy of the University of California, Berkeley)

However, in the summer of 1942, the cyclotron was the only means of producing plutonium, and the amounts produced were so small that they could not be seen or weighed with existing balances. Calculations showed that long periods of neutron bombardment of uranium in the cyclotron would produce only a few micrograms of Pu-239, much less than that normally required to determine the physical and chemical properties of a new element. The preparation and measurement of such small quantities of plutonium required the development of "ultramicrochemical" techniques and equipment.

The first weighing of a plutonium compound occurred in September of 1942, when 2.77 mg of PuO2 was placed on a balance especially designed for small masses and calibrated with platinum wire. Liquid volumes in the range of 0.10 to 10-5 mL were delivered with an error of less than one percent using calibrated capillary pipettes. Glassware, such as beakers and test tubes, was made from capillary tubing and handled under a microscope with micromanipulators. In November 1943, the first pure plutonium metal was prepared by reducing 35 mg of PuF4 with barium metal at 1,4000 C0. The plutonium metal appeared as silvery globules weighing about 3 mg each and having an estimated density of 16 g/mL.

Although the Pu-239 isotope had the potential to be fissionable material for bombs or power generation, realization of this potential required larger amounts of this isotope. Large-scale production of Pu-239 required a controlled nuclear chain reaction of uranium, a feat that would soon be achieved by Enrico Fermi and Leo Szilard in Chicago.

Pearl Harbor

Pearl Harbor is a U.S. naval base near Honolulu, Hawaii, that was the scene of a devastating surprise attack by Japanese forces on December 7, 1941. Just before 8 a.m. on that Sunday morning, hundreds of Japanese fighter planes descended on the base, where they managed to destroy or damage nearly 20 American naval vessels, including eight battleships, and over 300 airplanes. More than 2,400 Americans died in the attack, including civilians, and another 1,000 people were wounded. The day after the assault, President Franklin D. Roosevelt asked Congress to declare war on Japan.

The attack on Pearl Harbor was a surprise, but Japan and the United States had been edging toward war for decades. The United States was particularly unhappy with Japan’s increasingly belligerent attitude toward China. The Japanese government believed that the only way to solve its economic and demographic problems was to expand into its neighbor’s territory and take over its import market. To this end, Japan declared war on China in 1937, resulting in the Nanking Massacre and other atrocities. American officials responded to this aggression with a battery of economic sanctions and trade embargoes. They reasoned that without access to money and goods, and especially essential supplies like oil, Japan would have to rein in its expansionism. Instead, the sanctions made the Japanese more determined to stand their ground. During months of negotiations between Tokyo and Washington, D.C., neither side would budge. It seemed that war was all but inevitable.
Pearl Harbor, Hawaii, is located near the center of the Pacific Ocean, roughly 2,000 miles from the U.S. mainland and about 4,000 miles from Japan. Therefore, no one believed that the Japanese would start a war with an attack on the distant islands of Hawaii. Additionally, American intelligence officials were confident that any Japanese attack would take place in one of the (relatively) nearby European colonies in the South Pacific: the Dutch East Indies, Singapore or Indochina. Because American military leaders were not expecting an attack so close to home, the naval facilities at Pearl Harbor were relatively undefended. Almost the entire Pacific Fleet was moored around Ford Island in the harbor, and hundreds of airplanes were squeezed onto adjacent airfields. To the Japanese, Pearl Harbor was an irresistibly easy target.

The Japanese plan was simple: Destroy the Pacific Fleet. That way, the Americans would not be able to fight back as Japan’s armed forces spread across the South Pacific. On December 7, after months of planning and practice, the Japanese launched their attack. At about 8 a.m., Japanese planes filled the sky over Pearl Harbor. Bombs and bullets rained onto the vessels moored below. At 8:10, a 1,800-pound bomb smashed through the deck of the battleship USS Arizona and landed in her forward ammunition magazine. The ship exploded and sank with more than 1,000 men trapped inside. Next, torpedoes pierced the shell of the battleship USS Oklahoma. With 400 sailors aboard, the Oklahoma lost her balance, rolled onto her side and slipped underwater.
Less than two hours later, the surprise attack was over, and every battleship in Pearl Harbor—USS Arizona, USS Oklahoma, USS California, USS West Virginia, USS Utah, USS Maryland, USS Pennsylvania, USS Tennessee and USS Nevada—had sustained significant damage. (All but USS Arizona and USS Utah were eventually salvaged and repaired.)
In all, the Japanese attack on Pearl Harbor crippled or destroyed nearly 20 American ships and more than 300 airplanes. Dry docks and airfields were likewise destroyed. Most important, 2,403 sailors, soldiers and civilians were killed and about 1,000 people were wounded.
But the Japanese had failed to cripple the Pacific Fleet. By the 1940s, battleships were no longer the most important naval vessel: Aircraft carriers were, and as it happened, all of the Pacific Fleet’s carriers were away from the base on December 7. (Some had returned to the mainland and others were delivering planes to troops on Midway and Wake Islands.)Moreover, the Pearl Harbor assault had left the base’s most vital onshore facilities—oil storage depots, repair shops, shipyards and submarine docks—intact. As a result, the U.S. Navy was able to rebound relatively quickly from the attack.
President Franklin D. Roosevelt addressed a joint session of the U.S. Congress on December 8, the day after the crushing attack on Pearl Harbor.
“Yesterday, December 7, 1941—a date which will live in infamy—the United States of America was suddenly and deliberately attacked by naval and air forces of the Empire of Japan.” He went on to say, “No matter how long it may take us to overcome this premeditated invasion, the American people in their righteous might will win through to absolute victory. I believe I interpret the will of the Congress and of the people when I assert that we will not only defend ourselves to the uttermost, but will make very certain that this form of treachery shall never endanger us again.”

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After the Pearl Harbor attack, and for the first time during years of discussion and debate, the American people were united in their determination to go to war. The Japanese had wanted to goad the United States into an agreement to lift the economic sanctions against them; instead, they had pushed their adversary into a global conflict that ultimately resulted in Japan’s first occupation by a foreign power. On December 8, Congress approved Roosevelt’s declaration of war on Japan. Three days later, Japan’s allies Germany and Italy declared war against the United States. For the second time, Congress reciprocated, declaring war on the European powers. More than two years after the start of World War II, the United States had entered the conflict.

Pearl Harbor Facts_promo

After the sudden and deliberate attack on Pearl Harbor by the Japanese during World War II, President Roosevelt spoke to Congress and the American people.

The Chicago Pile

Although fission had been observed on a small scale in many laboratories, no one had carried out a controlled chain reaction that would provide continuous production of plutonium for isolation. Enrico Fermi thought that he could achieve a controlled chain reaction using natural uranium. He had started this work with Leo Szilard at Columbia University, but moved to the University of Chicago in early 1942.

As was noticed before, the first important fact to be realized was that the cross section for neutron fission was higher for low energy neutrons, while the second one was that the key isotope of uranium involved in the fission induced by slow neutrons was the rare one of mass 235, instead of the most abundant U-238. The problem was, however, complicated by the fact that, besides producing fission, slow neutrons can also give rise to the production of the radioactive isotope-239 by simple capture resonance . Such a process competes with fission in taking up the neutrons which are needed to sustain a chain reaction, so that a major problem in making the chain reaction to be effective was to avoid losses due to this absorption. In any case, the first basic point to be cleared up was the choice of the fissile material to be used and, in this respect, two alternatives were opened at the end of 1939.

The first one was the separation of U-235 from the natural uranium, thus eliminating the absorption by the most abundant isotope U-238. Obviously, for this method to work, the major difficulty for that time was to produce large quantities of the isotope needed.

The alternative choice was, instead, to use directly natural uranium, with the evident drawback caused by the undesirable absorption of neutrons by the most abundant isotope

The problem with both the alternative methods were serious, and Fermi chose to work out the one where more physical effects should be understood and kept under control, i.e. he decided to study the possibility of a chain reaction with natural uranium in conjunction with a readily available light element that would slow the neutrons to thermal energies, minimizing the loss by U-238 absorption.

It was recognized that the most efficient way to slow down neutrons was to pass them through hydrogen, the lightest chemical elements present in water, paraffin, etc., so that the obvious conclusion for getting a reproduction factor high enough for a chain reaction was to disseminate uranium powder in water. However, measurements revealed that thermal neutron absorption by hydrogen was too large for water to make it a usable medium for slowing down neutrons in a chain reaction. Thus, other light elements should be taken into consideration. Out of Szilard’s thinking came the idea of using graphite instead of water to slow down the neutron. Fermi had also been thinking about graphite. Measurements showed that the absorption of neutrons on graphite was small enough to make it the obvious choice for a material for slowing down the neutrons.
The next step was to design a chain reacting pile that would work. Fermi and Szilard suggested placing uranium in a matrix of the moderating material, thus forming a cubical lattice of uranium.
The study of graphite-uranium lattice reactors was started at Colombia university in July, 1941, abut after reorganization of Uranium Project it was decided that the chain reactor program will be concentrated at the University of Chicago.

The first nuclear reactor, called a pile, was a daring and sophisticated experiment that required nearly 50 tons of machined and shaped uranium and uranium oxide pellets along with 385 tons - the equivalent of four railroad coal hoppers - of graphite blocks, machined on site.

The pile itself was assembled in a squash court under the football field at the University of Chicago from the layered graphite blocks and uranium and uranium oxide lumps (Fermi's term) arranged roughly in a sphere with an anticipated 13 foot radius. Neutron absorbing, cadmium coated control rods were inserted in the pile.(By absorbing neutrons cadmium rods control and prevent from burning the pile to complete destruction.) By slowly withdrawing the rods, neutron activity within the pile was expected to increase and at some point, Fermi predicted, there would be one neutron produced for each neutron absorbed in either producing fission or by the control rods.

https://chem.libretexts.org/@api/deki/files/106161/pile.jpg?revision=1&size=bestfit&width=426&height=309
CP-1 - Graphite blocks with 3 inch diameter uranium cylinders inserted - part of a layer of CP-1, the first nuclear reactor. A layer of graphite blocks without inserted uranium is seen covering the active layer.

On December 2, 1942, with 57 of the anticipated 75 layers in place, Fermi began the first controlled nuclear chain reaction. Initial experiments indicated that an ideal reactor design consisted not of a uniform distribution of uranium throughout the graphite moderator but, instead, of a lattice consisting of "lumps" of uranium in the graphite. This approach led to minimal parasitic absorption of neutrons by U-238. Furthermore, it was discovered that loss of neutrons by diffusion out of the reactor could be minimized by simply making the reactor very large, thus trapping neutrons within the reactor. With these preliminary findings in hand, models were constructed at Columbia University in 1941 and thereafter at the University of Chicago. These models led to greater insight into, and mathematical models for, the behavior of neutrons in such a matrix. Despite the risk of conducting an unprecedented nuclear fission experiment in a densely populated major city, construction of the CP-1 began in October, 1942, on the campus of the University of Chicago. As its name implies, the CP-1 consisted of a spherical arrangement of "piles" of alternating uranium and graphite layers. Embedded into this matrix were three control rods composed of cadmium, a strong neutron absorber, which were used to control the reactor.

As the size of the pile was increased, researchers carefully monitored the neutron output until the measurements indicated that the pile was close to becoming self-sustaining ("critical"). The late-stage construction was performed with the cadmium rods in place, and on the morning of December 2, 1942, it was calculated that the pile would reach criticality if the rods were removed. During the key criticality experiment, all but one of the rods were removed. One control rod was automatically set to be inserted if the reactivity passed a preset level. Another was suspended by a rope and pulley and could be released manually in an emergency. The final rod was controlled manually by George Weil, who was tasked with carefully removing the rod while Fermi and others measured the increasing neutron output, as the reactor approached criticality. With each measured extension of the control rod, the reactor output increased in intensity but soon stabilized at constant output. This indicated that although the fission reaction was occurring, it was not yet self-sustaining. Finally, as Weil withdrew the rod a further distance, Fermi and others observed the neutron count rise at a slow but increasing rate -- evidence that the reactor had become self-sustaining and had reached criticality. After this event, the researchers re-inserted the control rod, ending the self-sustaining reaction.

At around 3:20 p.m. the reactor went critical; that is, it produced one neutron for every neutron absorbed by the uranium nuclei. Fermi allowed the reaction to continue for the next 27 minutes before inserting the neutron-absorbing control rods. The energy releasing nuclear chain reaction stopped as Fermi predicted it would.

In addition to excess neutrons and energy, the pile also produced a small amount of Pu-239, the other known fissionable material.

https://chem.libretexts.org/@api/deki/files/106159/1024px-Stagg_Field_reactor.jpg?revision=1&size=bestfit&width=506&height=396
The first controlled chain reaction, Stagg Field, Chicago, Dec. 2, 1942. The first nuclear reactor was erected in 1942 in the West Stands section of Stagg Field at the University of Chicago. On December 2, 1942 a group of scientists achieved the first self-sustaining chain reaction and thereby initiated the controlled release of nuclear energy. The reactor consisted of uranium and uranium oxide lumps spaced in a cubic lattice embedded in graphite. In 1943 it was dismantled and reassembled at the Palos Park unit of the Argonne National Laboratory. (Public Domain; Courtesy of the Argonne National Laboratory)

The achievement of the first sustained nuclear reaction was the beginning of a new age in nuclear physics and the study of the atom. Humankind could now use the tremendous potential energy contained in the nucleus of the atom.

However, while a controlled chain reaction was achieved with natural uranium, and could produce plutonium, it would be necessary to separate U-235 from U-238 to build a uranium bomb.

On December 28, 1942, upon reviewing a report from his advisors, President Franklyn Roosevelt recommended building full-scale plants to produce both U-235 and Pu-239. This changed the effort to develop nuclear weapons from experimental work in academic laboratories administered by the U.S. Office of Scientific Research and Development to a huge effort by private industry. This work, supervised by the U.S. Army Corps of Engineers, was codenamed the Manhattan Project. It spread throughout the entire United States, with the facilities for uranium and plutonium production being located at Oak Ridge, Tennessee, and Hanford, Washington, respectively. Work on plutonium production continued at the University of Chicago, at what became known as the Metallurgical Laboratory or Met Lab. A new laboratory at Los Alamos, New Mexico, became the focal point for development of the uranium and plutonium bombs.

Separations of the Isotopes of Uranium

Once the power that was hidden in uranium became evident, the emphasis shifted to methods to separate the much more potent U-235 from its abundant relative, U-238. This question consumed thousands of hours and millions of dollars.

Scientists had concluded that enriched samples of uranium-235 were necessary for further research and that the isotope might serve as an efficient fuel source for an explosive device. "Enrichment" meant increasing the proportion of U-235, relative to U-238, in a uranium sample. This required separating the two isotopes and discarding U-238. Uranium-235 occurred in a ratio of 1:139 in natural uranium ore. Since they were chemically identical, they could not be separated by chemical means. Furthermore, with their masses differing by less than 1 percent, separation by physical means would be extremely difficult and expensive.

Nevertheless, scientists pressed forward on several complicated techniques of physical separation, all based on the small difference in atomic weight between the uranium isotopes. Manhattan Project director General Leslie Groves wanted to investigate as many possibilities as possible, and had the resources to simultaneously pursue multiple speculative projects.

 Centrifuge



A centrifuge was the first device to separate chemical isotopes, used by Jesse Beams of the University of Virginia to separate chlorine-35 from chlorine-37 in 1934. In 1940, American physicists thought that the centrifuge was the best possibility for large-scale enrichment, and Beams received government money to attempt uranium enrichment via centrifuge.

Centrifugal force in a cylinder spinning rapidly on its vertical axis would separate a gaseous mixture of two isotopes. This is because the lighter U-235 isotope would be less affected by the action and could be drawn off at the top center of the cylinder. A cascade system composed of thousands of centrifuges could produce a rich mixture. Beams pioneered this method and received much of the early funding.

A high-speed centrifuge initially seemed promising for uranium enrichment, but the Manhattan Project failed to produce a workable model, and research stopped during the war. The centrifuge beams constructed could separate U-235 from U-238, but required huge amounts of energy and could only sustain a short run before breaking down; in other words, it was not suited for industrial production. Manhattan Project scientists opted to pursue gaseous diffusion over gas centrifuges as the primary method for uranium isotope separation, and in January 1944 Army support for the gas centrifuge method was dropped.

Electromagnetic Separation

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The electromagnetic method, pioneered by Alfred Nier of the University of Minnesota, used a mass spectrometer, or spectrograph, to send a stream of charged particles through a strong magnetic field. Atoms of the lighter isotope (U-235) would be deflected more by the magnetic field than those of the heavier isotope (U-238), resulting in two streams that could then be collected by different receivers.

The electromagnetic method as it existed in 1940, however, would have taken too long to separate quantities sufficient to be used in the current war. In fact, 27,000 years would have been required for a single spectrometer to separate 1 gram of uranium-235.

Ernest O. Lawrence of the Radiation Lab at the University of California at Berkeley favored this method and converted his giant cyclotron to accomplish this form of separation more efficiently. This model led to the eventual design and construction of the huge Y-12 Plant complex at Oak Ridge, Tennessee. Because of its exorbitant cost, electromagnetic separation was largely abandoned after the war for weapons production, 

Gaseous Diffusion

Gaseous Diffusion circuit

Gaseous diffusion’s principle was simple: molecules of a lighter isotope would pass through a porous barrier more readily than those of a heavier isotope. The tiny weight difference between U-235 and U-238 meant that initial separation would be negligible. Repeat the process hundreds of times in sequential "cascades," though, and the end product would be significantly enriched uranium.

Gaseous diffusion seemed promising in theory, but would clearly be difficult to implement on an industrial scale. Besides the massive scale involved, it was completely unclear in 1940 and 1941 how to construct a satisfactory apparatus for gaseous diffusion. Because of these problems, the S-1 committee did not make this method a priority.

Significant early work on gaseous diffusion happened in Britain, primarily at chemical conglomerate Imperial Chemical Industries (ICI) and the University of Birmingham. Both succeeded in producing a prototype of gaseous diffusion. Starting in 1941, a team at Columbia University in New York also researched gaseous diffusion, and produced a slightly different model.

Centrifuge research faltered in 1942, and General Leslie Groves pushed for more research into gaseous diffusion. In mid-1943, Roosevelt and Churchill signed the Quebec Agreement, under which British nuclear research was subsumed into the Manhattan Project; this meant full exchange of information. But serious problems remained. Besides the need for a porous barrier, component design would have to accommodate uranium hexafluoride, one of the most corrosive gases in the world.

Groves soon ordered the construction of K-25, but as construction crews cleared the site and poured the foundation it was still unclear what exactly was going to go inside the plant. To protect the pipes from corrosion, contractors undertook the new process of nickel coating. Sealing the pipes was also a problem, as grease would interfere with the process and uranium hexafluoride could not be permitted to leak out. The eventual solution was a completely novel seal material: Teflon.

Kellex Corporation undertook the barrier problem: developing a thin metal membrane with millions of tiny openings. After months of research and competition with an incomplete design from the team at Columbia, Groves accepted the Kellex design and ordered it installed in K-25. K-25, the largest roofed building in the world upon construction, would eventually serve as an intermediary step between S-50 and Y-12. Gaseous diffusion was important throughout the Cold War, but uses far more energy than centrifuges, and is now nearly obsolete.

 

Liquid Thermal Diffusion

Liquid Thermal Diffusion

The Uranium Committee briefly demonstrated an interest in a fourth enrichment process during 1940, only to conclude that it would not be worth pursuing. This process, liquid thermal diffusion, was being investigated by Philip Abelson at the Carnegie Institution in Washington. Abelson placed pressurized liquid uranium hexafluoride into the space between two concentric vertical pipes. With the outer wall cooled by a circulating water jacket and the inner wall heated by high-pressure steam, the lighter U-235 isotope tended to concentrate near the hot wall and the heavier U-238 near the cold. Over time, convection would carry the lighter isotope to the top of the column where it could be drawn off. Taller columns would produce more separation.

Like other enrichment methods, liquid thermal diffusion was at an early stage in 1940. Abelson eventually relocated his experimentation to the Naval Research Laboratory in Washington, DC, whereupon money was obtained to construct a pilot plant at the Philadelphia Navy Yard. When Oppenheimer learned the Navy was using liquid thermal diffusion for enriched uranium in submarines, the Manhattan Project subsequently built the S-50 Plant, which enriched uranium slightly before it was sent to K-25 and Y-12 for further enrichment. The inefficiency of this method meant that, like electromagnetic separation, it was obsolete after the war.

Manhattan Project Chronology

1919

Ernest Rutherford discovers the proton by artificially transmuting an element (nitrogen into oxygen).

1930

Ernest O. Lawrence builds the first cyclotron in Berkeley.

1931

Robert J. Van de Graaff develops the electrostatic generator.

1932

James Chadwick discovers the neutron.

1932

J. D. Cockroft and E. T. S. Walton first split the atom.

1932

Lawrence, M. Stanley Livingston, and Milton White operate the first cyclotron.

1934

Enrico Fermi produces fission.

December 1938

Otto Hahn and Fritz Strassman discover the process of fission in uranium.

December 1938

Lise Meitner and Otto Frisch confirm the Hahn-Strassman discovery and communicate their findings to Niels Bohr.

January 26,1939

Bohr reports on the Hahn-Strassman results at a meeting on theoretical physics in Washington, D. C.

August 2, 1939

Albert Einstein writes President Franklin D. Roosevelt, alerting the President to the importance of research on chain reactions and the possibility that research might lead to developing powerful bombs.

September 1, 1939

Germany invades Poland.

October 11-12, 1939

Alexander Sachs discusses Einstein's letter with President Roosevelt. Roosevelt decides to act and appoints Lyman J. Briggs head of the Advisory Committee on Uranium.

October 19,1939

Roosevelt informs Einstein that he has set up a committee to study uranium.

October 21, 1939

The Uranium Committee meets for the first time.

November 1, 1939

The Uranium Committee recommends that the government purchase graphite and uranium oxide for fission research.

March 1940

John R. Dunning and his colleagues demonstrate that fission is more readily produced in the rare uranium-235 isotope, not the more plentiful uranium-238.

Spring-Summer 1940

Isotope separation methods are investigated.

June 1940

Vannevar Bush is named head of the National Defense Research Committee. The Uranium Committee becomes a scientific subcommittee of Bush's organization.

February 24, 1941

Glenn T. Seaborg's research group discovers plutonium.

March 28, 1941

Seaborg's group demonstrates that plutonium is fissionable.

May 3, 1941

Seaborg proves plutonium is more fissionable than uranium-235.

May 17, 1941

A National Academy of Sciences report emphasizes the necessity of further research.

June 22, 1941

Germany invades the Soviet Union.

June 28, 1941

Bush is named head of the Office of scientific Research and Development. James B. Conant replaces Bush at the National Defense Research Committee, which becomes an advisory body to the Office of Scientific Research and Development.

July 2, 1941

The British MAUD report concludes that an atomic bomb is feasible.

July 11, 1941

A second National Academy of Sciences report confirms the findings of the first.

July 14, 1941

Bush and Conant receive the MAUD report.

October 9, 1941

Bush briefs Roosevelt and Vice President Henry A. Wallace on the state of atomic bomb research. Roosevelt instructs Bush to find out if a bomb can be built and at what cost. Bush receives permission to explore construction needs with the Army.

November 9, 1941

A third National Academy of sciences report agrees with the MAUD report that an atomic bomb is feasible,

November 27, 1941

Bush forwards the third National Academy of sciences report to the President.

December 7, 1941

The Japanese attack Pearl Harbor.

December 10, 1941

Germany and Italy declare war on the United States.

December 16, 1941

The Top Policy Committee becomes primarily responsible for making broad policy decisions relating to uranium research.

December 18, 1941

The S-1 Executive committee (which replaced the Uranium Committee in the Office of Scientific and Research Development) gives Lawrence $400,000 to continue electromagnetic research.

January 19, 1942

Roosevelt responds to Bush's November 27 report and approves production of the atomic bomb.

March 9,1942

Bush gives Roosevelt an optimistic report on the possibility of producing a bomb.

May 23,1942

The S-1 Executive Committee recommends that the project move to the pilot plant stage and build one or two piles (reactors) to produce plutonium and electromagnetic, centrifuge, and gaseous diffusion plants to produce uranium-235.

June 1942

Production pile designs are developed at the Metallurgical Laboratory in Chicago.

June 17 1943

President Roosevelt approves the S-1 Executive Committee recommendation to proceed to the pilot plant stage and instructs that plant construction be the responsibility of the Army. The Office of Scientific Research and Development continues to direct nuclear research, while the Army delegates the task of plant construction to the Corps of Engineers.

July 1942

Kenneth Cole establishes the health division at the Metallurgical Laboratory.

August 7, 1942

The American island-hopping campaign in the Pacific begins with the landing at Guadalcanal.

August 13, 1942

The Manhattan Engineer District is established in New York City, Colonel James C. Marshall commanding.

August 1942

Seaborg produces a microscopic sample of pure plutonium.

September 13,1942

The S-1 Executive Committee visits Lawrence's Berkeley laboratory and recommends building an electromagnetic pilot plant and a section of a full scale plant in Tennessee.

September 17, 1942

Colonel Leslie R. Groves is appointed head of the Manhattan Engineer District. He is promoted to Brigadier General six days later.

September 19, 1942

Groves selects the Oak Ridge, Tennessee site for the pilot plant.

September 23, 1942

Secretary of War Henry Stimson creates a Military Policy Committee to help make decisions for the Manhattan Project.

October 3, 1942

E. I. du Pont de Nemours and Company agrees to build the chemical separation plant at Oak Ridge.

October 5, 1942

Compton recommends an intermediate pile at Argonne.

Fall 1942

J. Robert Oppenheimer and the luminaries report from Berkeley that more fissionable material may be needed than previously thought.

October 19, 1942

Groves decides to establish a separate scientific laboratory to design an atomic bomb.

October 26, 1942

Conant recommends dropping the centrifuge method.

November 22, 1942

On the recommendation of Groves and Conant, the Military Policy Committee decides to skip the pilot plant stage on the plutonium, electromagnetic, and gaseous diffusion projects and go directly from the research stage to industrial-scale production. The Committee also decides not to build a centrifuge plant.

November 14, 1942

The S-1 Executive Committee endorses the recommendations of the Military Policy committee.

November 1942

The Allies invade North Africa.

November 25, 1942

Groves selects Los Alamos, New Mexico as the bomb laboratory (codenamed Project Y). Oppenheimer is chosen laboratory director.

December 2, 1942

Scientists led by Enrico Fermi achieve the first self-sustained nuclear chain reaction in Chicago.

December 10, 1942

The Lewis committee compromises on the electromagnetic method. The Military policy Committee decides to build the plutonium production facilities at a site other than Oak Ridge.

December 28, 1942

Roosevelt approves detailed plans for building production facilities and producing atomic weapons.

January 13-14, 1943

Plans for the Y-12 electromagnetic plant are discussed. Groves insists that Y-12's first racetrack be finished by July 1.

January 14-24, 1943

At the Casablanca Conference, Roosevelt and British Prime Minister Churchill agree upon unconditional surrender for the h powers.

January 16, 1943

Groves selects Hanford, Washington as the site for the plutonium production facilities. Eventually three reactors, called B, D, and F, are built at Hanford.

January 1943

Bush encourages Philip Abelson's research on the thermal diffusion process.

February 18, 1943

Construction of Y-12 begins at Oak Ridge.

February 1943

Groundbreaking for the X-10 plutonium pilot plant takes place at Oak Ridge.

March 1943

Researchers begin arriving at Los Alamos.

April 1943

Bomb design work begins at Los Alamos.

June 1943

Site preparation for the K-25 gaseous diffusion plant commences at Oak Ridge.

Summer 1943

The Manhattan Engineer District moves its headquarters to Oak Ridge.

July 1943

Oppenheimer reports that three times as much fissionable material maybe necessary than thought nine months earlier.

August 27, 1943

Groundbreaking for the 100-B plutonium production pile at Hanford takes place.

September 8, 1943

Italy surrenders to Allied forces.

September 9, 1943

Groves decides to double the size of Y-12.

September 27, 1943

Construction begins on K-25 at Oak Ridge.

November 4, 1943

The X-10 pile goes critical and produces plutonium by the end of the month.

Late 1943

John von Neumann visits Los Alamos to aid implosion research.

December 15, 1943

The first Alpha racetrack is shut down due to maintenance problems.

January 1944

The second Alpha racetrack is started and demonstrates maintenance problems similar to those that disabled the first.

January 1944

Construction begins on Abelson's thermal diffusion plant at the Philadelphia Naval Yard.

February 1944

Y-12 sends 200 grams of uranium-235 to Los Alamos.

March 1944

The Beta building at Y-12 is completed.

March 1944

Bomb models are tested at Los Alamos.

April 1944

Oppenheimer informs Groves about Abelson's thermal diffusion research in Philadelphia.

June 6,1944

Allied forces launch the Normandy invasion.

June 21,1944

Groves orders the construction of the S-50 thermal diffusion plant at Oak Ridge.

July 4, 1944

The decision is made to work on a calutron with a 30-beam source for use in Y-12.

July 17, 1944

The plutonium gun bomb (code named Thin Man) is abandoned.

July 1944

A major reorganization to maximize implosion research occurs at Los Alamos.

July 1944

Scientists at the Metallurgical Laboratory issue the "Prospectus on Nucleonics," concerning the international control of atomic energy.

August 7, 1944

Bush briefs General George C. Marshall, informing him that small implosion bombs might be ready by mid-1945 and that a uranium bomb will almost certainly be ready by August 1, 1945.

September 1944

Colonel Paul Tibbets' 393rd Bombardment Squadron begins test drops with dummy bombs called Pumpkins.

September 13, 1944

The first slug is placed in pile 100-B at Hanford.

September 1944

Roosevelt and Churchill meet in Hyde Park and sign an "aide memoire" pledging to continue bilateral research on atomic technology.

Summer 1944-Spring 1945

The Manhattan Project's chances for success advance from doubtful to probable as Oak Ridge and Hanford produce increasing amounts of fissionable material, and Los Alamos makes progress in chemistry, metallurgy, and weapon design.

September 27, 1944

The 1OO-B reactor goes critical and begins operation.

September 30, 1944

Bush and Conant advocate international agreements on atomic research to prevent an arms race.

December 1944

The chemical separation plants (Queen Mary) are finished at Hanford.

February 2, 1945

Los Alamos receives its first plutonium.

February 4-11, 1945

Roosevelt, Churchill, and Soviet Premier Joseph Stalin meet at Yalta.

March 1945

S-50 begins operation at Oak Ridge.

March 1945

Tokyo is firebombed, resulting in 100,000 casualties.

March 12,1945

K-25 begins production at Oak Ridge.

April 12, 1945

President Roosevelt dies.

April 25,1945

Stimson and Groves brief President Truman on the Manhattan Project.

May 1945

Stalin tells Harry Hopkins that he is willing to meet with Truman and proposes Berlin as the location.

May 7, 1945

The German armed forces in Europe surrender to the Allies.

May 23, 3945

Tokyo is firebombed again, this time resulting in 83,000 deaths.

May 31 - June 1, 1945

The Interim Committee meets to make recommendations on wartime use of atomic weapons, international regulation of atomic information, and legislation regarding domestic control of the atomic enterprise (the Committee's draft legislation becomes the basis for the May- Johnson bill).

June 6, 1945

Stimson informs President Truman that the Interim Committee recommends keeping the atomic bomb a secret and using it as soon as possible without Warning.

June 1945

Scientists at the Metallurgical Laboratory issue the Franck Report, advocating international control of atomic research and proposing a demonstration of the atomic bomb prior to its combat use.

June 14,1945

Groves submits the target selection group's recommendation to Marshall.

June 21,1945

The Interim Committee, Supporting its Scientific Panel, rejects the Franck Report recommendation that the bomb be demonstrated prior to combat.

July 2-3, 1945

Stimson briefs Truman on the Interim Committee's deliberations and outlines the peace terms for Japan.

July 16, 1945

Los Alamos scientists successfully test a plutonium implosion bomb in the Trinity shot at Alamogordo, New Mexico.

July 17 - August 2, 1945

Truman, Churchill, and Stalin meet in Potsdam.

July 21, 1945

Groves sends Stimson a report on the Trinity test.

July 24,1945

Stimson again briefs Truman on the Manhattan Project and peace terms for Japan. In an evening session, Truman informs Stalin that the United States has tested a powerful new weapon.

July 25, 1945

The 509th Composite Group is ordered to attack Japan with an atomic bomb "after about" August 3.

July 26, 1945

Truman, Chinese President Chiang Kai-Shek, and new British Prime Minister Clement Atlee issue the Potsdam Proclamation, calling for Japan to surrender unconditionally.

July 29, 1945

The Japanese reject the Potsdam Proclamation.

August 6, 1945

The gun model uranium bomb, called Little Boy, is dropped on Hiroshima. Truman announces the raid to the American public.

August 8, 1945

Russia declares war on Japan and invades Manchuria.

August 9, 1945

The implosion model plutonium bomb, called Fat Man, is dropped on Nagasaki.

August 12, 1945

The Smyth Report, containing unclassified technical information on the bomb project, is released.

August 14, 1945

Japan surrenders.

September 2, 1945

The Japanese sign articles of surrender aboard the U.S.S. Missouri

September 9, 1945

S-50 shuts down.

September 1945

Y-12 shutdown begins.

October 3, 1945

Operation Crossroads continues with Shot Baker, a plutonium bomb detonated underwater, at Bikini

.

August 15, 1947

The Manhattan Engineer District is abolished.

December 31, 1947

The National Defense Research Committee and the Office of Scientific Research and Development are abolished. Their functions are transferred to the Department of Defense.

The Making of the Atomic Bomb

Topic 11: Years 1942-1945: General Leslie Groves, Robert Oppenheimer, and the Manhattan Project. Oak Ridge, Hanford, and Los Alamos. The separation of U-235 and the production of plutonium. The development of the implosion lens.

The Manhattan Project

The Manhattan Project was the code name for the American-led effort to develop a functional atomic weapon during World War II. The controversial creation and eventual use of the atomic bomb engaged some of the world’s leading scientific minds, as well as the U.S. military—and most of the work was done in Los Alamos, New Mexico, not the borough of New York City for which it was originally named. The Manhattan Project was started in response to fears that German scientists had been working on a weapon using nuclear technology since the 1930s—and that Adolf Hitler was prepared to use it.
The agencies leading up to the Manhattan Project were first formed in 1939 by President Franklin D. Roosevelt, after U.S. intelligence operatives reported that scientists working for Adolf Hitler were already working on a nuclear weapon.
At first, Roosevelt set up the Advisory Committee on Uranium, a team of scientists and military officials tasked with researching uranium’s potential role as a weapon. Based on the committee’s findings, the U.S. government started funding research by Enrico Fermi and Leo Szilard at Columbia University, which was focused on radioactive isotope separation (also known as uranium enrichment) and nuclear chain reactions.
The Advisory Committee on Uranium’s name was changed in 1940 to the National Defense Research Committee, before finally being renamed the Office of Scientific Research and Development (OSRD) in 1941 and adding Enrico Fermi to its list of its members.
The Army Corps of Engineers joined the OSRD in 1942 with President Roosevelt’s approval, and the project officially morphed into a military initiative, with scientists serving in a supporting role.
Summer 1942--during which the American island hopping campaign in the Pacific began at Guadalcanal-proved to be a troublesome one for the fledgling bomb project. Colonel James C. Marshall received the assignment of directing the Laboratory for the Development of Substitute Metals, or DSM. Marshall immediately moved from Syracuse to New York City, where he set up the Manhattan Engineer District, established by general order on August 13. Marshall, like most other Army officers, knew nothing of nuclear physics. Furthermore, Marshall and his Army superiors were disposed to move cautiously. In one case, for instance, Marshall delayed purchase of an excellent production site in Tennessee pending further study, while the scientists who had been involved in the project from the start were pressing for immediate purchase.

General Leslie Groves

Decisions made in September provided administrative clarity and renewed the project's sense of urgency. Project Started to be called as the Manhattan Project. On September 17, the Army appointed Colonel Leslie R. Groves (promoted to Brigadier General six days later) to head the effort. Groves was an engineer with impressive credentials, including building of the Pentagon, and, most importantly, had strong administrative abilities. Within two days Groves acted to obtain the Tennessee site and secured a higher priority rating for project materials. In addition, Groves moved the Manhattan Engineer District headquarters from New York to Washington.

Photograph of General Groves

General Leslie R. Groves

During summer and fall 1942 technical and administrative difficulties were still severe. Each of the four isotope separation processes remained under consideration, but a full-scale commitment to all four posed serious problems even with the project's high priority. When Groves took command in mid-September, he made it clear that by late 1942 decisions would be made as to which process or processes promised to produce a bomb in the shortest amount of time. The exigencies of war, Groves held, required scientists to move from laboratory research to development and production in record time. Though traditional scientific caution might be short-circuited in the process, there was no alternative if a bomb was to be built in time to be used in the current conflict. As everyone involved in the Manhattan Project soon learned, Groves never lost sight of this goal and made all his decisions accordingly.

Robert Oppenheimer

J. Robert Oppenheimer

J. Robert Oppenheimer

While each of the four processes fought to demonstrate its "workability" during summer and fall 1942 equally important theoretical studies were being conducted that greatly influenced the decisions made in November. Robert Oppenheimer headed the work of a group of theoretical physicists he called the luminaries, which included Felix Bloch, Hans Bethe, Edward Teller, and Robert Serber, while John H. Manley assisted him by coordinating nationwide fission research and instrument and measurement studies from the Metallurgical Laboratory in Chicago. Despite inconsistent experimental results, the consensus emerging at Berkeley was that approximately twice as much fissionable material would be required for a bomb than had been estimated six months earlier. This was disturbing, especially in light of the military's view that it would take more than one bomb to win the war. The goal of mass-producing fissionable material, which still appeared questionable in late 1942, seemed even more unrealistic given Oppenheimer's estimates. Oppenheimer did report, with some enthusiasm, that fusion explosions using deuterium (heavy hydrogen) might be possible.

On December 28, 1942, President Roosevelt approved the establishment of what ultimately became a government investment in excess of $2 billion. The Manhattan Project was authorized to build full-scale gaseous diffusion and plutonium plants and the compromise electromagnetic plant, as well as heavy water production facilities. No one could guarantee that the United States would overtake Germany in the race for the bomb, but by the beginning of 1943 the Manhattan Project had the complete support of President Roosevelt and the military leadership, the services of some of the nation's most distinguished scientists, and a sense of urgency driven by fear. Much had been achieved in the year between Pearl Harbor and the end of 1942.

No single decision created the American atomic bomb project. Roosevelt's December 28 decision was inevitable in light of numerous earlier ones that, in incremental fashion, committed the United States to pursuing atomic weapons. At that time, there was a science organization at the highest level of the federal government and a Top Policy Group with direct access to the President. Funds were authorized, and the participation of the Corps of Engineers had been approved in principle. In addition, the country was at war and its scientific leadership-as well as its President-had the belief, born of the MAUD report that the project could result in a significant contribution to the war effort. Roosevelt's approval of $500 million in late December 1942 was a step that followed directly from the commitments made in January of that year and stemmed logically from the President's earliest tentative decisions in late 1939.

In many ways the Manhattan Engineer District operated like any other large construction company. It purchased and prepared sites, let contracts, hired personnel and subcontractors, built and maintained housing and service facilities, placed orders for materials, developed administrative and accounting procedures, and established communications networks. By the end of the war Groves and his staff had spent approximately $2.2 billion on production facilities and towns built in the states of Tennessee, Washington, and New Mexico, as well as on research in university laboratories from Columbia to Berkeley. What made the Manhattan Project unlike other companies performing similar functions was that, because of the necessity of moving quickly, it invested hundreds of millions of dollars in unproven and hitherto unknown processes and did so entirely in secret. Speed and secrecy were the watchwords of the Manhattan Project.

Secrecy proved to be very high. Although it served as a constant irritant to the academic scientists on the project, it had one overwhelming advantage: Secrecy made it possible to make decisions with little regard for normal peacetime political considerations. Groves knew that as long as he had the backing of the White House money would be available and he could devote his considerable energies entirely to running the bomb project. Secrecy in the Manhattan Project was so complete that many people working for the organization did not know what they were, working on until they heard about the bombing of Hiroshima on the radio.

Unfinished research on three separate, unproven processes had to be used to freeze design plans for production facilities, even though it was recognized that later findings inevitably would dictate changes. The pilot plant stage was eliminated entirely, violating all manufacturing practices and leading to intermittent shutdowns and endless troubleshooting during trial runs in production facilities. The inherent problems of collapsing the stages between the laboratory and full production created an emotionally charged atmosphere with optimism and despair alternating with confusing frequency.

Nobody knew when the bomb will be ready. For any large organization to take laboratory research into design, construction, operation, and product delivery in two-and-a-half years (from early 1943 to Hiroshima) would be a major industrial achievement. Whether the Manhattan Project would be able to produce bombs in time to affect the current conflict was an open question as 1943 began.

Manhattan Project Signature Facilities

Following is the list of the Department of Energy's Manhattan Project "Signature Facilities" bombs during World War II

Metallurgical Laboratory, University of Chicago (Chemistry Building and CP-1 site)

In August 1942, Met Lab isolated first weighable amount of plutonium. Chemistry Building now a National Landmark with plaque and interpretive display. On December 2, 1942, CP-1 (Fermi’s "pile" at Stagg Field) produced the first self-sustaining nuclear reaction. Site commemorated with plaque and sculpture. Argonne National Laboratory lineal descendant of Met Lab.

X-10 Graphite Reactor, Oak Ridge

Built in 1943, was designed as pilot for the Hanford production reactors. Produced first significant amounts of plutonium. A National Historic Landmark, control room and reactor face are accessible to the public.

K-25 Gaseous Diffusion Process Building, Oak Ridge

Completed in 1945, U-shaped building measures half a mile by 1,000 feet. Gaseous diffusion one of three isotope separations processes that provided U 235 for the Hiroshima weapon (Little Boy). Gaseous diffusion only uranium enrichment process used during Cold War. K-25 prototype for later Oak Ridge plants and those at Paducah and Portsmouth.

Y-12 Beta-3 Racetracks, Oak Ridge

Produced U-235 for the Hiroshima weapon. Only surviving production-level electromagnetic isotope separations facility in U.S. (Comparable facility in Sverdlovsk, Russia).

B Reactor, Hanford

Completed in 1944, was world's first large-scale plutonium production reactor. Produced plutonium for Trinity device, Nagasaki weapon (Fat Man), and Cold War weapons. Interior of reactor building currently accessible by appointment only. A National Historic Mechanical Engineering Landmark.

Chemical Separations Building (T Plant), Hanford

Completed in 1944-45, separated plutonium out of production reactor fuel rods. Massive canyon-like structure 800 feet long, 65 feet wide, and 80 feet high. Contamination precludes access to interior, though interior can be viewed through closed-circuit television system.

V-Site Assembly Building, Los Alamos

Among last remaining Manhattan Project buildings at Los Alamos. Trinity device and later weapons assembled here. Other buildings at site destroyed by the Cerro Grande fire in May 2000.

Trinity Site, Alamogordo

July 16, 1945, test began the atomic age. Site, now owned by DOD (part of White Sands Missile Range), contains commemorative sign and other artifacts as well as the McDonald Ranch House and remnants of base camp. Currently open to public twice a year. A National Historic Landmark.

Source: Department of Energy

Oak Ridge

Oak Ridge was the home of the uranium enrichment plants (K-25 and Y-12), the liquid thermal diffusion plant (S-50), and the pilot plutonium production reactor (X-10 Graphite Reactor).
 

Site Selection

In 1942, General Leslie Groves approved Oak Ridge, Tennessee, as the site for the pilot plutonium plant and the uranium enrichment plant. Manhattan Project engineers had to quickly build a town to accommodate 30,000 workers--as well as build the enormously complex plants.

By the time President Roosevelt authorized the Manhattan Project on December 28, 1942, work on the east Tennessee site where the first production facilities were to be built was already underway. On Saturday, September 19, Groves had approved the acquisition of 59,000 acres of land along the Clinch River, 20 miles west of Knoxville, Tennessee. Also approved was the removal of the relatively few families on the marginal farmland and extensive site preparation to provide the transportation, communications, and utility needs of the town and production plants that would occupy the previously undeveloped area. At first, this location was known as "Site X" and later changed to the Clinton Engineer Works, named after the nearest town. After the war, the name was again changed officially to Oak Ridge.

Building Oak Ridge.

Original plans called for the military reservation to house approximately 13,000 people in prefabricated housing, trailers, and wood dormitories. By the time the Manhattan Engineer District headquarters were moved from Washington, DC to Tennessee in the summer of 1943 (Groves kept the Manhattan Project's office in Washington and placed Col. Kenneth D. Nichols in command at Tennessee), estimates for the town of Oak Ridge had been revised upward to 45,000 people. By the end of the war, Oak Ridge was the fifth largest city in Tennessee. While the Army and its contractors tried desperately to keep up with the rapid influx of workers and their families, services always lagged behind demand.

The town site was in the northeast corner of the reservation, a strip less than one mile wide and six miles long with hilly terrain descending from the Black Oak Ridge in the north. Town planners were originally to provide housing for an estimated 30,000 people, but by 1945, the population had reached 75,000. Architectural firm Skidmore, Owings & Merrill (SOM) envisioned pleasant neighborhood communities with libraries, schools and shopping centers. However, wartime constraints limited the availability of labor and materials. Rather than performing time-consuming grading, houses were adjusted to fit the contours of the land. Most of Oak Ridge’s kitchens faced the street to minimize the length of plumbing and utility lines.

An aerial view of Oak Ridge

View of Oak Ridge

Materials were in short supply, so the first houses were built of prefabricated panels of cement and asbestos or cement board. They were known as “alphabet houses” because each of the handful of home designs was assigned a letter of the alphabet. There were small, two bedroom “A” houses, “C” houses with extra bedrooms, “D” houses with a dining room, and so forth for a total of 3,000 cement-type homes. Later, thousands of prefabricated houses were sent to Oak Ridge in sections complete with walls, floors, room partitions, plumbing and wiring. Workers turned over 30 or 40 houses to occupants each day. The Roane-Anderson Company administered all housing facilities.

Part of Oak Ridge’s appeal to Manhattan Project planners was nearby Knoxville with its population of 111,000. However, the top-secret project was not warmly welcomed in Knoxville, arousing both suspicion and resentment. Many saw the people flooding into East Tennessee from all over the country—and the world—as “furriners” who could not be questioned. In a time of austerity and rationing, others resented Oak Ridge residents arriving with unlimited ration stamps and fistfuls of cash. Oak Ridgers who ventured into Knoxville were easy to spot. The quickly constructed secret city was blanketed in a thick layer of mud. As a result, its residents’ muddy shoes were a dead giveaway as to their origin.
The Plants

The four production facility sites were located in valleys away from the town. This provided security and containment in case of accidental explosions. The Y-12 area, home of the electromagnetic plant, was closest to Oak Ridge, being one ridge away to the south. Farther to the south and west lay both the X-10 area, which contained the experimental plutonium pile and separation facilities, and K-25, site of the gaseous diffusion plant and later the S-50 thermal diffusion plant.

X-10 Graphite Reactor

The X-10 Graphite Reactor

The X-10 Graphite Reactor was the first reactor built after the successful experimental “Chicago Pile I” at the University of Chicago. On December 2, 1942, using a lattice of graphite blocks and uranium rods, Enrico Fermi proved that a nuclear chain reaction could be controlled. Scientists knew that it would only be a matter of time before the energy of the atom could be harnessed for a bomb. The X-10 Graphite Reactor was a semi-works (pilot plant) facility based on design and engineering information developed at the Metallurgical Laboratory at the University of Chicago. Based on Fermi's pile (CP-1), the X-10 Reactor and associated chemical extraction facility, produced the world's first plutonium outside the laboratory. Technology developed at Oak Ridge formed the foundation upon which the giant plutonium producing facilities at Hanford were based.

DuPont broke ground for the X-10 complex at Oak Ridge in February 1943. The site would include an air-cooled experimental pile reactor, a pilot chemical separation plant, and various support facilities. Cooper produced blueprints for the chemical separation plants in time for construction to begin in March. A series of huge underground concrete cells, the first of which sat under the pile, extended to one story above ground. Aluminum cans containing uranium slugs would drop into the first cell of the chemical separation facility and dissolve and then begin the extraction process.

The X-10 Graphite Reactor

The pile building itself went up during the spring and summer of 1943, a huge concrete shell seven feet thick with hundreds of holes for uranium slug placement. Slugs were to plutonium piles what barrier was to gaseous diffusion; that is, an obstacle that could shut down the entire process. ALCOA (Aluminum Company of America) was the only firm left working on a process to enclose uranium 235 within aluminum sheaths, and it was still having problems. Initial production provided mixed results, with many cans failing vacuum tests because of faulty seams.

 The moment everyone had been waiting for came in late October 1943 when DuPont completed construction and tests of the X-10 pile. After thousands of uranium slugs were loaded, the pile went critical in the early morning of November 4th and produced its first plutonium by the end of the month. Criticality was achieved with only half of the channels filled with uranium. During the next several months, Compton gradually raised the power level of the pile and increased plutonium yield.

Chemical separation techniques using the bismuth phosphate process were so successful that Los Alamos received its first plutonium samples beginning in the spring of 1944. Fission studies of these samples at Los Alamos during the summer heavily influenced bomb design.

Y-12 Plant

Calutron Girls at work in Y-12

The Y-12 Plant in Oak Ridge used the electromagnetic separation method, developed by Ernest Lawrence at University of California-Berkeley, to separate uranium isotopes, and was the most developed way to produce fissile material at the start of the Manhattan Project.  When an electrically-charged atom was placed in a magnetic field, it would trace a circular path with a radius determined by the atom’s mass. U-235 was lighter than U-238 and could be isolated by placing a collecting pocket in its path.

Although it was decided, that gaseous diffusion is more effective and it was placed ahead of the electromagnetic approach, many were still betting in early 1943 that Lawrence and his mass spectrograph would eventually predominate. Lawrence and his laboratory of mechanics at Berkeley continued to experiment with the giant 184-inch magnet, trying to reach a consensus on which shims, sources, and collectors to incorporate into the Y-12 design for the Oak Ridge plant. Research on magnet size and placement and beam resolution eventually led to a "racetrack" configuration of two magnets with forty-eight gaps containing two vacuum tanks each per building, with ten buildings being necessary to provide the 2,000 sources and collectors needed to separate 100 grams of uranium 235 daily. It was hoped that improvements in calutron design, or placing multiple sources and collectors in each tank, might increase efficiency and reduce the number of tanks and buildings required, but experimental results were inconclusive even as Stone & Webster of Boston, the Y-12 contractor, prepared to break ground.

Workers at Y-12

At a meeting of Groves, Lawrence, and John Lotz of Stone & Webster in Berkeley late in December 1942, Y-12 plans took shape. It was agreed that Stone & Webster would take over design and construction of a 500-tank facility, while Lawrence's laboratory would play a supporting role by supplying experimental data. By the time another summit conference on Y-12 took place in Berkeley on January 13 and 14, Groves had persuaded the Tennessee Eastman Company to sign on as plant operator and arranged for various parts of the electromagnetic equipment to be manufactured by the Westinghouse Electric Company, the Allis-Chalmers Manufacturing Company and the Chapman Valve Manufacturing Company. At the same time, General Electric agreed to provide electrical equipment.

On January 14, after a day of presentations and a demonstration of the experimental tanks in the cyclotron building, Groves stunned the Y-12 contractors by insisting that the first racetrack of ninety-six tanks be in operation by July 1 and that 500 tanks be delivered by year's end. Given that each racetrack assembly was 122 feet long, 77 feet wide and 15 feet high, the completed plant was to be the size of three, large two-story buildings, tank design was still in flux, and chemical extraction facilities also would have to be built, Groves' demands were little less than shocking. Nonetheless, Groves maintained that his schedule could be met.

For the next two months Lawrence, the contractors, and the Army negotiated over the final design. While all involved could see possible improvements, there simply was not enough time to incorporate every suggested modification. Y-12 design was finalized at a March 17 meeting in Boston, with one major modification - the inclusion of a second stage of the electromagnetic process. The purpose of this second stage was to take the enriched uranium 235 derived from several runs of the first stage and use it as the "feed material" for a second stage of racetracks containing tanks approximately half the size of those in the first. Groves approved this arrangement and work began on both the Alpha (first-stage) and Beta (second-stage) tracks.

Y-12 interior

Groundbreaking for the Alpha plant took place on February 18, 1943. Soon blueprints could not be produced fast enough to keep up with construction as Stone & Webster labored to meet Groves' deadline. The Beta facility was actually begun before formal authorization. While laborers were aggressively recruited, there was always a shortage of workers skilled enough to perform jobs according to the rigid specifications. (A further complication was that some tasks could be performed only by workers with special security clearances). Huge amounts of material had to be obtained (38 million board feet of lumber, for instance), and the magnets needed so much copper for windings that the Army had to borrow close to 15,000 tons of silver bullion from the United States Treasury to fabricate into strips and wind onto coils as a substitute for copper. Treasury silver was also used to manufacture the busbars that ran around the top of the racetracks.

Replacing copper with silver solved the immediate problem of the magnets and busbars, but persistent shortages of electronic tubes, generators, regulators, and other equipment plagued the electromagnetic project and posed the most serious threat to Groves' deadline. Furthermore, last minute design changes continued to frustrate equipment manufacturers. Nonetheless, when Lawrence toured with Y-12 contractors in May of 1943, he was impressed by the scale of operations. Lawrence returned to Berkeley rededicated to the "awful job" of finishing the racetracks on time.

But in the midst of encouraging progress in construction and research on the electromagnetic process in July came discouraging news from Oppenheimer's isolated laboratory in Los Alamos, set up in 1943 to consolidate work on atomic weapons. Oppenheimer warned that three times more fissionable material would be required for a bomb than earlier estimates had indicated. Even with satisfactory performance of the racetracks, it was now possible that they might not produce enough purified uranium 235 in time.

Groves let Lawrence talk him into building a new plant - in effect, doubling the size of the Y-12 complex. The new facility, Groves reported to the Military Policy Committee on September 9, would consist of two buildings, each with two rectangular racetracks of ninety-six tanks operating with four-beam sources.

K-25 Plant

The K-25 Plant in Oak Ridge, TN housed the massive gaseous diffusion apparatus used to partially enrich uranium before it was sent to the nearby Y-12 Plant. "K-25" comes from Kellex Corporation, the contractor that designed and built the plant, and uranium-235, often shortened to "25."  The K-25 plant was an enormously ambitious and risky undertaking. A mile-long, U-shaped building, the K-25 plant was the world’s largest roofed building at the time. British scientists working on the “tube alloy,” code for the atomic bomb project, first advocated the gaseous diffusion method in March 1941. Because of the Nazi bombing of England, any production plants had to be located elsewhere.

Columbia University’s John R. Dunning and Eugene Booth began working in 1941 on the gaseous diffusion process. The goal was to separate the isotopes of U-235 from U-238 by turning uranium metal into uranium hexafluoride gas and pumping it through a barrier material that had millions of microscopic holes.  Developing an effective barrier material was the greatest challenge. Columbia University’s SAM Labs, Kellex Corporation, and Union Carbide all pursued major programs addressing this very difficult problem, and all contributed to its final solution. General Groves ordered construction to begin and the plant was one-third complete before a solution was found.

The K-25 Plant

Eleven miles southwest of Oak Ridge on the Clinch River was the site of the planned K-25 Gaseous Diffusion plant upon which so much hope had rested when it was authorized in late 1942. Championed by the British and placed first by the Lewis Committee, gaseous diffusion seemed to be based on sound theory but had not yet produced any samples of enriched uranium-235.

At Oak Ridge, on a relatively flat area of about 5,000 acres, site preparation for the K-25 power plant began in June. Throughout the summer, contractors contended with primitive roads as they shipped in the materials needed to build what became the world's largest steam electric plant. In September work began on the cascade building, plans for which had changed dramatically since the spring. Now there were to be fifty four-story buildings (2,000,000 square feet) in a U-shape measuring a half-mile long by 1,000 feet wide. Innovative foundation techniques were required to avoid setting thousands of concrete piers to support load-bearing walls.

Since it was eleven miles from the headquarters at Oak Ridge, the K-25 site developed into a satellite town. Housing was supplied, as was a full array of service facilities for the population that eventually reached 15,000. Dubbed "Happy Valley" by the inhabitants, the town had housing similar to that in Oak Ridge, but, like headquarters, it too experienced chronic shortages. Even with a contractor camp with facilities for 2,000 workers nearby, half of the construction force had to commute to the site daily.

K-25 cost $512 million to build, or about $7 billion in 2019 dollars. The mile-long, U-shaped plant covered forty-four acres, was four stories high and up to 400 feet wide. Engineers developed special coatings for the hundreds of miles of pipes and equipment to withstand the corrosive uranium hexafluoride gas that would pass through the plant’s 3,000 repetitive diffusion stages (together making up a cascade).The entire process was hermetically sealed like a thermos bottle, as any moisture could cause a violent reaction with the uranium hexafluoride. Even minute pinhole leaks and contamination from fingerprints were major concerns. A special leak detector was invented and every component of the entire system underwent a “cleanliness control” procedure before it was installed.

Sign for the historic K-25 Plant

In late summer of 1943 it was decided that K-25 would play a lesser role than originally intended. Instead of producing fully enriched uranium 235, the new gaseous diffusion plant would provide around fifty-percent enrichment for use as feeder material for Y-12. This would be accomplished by eliminating the more troublesome upper part of the cascade. Even this level of enrichment was not assured since a suitable barrier for the diffusion process still did not exist. The decision to downgrade K-25 was part of the larger decision to double the capacity of Y-12 and fit with Groves' new strategy of utilizing a combination of separation methods to produce enough fissionable material for bombs as soon as possible.

There was no doubt in Groves' mind that gaseous diffusion still had to be pursued vigorously. Not only had major resources already been expended on the program, but there was also the possibility that it might yet prove successful. Y-12 was in trouble as 1944 began, and the plutonium pile projects (X-10) were just getting underway. A workable barrier design might put K-25 ahead in the race for the bomb. Unfortunately, no one had been able to fabricate barrier material of sufficient quality. The only alternative remaining was to increase production enough to compensate for the low percentage of barrier that met specifications. As Lawrence prepared to throw everything he had into a thirty-beam source for Y-12, Groves ordered a crash barrier program, hoping to prevent K-25 from standing idle as the race for the bomb continued.

S-50 Plant.

The theory behind investing in S-50 was that the enrichment process might work best if the three plants were used in a series. In practice, this proved to be correct. The uranium product was slightly enriched at S-50 (one to two percent U-235) and this was fed into the K-25 plant. The gaseous diffusion process raised the enrichment to about 20 percent. This was fed into the Y-12 plant for the final enrichment cycle. Through this serial approach, the first atomic bomb received its enriched uranium.

The S-50 Plant

As problems with both Y-12 and K-25 reached crisis proportions in the spring and summer of 1944, the Manhattan Project received help from an unexpected source - the United States Navy. President Roosevelt had instructed that the atomic bomb effort be an Army program and that the Navy be excluded from deliberations. Navy research on atomic power, conducted primarily for submarines, received no direct aid from Groves, who, in fact, was not up-to-date on the state of navy efforts when he received a letter on the subject from Oppenheimer in April 1944.

Oppenheimer informed Groves that Philip Abelson's experiments on thermal diffusion at the Philadelphia Navy Yard deserved a closer look. Abelson was building a plant to produce enriched uranium to be completed by early July 1944. It might be possible, Oppenheimer thought, to help Abelson complete and expand his plant and use its slightly enriched product as feed material for Y-12 until the problems plaguing K-25 could be resolved.

A thorough review of Abelson's project early in 1943, however, concluded that thermal diffusion work should be expanded but should not be considered as a replacement for gaseous diffusion, which was better understood theoretically. Abelson continued his work independently of the Manhattan Project. He obtained authorization to build a new plant at the Philadelphia Navy Yard, where construction began in January 1944.

(Note: The Navy, and specifically the Philadelphia Navy Yard, was chosen by Abelson because of their experience dealing with huge ship boilers which produced steam. Steam was the essential source of heat required for the liquid thermal diffusion process.)

Groves immediately saw the value of Oppenheimer's suggestion and sent a group to Philadelphia to visit Abelson's facility. A quick analysis demonstrated that a thermal diffusion plant could indeed be built at Oak Ridge and placed in operation by early 1945. The steam required in the convection columns was already at hand in the form of the almost completed K-25 power plant (the largest in the world). It would be relatively simple to provide steam to the thermal diffusion plant and produce enriched uranium, while providing electricity for the K-25 plant when it was finished. Groves gave the contractor, the H. K. Ferguson Company of Cleveland, just ninety days from September 27 to bring a 2,142 column plant on line (In comparison, Abelson's plant in Philadelphia contained 100 columns). There was no time to waste as Happy Valley in Oak Ridge braced itself for a new influx of 10,000 workers.

 Closely patterned on the Navy pilot plant in Philadelphia, the S-50 plant consisted of 2,142 uniform columns, each 48 feet high. Manufacturing this plant to exacting specifications within 90-days would be no small feat. Indeed, 21 firms turned down the assignment before the H. K. Ferguson Company, an engineering firm in Cleveland, accepted the challenge.

The construction of the plant demanded a high level of precision. It required nearly perfectly round columns with a uranium hexafluoride layer spacing of only 0.010 inches (3 sheets of paper) thick! In order to meet the nearly impossible deadline, operators, electricians and welders scrambled to complete the project and even used passenger trains to transport construction materials. In the end, the contractors beat the deadline and completed the S-50 plant in just 69 days.

The S-50 production plant required an enormous amount of energy and was shut down in 1946. The K-25 plant was most effective.

Glen Seaborg and Plutonium Chemistry

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The Metallurgical laboratory.

One of the most important branches of the far flung Manhattan Project was the Metallurgical Laboratory (Met Lab) in Chicago, which was counted on to design a production pile for plutonium. Here again the job was to design equipment for a technology that was not well understood even in the laboratory. The Fermi pile, important as it was historically, provided little technical guidance other than to suggest a lattice arrangement of graphite and uranium. Any pile producing more power than the few watts generated in Fermi's famous experiment would require elaborate controls, radiation shielding, and a cooling system. These engineering features would all contribute to a reduction in neutron multiplication (neutron multiplication being represented by k); so it was imperative to determine which pile design would be safe and controllable and still have a k high enough to sustain an ongoing reaction. 
 Group headed by, Thomas V. Moore, began designing the production pile in June 1942. Moore's first goals were to find the best methods of extracting plutonium from the irradiated uranium and for cooling the uranium. It quickly became clear that a production pile would differ significantly in design from Fermi's experimental reactor, possibly by extending uranium rods into and through the graphite next to cooling tubes and building a radiation and containment shield. Although experimental reactors like Fermi's did not generate enough power to need cooling systems, piles built to produce plutonium would operate at high power levels and require coolants. The Met Lab group considered the full range of gases and liquids in a search to isolate the substances with the best nuclear characteristics, with hydrogen and helium standing out among the gases and water-even with its marginal nuclear properties and tendency to corrode uranium-as the best liquid.

During the summer, Moore and his group began planning a helium-cooled pilot pile for the Argonne Forest Preserve near Chicago, built by Stone & Webster, and on September 25 they reported to Compton. The proposal was for a 460-ton cube of graphite to be pierced by 376 vertical columns containing twenty-two cartridges of uranium and graphite. Cooling would be provided by circulating helium from top to bottom through the pile. A wall of graphite surrounding the reactor would provide radiation containment, while a series of spherical segments that gave the design the nickname Mae West would make up the outer shell.

By the time there were two other pile designs to consider. One was a water-cooled model developed by Eugene Wigner and Gale Young. Wigner and Young proposed a twelve-foot by twenty-five-foot cylinder of graphite with pipes of uranium extending from a water tank above, through the cylinder, and into a second water tank underneath. Coolant would circulate continuously through the system, and corrosion would be minimized by coating interior surfaces or lining the uranium pipes.

A second alternative to Mae West was more daring. Szilard thought that liquid metal would be such an efficient coolant that, in combination with an electromagnetic pump having no moving parts (adapted from a design he and Einstein had created), it would be possible to achieve high power levels in a considerably smaller pile. Szilard had trouble obtaining supplies for his experiment, primarily because bismuth, the metal he preferred as the coolant, was rare. The decision was made that the helium cooled Mae West, designed to produce 100 grams of plutonium a day, would be built and operating by March 1944. Studies on liquid-cooled reactors would continue, including Szilard's work on liquid metals.
While the Met Lab labored to make headway on pile design, Glenn Seaborg and his coworkers tried to gain enough information about transuranium chemistry to insure that plutonium produced could be successfully extracted from the irradiated uranium. Using lanthanum fluoride as a carrier, Seaborg isolated a weighable sample of plutonium in August 1942.Seaborg's discovery and subsequent isolation of plutonium were major events in the history of chemistry, but, like Fermi's achievement, it remained to be seen whether they could be translated into a production process useful to the bomb effort. In fact, Seaborg's challenge seemed even more daunting, for while piles had to be scaled up ten to twenty times, a separation plant for plutonium would involve a scale-up of the laboratory experiment on the order of a billion-fold.

Collaboration with DuPont's Charles M. Cooper and his staff on plutonium separation facilities began even before Seaborg succeeded in isolating a sample of plutonium. Seaborg was reluctant to drop any of the approaches then under consideration, and Cooper agreed. The two decided to pursue all four methods of plutonium separation but put first priority on the lanthanum fluoride process Seaborg had already developed. Cooper's staff ran into problems with the lanthanum fluoride method in late 1942, but by then Seaborg had become interested in phosphate carriers. Work led by Stanley G. Thompson found that bismuth phosphate retained over ninety-eight percent plutonium in a precipitate. With bismuth phosphate as a backup for the lanthanum fluoride, Cooper moved ahead on a works near Stagg Field (Chicago University).

Hanford

December 16, 1942, found Colonel Franklin T. Matthias of Groves' staff and two DuPont engineers headed for the Pacific Northwest and southern California to investigate possible production sites. Of the possible sites available, none had a better combination of isolation, long construction season, and abundant water for hydroelectric power than those found along the Columbia and Colorado Rivers. After viewing six locations in Washington, Oregon, and California, the group agreed that the area around Hanford, Washington, best met the criteria established by the Met Lab scientists and DuPont engineers. The Grand Coulee and Bonneville Dams offered substantial hydroelectric power, while the flat but rocky terrain would provide excellent support for the huge plutonium production buildings. The ample site of nearly one-half million acres was far enough inland to meet security requirements, while existing transportation facilities could quickly be improved and labor was readily available. Pleased with the committee's unanimous report, Groves accepted its recommendation and authorized the establishment of the Hanford Engineer Works, codenamed Site W.
DuPont established the general specifications for the air-cooled semiworks and chemical separation facilities in early 1943. A massive graphite block, protected by several feet of concrete, would contain hundreds of horizontal channels filled with uranium slugs surrounded by cooling air. New slugs would be pushed into the channels on the face of the pile, forcing irradiated ones at the rear to fall into an underwater bucket. The buckets of irradiated slugs would undergo radioactive decay for several weeks, then be moved by underground canal into the chemical separation facility where the plutonium would be extracted with remote control equipment..
Colonel Matthias returned to the Hanford area to set up a temporary office on February 22, 1943. His orders were to purchase half a million acres in and around the Hanford-Pasco-White Bluffs area, a sparsely populated region where sheep ranching and farming were the main economic activities. Many of the area's landowners rejected initial offers on their land and took the Army to court seeking more acceptable appraisals. Matthias adopted a strategy of settling out of court to save time, time being a more important commodity than money to the Manhattan Project.

Matthias received his assignment in late March. The three water-cooled piles, designated by the letters B, D, and F, would be built about six miles apart on the south bank of the Columbia River. The four chemical separation plants, built in pairs, would be nearly ten miles south of the piles, while a facility to produce slugs and perform tests would be approximately twenty miles southeast of the separation plants near Richmond. Temporary quarters for construction workers would be put up in Hanford, while permanent facilities for other personnel would be located down the road in Richland, safely removed from the production and separation plants.

During summer 1943, Hanford became the Manhattan Project's newest atomic boomtown. Thousands of workers poured into the town, many of them to leave in discontent. Well situated from a logistical point of view, Hanford was a sea of tents and barracks where workers had little to do and nowhere to go. DuPont and the Army coordinated efforts to recruit laborers from all over the country for Hanford, but even with a relative labor surplus in the Pacific Northwest, shortages plagued the project. Conditions improved significantly during the second half of the year, with the addition of recreational facilities, higher pay, and better overall service for Hanford's population, which reached 50,000 by summer 1944. Hanford still resembled the frontier and mining towns once common in the west, but the rate of worker turnover dropped substantially.

Groundbreaking for the water-cooling plant for the 100-B pile, the westernmost of the three, took place on August 27, less than two weeks before Italy's surrender to the Allies on September 8. Work on the pile itself began in February, with the base and shield being completed by mid-May. It took another month to place the graphite pile and install the top shield and two more months to wire and pipe the pile and connect it to the various monitoring and control devices.

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Pile D at Hanford. Pile in Foreground, Water Treatment Plant in Rear.

At Hanford, irradiated uranium slugs would drop into water pools behind the piles and then be moved by remote controlled rail cars to a storage facility five miles away for transportation to their final destination at one of the two chemical separation locations, designated 200-West and 200-East. The T and U plants were located at 200-West, while a single plant, the B unit, made up the 200-East complex (the planned fourth chemical separation plant was not built).
Both 221T and 221U, the chemical separation buildings in the 200-West complex, were finished by December 1944. 221B, their counterpart in 200-East, was completed in spring 1945. Nicknamed Queen Mary by the workers who built them, the separation buildings were awesome canyon-like structures 800 feet long, 65 feet wide, and 80 feet high containing forty process pools. The interior had an eerie quality as operators behind thick concrete shielding manipulated remote control equipment by looking through television monitors and periscopes from an upper gallery. Even with massive concrete lids on the process pools, precautions against radiation exposure were necessary and influenced all aspects of plant design. 

Queen Mary

Chemical Separation Plant (Queen Mary) at Hanford.

Construction of the chemical concentration buildings (224-T, -U, and -B) was a less daunting task because relatively little radioactivity was involved, and the work was not started until very late 1944. The 200-West units were finished in early October, the East unit in February 1945. In the Queen Marys, bismuth phosphate carried the plutonium through the long succession of process pools. The concentration stage was designed to separate the two chemicals. The normal relationship between pilot plant and production plant was realized when the Oak Ridge pilot plant reported that bismuth phosphate was not suitable for the concentration process but that Seaborg's original choice, lanthanum fluoride, worked quite well. Hanford, accordingly, incorporated this suggestion into the concentration facilities. The final step in plutonium extraction was isolation, performed in a more typical laboratory setting with little radiation present. Here Perlman's earlier research on the peroxide method paid off and was applied to produce pure plutonium nitrate. The nitrate would be converted to metal in Los Alamos, New Mexico.

Los Alamos

The final link in the Manhattan Project's far flung network was the Los Alamos Scientific Laboratory in Los Alamos, New Mexico. The laboratory that designed and fabricated the first atomic bombs, codenamed Project Y, began to take shape in spring 1942 By the time of his appointment in late September, Groves had orders to setup a committee to study military applications of the bomb. Meanwhile, sentiment was growing among the Manhattan scientists that research on the bomb project needed to be better coordinated. Oppenheimer, among others, advocated a central facility where theoretical and experimental work could be conducted according to standard scientific protocols. This would insure accuracy and speed progress. Oppenheimer suggested that the bomb laboratory operate secretly in an isolated area but allow free exchange of ideas among the scientists on the staff. Groves accepted Oppenheimer's suggestion and began seeking an appropriate location.

Los Alamos

The search for a bomb laboratory site quickly narrowed to two places in northern New Mexico, Jemez Springs and the Los Alamos Boys Ranch School, locations Oppenheimer knew well since he had a ranch nearby in the Pecos Valley of the Sangre de Cristo Mountains. In mid-November, Oppenheimer, Groves, Edwin M. McMillan, and Lieutenant Colonel W. H. Dudley visited the two sites and chose Los Alamos. Located on a mesa about thirty miles northwest of Santa Fe, Los Alamos was virtually inaccessible. It would have to be provided with better water and power facilities, but the laboratory community was not expected to be very large. The boys' school occupying the site was eager to sell, and Groves was equally eager to buy. By the end of 1942 the district engineer in Albuquerque had orders to begin construction, and the University of California had contracted to provide supplies and personnel.

Oppenheimer and Groves

Oppenheimer, selected to head the new laboratory, proved to be an excellent director despite initial concerns about his administrative inexperience, leftist political sympathies, and lack of a Nobel Prize when several scientists he would be directing were prize winners. Groves worked well with Oppenheimer although the two were fundamentally different in temperament. Groves was a practical-minded military man, brusque and goal oriented. His aide, Colonel Nichols, characterized his heavyset boss as ruthless, egotistical, and confident, "the biggest S.O.B. I have ever worked for. He is most demanding. He is most critical. He is always a driver, never a praiser. He is abrasive and sarcastic," Nichols admitted, however, that if he had it to do over again, he would once again "pick General Groves [as his boss]" because of his unquestioned ability. Groves demanded that the Manhattan Project scientists spend all their time on the bomb and resist the temptation, harmless enough in peacetime, to follow lines of research that had no direct applicability to immediate problems. In contrast to Groves, Oppenheimer was a philosophical man, attracted to Eastern mysticism and of a decidedly theoretical inclination and sensitive nature. A chain-smoker given to long working hours, Oppenheimer appeared almost emaciated. The Groves-Oppenheimer alliance, though not one of intimacy, was marked by mutual respect and was a major factor in the success of the Manhattan Project.

General Leslie Groves and J. Robert Oppenheimer

General Leslie Groves and J. Robert Oppenheimer

Oppenheimer insisted, with some success, that scientists at Los Alamos remain as much an academic community as possible, and he proved adept at satisfying the emotional and intellectual needs of his highly distinguished staff. Hans Bethe, head of the theoretical division, remembered that nobody else in that laboratory "even came close to him. In his knowledge. There was human warmth as well. Everybody certainly had the impression that Oppenheimer cared what each particular person was doing. In talking to someone he made it clear that that person's work was important for the success of the whole project." 
Oppenheimer had a chance to display his persuasive abilities early when he had to convince scientists, many of them already deeply involved in war related research in university laboratories, to join his new organization. Complicating his task were the early plans to operate Los Alamos as a military laboratory. Oppenheimer accepted Groves' rationale for this arrangement but soon found that scientists objected to working as commissioned officers and feared that the military chain of command was ill suited to scientific decision making. The issue came to a head when Oppenheimer tried to convince Robert F. Bacher and Isidor I. Rabi of the Massachusetts Institute of Technology's Radiation Laboratory to join the Los Alamos team. Neither thought a military environment was conducive to scientific research. At Oppenheimer's request, Conant and Groves wrote a letter explaining that the secret weapon-related research had presidential authority and was of the utmost national importance. The letter promised that the laboratory would remain civilian through 1943, when it was believed that the requirements of security would require militarization of the final stages of the project (in fact, militarization never took place). Oppenheimer would supervise all scientific work, and the military would maintain the post and provide security.


Recruiting the Staff


Oppenheimer spent the first three months of 1943 tirelessly crisscrossing the country in an attempt to put together a first-rate staff, an effort that proved highly successful.41 Even Becher signed on, though he promised to resign the moment militarization occurred; Rabi, though he did not move to Los Alamos, became a valuable consultant. As soon as Oppenheimer arrived at Los Alamos in mid-March, recruits began arriving from universities across the United States, including California, Minnesota, Chicago, Princeton, Stanford, Purdue, Columbia, Iowa State, and the Massachusetts Institute of Technology, while still others came from the Met Lab and the National Bureau of Standards. Virtually overnight Los Alamos became an ivory tower frontier boomtown, as scientists and their families, along with nuclear physics equipment, including two Van de Graff’s, a Cockroft-Walton accelerator, and a cyclotron, arrived caravan fashion at the Santa Fe railroad station and then made their way up to the mesa along the single primitive road. It was a most remarkable collection of talent and machinery that settled this remote outpost of the Manhattan Project.



Theory and the "Gadget"


The initial Spartan environment of "the Hill" (which included box lunches and temporary housing) was without doubt quite a contrast to the comfortable campus settings so familiar to many on the staff. But the laboratory's work began even as the Corps of Engineers struggled to provide the amenities of civilized life. The properties of uranium were reasonably well understood, those of plutonium less so, and knowledge of fission explosions entirely theoretical. That 2.2 secondary neutrons were produced when uranium-235 fissioned was accepted, but while Seaborg's team had proven in March 1941 that plutonium underwent neutron induced fission, it was not known yet if plutonium released secondary neutrons during bombardment. The theoretical consensus was that chain reactions took place with sufficient speed to produce powerful releases of energy and not simply explosions of the critical mass itself, but only experiments could test the theory. The optimum size of the critical mass remained to be established, as did the optimum shape. When enough data were gathered to establish optimum critical mass, optimum effective mass still had to be determined. That is, it was not enough simply to start a chain reaction in a critical mass; it was necessary to start one in a mass that would release the greatest possible amount of energy before it was destroyed in the explosion.

In addition to calculations on uranium and plutonium fission, chain reactions, and critical and effective masses, work needed to be done on the ordnance aspects of the bomb, or "gadget" as it came to be known. Two subcritical masses of fissionable material would have to come together to form a supercritical mass for an explosion to occur. Furthermore, they had to come together in a precise manner and at high speed. Measures also had to be taken to insure that the highly unstable subcritical masses did not predetonate because of spontaneously emitted neutrons or neutrons produced by alpha particles reacting with lightweight impurities. The chances of predetonation could be reduced by purification of the fissionable material and by using a high-speed firing system capable of achieving velocities of 3,000 feet per second. A conventional artillery method of firing one subcritical mass into the other was under consideration for uranium-235, but this method would work for plutonium only if absolute purification of plutonium could be achieved.

A variation of the artillery method was designed for uranium. Bomb designers, unable to solve the purification problem, turned to the relatively unknown implosion method for plutonium. With implosion, symmetrical shockwaves directed inward would compress a subcritical mass of plutonium packed in a nickel casing (tamper), releasing neutrons and causing a chain reaction.

Always in the background loomed the hydrogen bomb, a thermonuclear device considerably more powerful than either a uranium or plutonium device but one that needed a nuclear fission bomb as a detonator. Research on the hydrogen bomb, or Super, was always a distant second in priority at Los Alamos, but Oppenheimer concluded that it was too important to ignore. After considerable thought, he gave Teller permission to devote himself to the Super. To make up for Teller's absence, Rudolf Peierls, one of a group of British scientists who reinforced the Los Alamos staff at the beginning of 1944, was added to Bethe's theory group in mid-1944. Another member of the British contingent was the Soviet agent Klaus Fuchs, who had been passing nuclear information to the Russians since 1942 and continued doing so until 1949 when he was caught and convicted of espionage (and subsequently exchanged). 

Bomb design

Differences in the uranium and plutonium atomic bombs can be traced back to the elements’ differences in reactivity and isotopes. Uranium-235 has a “slow” critical insertion time of about one millisecond. Critical insertion time is the amount of time required to form more than enough fissile material to maintain a nuclear chain reaction, known as forming a critical mass. 

Isotope uranium-235, undergoes fission more readily and emits more neutrons per fission than other such isotopes. Plutonium-239 has these same qualities. These are the primary fissionable materials used in atomic bombs.

A small amount of uranium-235, cannot undergo a chain reaction and is thus termed a subcritical mass; this is because, on average, the neutrons released by a fission are likely to leave the assembly without striking another nucleus and causing it to fission. If more uranium-235 is added to the system, the chances that one of the released neutrons will cause another fission are increased, since the escaping neutrons must traverse more uranium nuclei and the chances are greater that one of them will bump into another nucleus and split it. At the point at which one of the neutrons produced by a fission will on average create another fission, critical mass has been achieved, and a chain reaction and thus an atomic explosion will result.

In practice, an assembly of fissionable material must be brought from a subcritical to a critical state extremely suddenly.

In the uranium “gun-type” bomb design, a critical mass is generated through firing a sub-critical uranium projectile through a gun barrel at a sub-critical uranium target. The formed uranium mass of both the projectile and the target becomes critical and the nuclear chain reaction is initiated.
This can be practically achieved by using high explosives to shoot two subcritical slugs of fissionable material together in a hollow tube.

C:\Users\LYUDMILA\Desktop\Capture bomb.PNG

Unlike uranium, plutonium-240 has a high rate of spontaneous fission and has a smaller critical insertion time of approximately ten nanoseconds. The gun-type design was not feasible for plutonium-240 because of this increased number of spontaneous neutrons. The spontaneity of these neutrons could have created nuclear pre-detonation fizzle. A pre-detonation fizzle is an explosion caused by the two sub-critical masses of the bomb being brought together too slowly.

Realizing the uranium gun-type designs were not applicable for a plutonium-based weapon, the Los Alamos scientists were forced to engineer a new design that employed implosion rather than explosion. Implosion is when a burst of force travels inward rather than outward like in an explosion. American physicist Seth Neddermeyer proposed the implosion design that employed the spherical, layered approach ultimately used in the “Gadget” in the Trinity Test and in Fat Man. 

In the design, a series of high explosives (lenses) surround a solid sphere of plutonium-239. The weapon was engineered so that “you are detonating explosives in such a way to produce a spherical shockwave going inward to compress plutonium to a critical point.” The shockwave brings plutonium to its critical mass point because the compression of the core with the explosives increases the plutonium’s pressure and density.

An important aid in achieving criticality is the use of a tamper; this is a jacket of beryllium oxide or some other substance surrounding the fissionable material and reflecting some of the escaping neutrons back into the fissionable material, where they can thus cause more fissions. In addition, “boosted fission” devices incorporate such fusion able materials as deuterium or tritium into the fission core. The fusion able material boosts the fission explosion by supplying a superabundance of neutrons.

The three main challenges of the implosion design were: generating enough pressure to compress the plutonium, perfecting the timing of the detonators, and achieving a symmetrical implosion. 

To overcome these major challenges, Los Alamos scientists and engineers developed new innovations in the field of electronics. Three essential innovations for the implosion design’s success were Exploding Bridgewire Detonators, the Spark Gap Switch, and Composition B explosive lenses.

fission bomb

The Making of the Atomic Bomb

Topic 12: Year 1945: The “Dragon” experiments on critical mass. The death of President Roosevelt. The Trinity test. Harry Truman and Potsdam. The decision to use the bomb. Hiroshima and Nagasaki. Final perspectives on war in the 20-th century, nuclear proliferation, and the challenge of nuclear terrorism


Dragon Experiment on Critical Mass

One of the most important problem that had to be solved in Manhattan Project was problem of critical mass of
bomb fuel.
The most famous experiment conducted at Los Alamos during the Manhattan Project, after the Trinity test itself, is the one with the most evocative name. “Tickling the Dragon’s Tail,” also known internally as just “Dragon,” is straightforward about its meaning, compared to the enigma of “Trinity.” Dragons don’t like to have their tails tickled — so watch out for the fire..

Otto Frisch wanted to work with full critical mass to determine by experiment what Los Alamos had so far been able to determine only theoretically: how much uranium Little Boy would need. The idea of experiment was that the compound of uranium-235, which by then had arrived to Los Alamos in amount enough to make an explosive device, should be assembled to make one, but leaving a big hole so that the central portion was missing. This will allow enough neutron to escape so that no chain reaction could develop. But the missing portion was to be made, ready to be dropped through the hole so that for a split second there was the condition for an atomic explosion, although only barely so.
Richard Feynman, young physicist also member of the Manhattan Project named the experiment: he said it would be like tickling the tail of a sleeping dragon.

With the Manhattan Project on the brink of success in spring 1945, the atomic bomb became an increasingly important element in American strategy. A long hoped-for weapon now seemed within reach at a time when hard decisions were being made, not only on ending the war in the Pacific, but also on the shape of the postwar international order.

From Roosevelt to Truman

On April 12, only weeks before Germany's unconditional surrender on May 7, President Roosevelt died suddenly in Warm Springs, Georgia, bringing Vice President Harry S. Truman, a veteran of the United States Senate, to the presidency. Truman was not privy to many of the secret war efforts Roosevelt had undertaken and had to be briefed extensively in his first weeks in office. One of these briefings, provided by Secretary of War Stimson on April 25, concerned S-1 (the Manhattan Project). Stimson, with Groves present during part of the meeting, traced the history of the Manhattan Project, summarized its status, and detailed the timetable for testing and combat delivery. Truman asked numerous questions during the forty-five minute meeting and made it clear that he understood the relevance of the atomic bomb to upcoming diplomatic and military initiatives.

By the time Truman took office, Japan was near defeat. American aircraft were attacking Japanese cities at will. A single fire bomb raid in March killed nearly 100,000 people and injured over a million in Tokyo. A second air attack on Tokyo in May killed 83,000. Meanwhile, the United States Navy had cut the islands' supply lines. But because of the generally accepted view that the Japanese would fight to the bitter end, a costly invasion of the home islands seemed likely, though some American policy makers held that successful combat delivery of one or more atomic bombs might convince the Japanese that further resistance was futile.

The Trinity Test

No one doubted that Little Boy would work if any design would. Otto Frisch Dragon experiments had proven the efficiency of the fast-neutron chain reaction in uranium. The gun mechanism was wasteful and inefficient but U-235 was forgiving.

It remained to test implosion. While doing so the physicists could also compare their theory of the progress of such unusual release of energy. Trinity would be the largest physics experiment ever attempted up to that time.

Meanwhile, the test of the plutonium weapon, named Trinity by Oppenheimer (a name inspired by the poems of John Donne), was rescheduled for July 16 at a barren site on the Alamogordo Bombing Range known as the Jomada del Muerto, or Journey of Death, 210 miles south of Los Alamos. A test explosion had been conducted on May 7 with a small amount of fissionable material to check procedures and fine-tune equipment. Preparations continued through May and June and were complete by the beginning of July. Three observation bunkers located 10,000 yards north, west, and south of the firing tower at ground zero would attempt to measure critical aspects of the reaction. Specifically, scientists would try to determine the symmetry of the implosion and the amount of energy released. Additional measurements would be taken to determine damage estimates, and equipment would record the behavior of the fireball. The biggest concern was control of the radioactivity the test device would release. Not entirely content to trust favorable meteorological conditions to carry the radioactivity into the upper atmosphere, the Army stood ready to evacuate the people in surrounding areas.

Tower for the Trinity Test

Tower for the Trinity Test.

On July 12 the plutonium core was taken to the test area in an army sedan. The non-nuclear components left for the test site at 12:01 a.m., Friday the 13th. During the day on the 13th, final assembly of the gadget took place in the McDonald ranch house. By 5:00 p.m. on the 15th, the device had been assembled and hoisted atop the one-hundred foot firing tower. Groves, Bush, Conant, Lawrence, Farrell, Chadwick (head of the British contingent at Los Alamos and discoverer of the neutron), and others arrived in the test area, where it was pouring rain. Groves and Oppenheimer, standing at the S-10,000 control bunker, discussed what to do if the weather did not break in time for the scheduled 4:00 a.m. test. At 3:30 they pushed the time back to 5:30; at 4:00 the rain stopped. Physicist Kistiakowsky and his team armed the device shortly after 5:00 a.m. and retreated to S-10,000. In accordance with his policy that each observe from different locations in case of an accident, Groves left Oppenheimer and joined Bush and Conant at base camp. Those in shelters heard the countdown over the public address system, while observers at base camp picked it up on an FM radio signal. 

Trinity Device Being Readied

Trinity Device, the "Gadget" Being Readied.

The Trinity Test

The mushroom cloud from the Trinity test.

At precisely 5:30 a.m. on Monday, July 16, 1945, the atomic age began. While Manhattan staff members watched anxiously, the device exploded over the New Mexico desert, vaporizing the tower and turning asphalt around the base of the tower to green sand. The bomb released approximately 18.6 kilotons of power, and the New Mexico sky was suddenly brighter than many suns. Some observers suffered temporary blindness even though they looked at the brilliant light through smoked glass. Seconds after the explosion came a huge blast, sending searing heat across the desert and knocking some observers to the ground. A steel container weighing over 200 tons, standing a half-mile from ground zero, was knocked ajar. (Nicknamed Jumbo, the huge container had been ordered for the plutonium test and transported to the test site but eliminated during final planning). As the orange and yellow fireball stretched up and spread, a second column, narrower than the first, rose and flattened into a mushroom shape, thus providing the atomic age with a visual image that has become imprinted on the human consciousness as a symbol of power and awesome destruction. 

At base camp, Bush, Conant, and Groves shook hands. Oppenheimer reported later that the experience called to his mind the legend of Prometheus, punished by Zeus for giving man fire. He also thought fleetingly of Alfred Nobel's vain hope that dynamite would end wars. The terrifying destructive power of atomic weapons and the uses to which they might be put were to haunt many of the Manhattan Project scientists for the remainder of their lives. The success of the Trinity test meant that a second type of atomic bomb could be readied for use against Japan. In addition to the uranium gun model, which was not tested prior to being used in combat, the plutonium implosion device detonated at Trinity now figured in American Far Eastern strategy. In the end Little Boy, the untested uranium bomb, was dropped first at Hiroshima on August 6, 1945, while the plutonium weapon Fat Man followed three days later at Nagasaki on August 9.

The remains of the tower after the test

Remains of Trinity Test Tower Footings. Oppenheimer and Groves at Center.

Potsdam

The American contingent to the Big Three conference, headed by Truman, Byrnes, and Stimson, arrived in Berlin on July 15 and spent most of the next two days. Grappling with the interrelated issues of Russian participation in the Far Eastern conflict and the wording of an early surrender offer that might be presented to the Japanese. This draft surrender document received considerable attention, the sticking point being the term "unconditional." It was clear that the Japanese would fight on rather than accept terms that would eliminate the Imperial House or demean the warrior tradition, but American policy makers feared that anything less than a more democratic political system and total demilitarization might lead to Japanese aggression in the future. Much effort went into finding the precise formula that would satisfy American war killed the Pacific without requiring a costly invasion of the Japanese mainland. In an attempt to achieve surrender with honor, the emperor had instructed his ministers to open negotiations with Russia. The United States intercepted and decoded messages between Tokyo and Moscow that made it unmistakably clear that the Japanese were searching for an alternative to unconditional surrender.

Reports on Trinity

Stalin arrived in Berlin a day late, leaving Stimson July 16 to mull over questions of postwar German administration and the Far Eastern situation. After sending Truman and Byrnes a memorandum advocating an early warning to Japan and setting out a bargaining strategy for Russian entry in the Pacific war, Stimson received a cable from George L. Harrison, his special consultant in Washington, that read:” Operated on this morning. Diagnosis not yet complete but results seem satisfactory and already exceed expectations. Local press release necessary as interest extends great distance. Dr. Groves pleased. He returns tomorrow. I will keep you posted.”

Stimson immediately informed Truman and Byrnes that the Trinity test had been successful. The next day Stimson informed Churchill of the test. The prime minister expressed great delight and argued forcefully against informing the Russians, though he later relented. On July 18, while debate continued over the wording of the surrender message, focusing on whether or not to guarantee the place of the emperor, Stimson received a second cable from Harrison:

“Doctor has just returned most enthusiastic and confident that the little boy is as husky as his big brother. The light in his eyes discernible from here to Highhold and I could have heard his screams from here to my farm.”

Translation: “Groves thought the plutonium weapon would be as powerful as the uranium device and that the Trinity test could be seen as far away as 250 miles and the noise heard for fifty miles.” Initial measurements taken at the Alamogordo site suggested a yield in excess of 5,000 tons of TNT. Truman went back to the bargaining table with a new card in his hand.

Further information on the Trinity test arrived on July 21 in the form of a long and uncharacteristically excited report from Groves. Los Alamos scientists now agreed that the blast had been the equivalent of between 15,000 and 20,000 tons of TNT, higher than anyone had predicted. Groves reported that glass shattered 125 miles away, that the fireball was brighter than several suns at midday, and that the steel tower had been vaporized. Though he had previously believed it impregnable, Groves stated that he did not consider the Pentagon safe from atomic attack.53 Stimson informed Marshall and then read the entire report to Truman and Byrnes. Stimson recorded that Truman was "tremendously pepped up" and that the document gave him an entirely new feeling of confidence."54 The next day Stimson, informed that the uranium bomb would be ready in early August, discussed Grove's report at great length with Churchill. The British prime minister was elated and said that he now understood why Truman had been so forceful with Stalin the previous day, especially in his opposition to Russian designs on Eastern Europe and Germany. Churchill then told Truman that the bomb could lead to Japanese surrender without an invasion and eliminate the necessity for Russian military help. He recommended that the President continue to take a hard line with Stalin. Truman and his advisors shared Churchill's views.

The success of the Trinity test stiffened Truman's resolve, and he refused to accede to Stalin's new demands for concessions in Turkey and the Mediterranean. On July 24 Stimson met with Truman. He told the President that Marshall no longer saw any need for Russian help, and he briefed the President on the latest S-1 situation. The uranium bomb might be ready as early as August 1 and was a certainty by August 10. The plutonium weapon would be available by August 6. Stimson continued to favor making some sort of commitment to the Japanese emperor, though the draft already shown to the Chinese was silent on this issue. American and British coordination for an invasion of Japan continued, with November 1 standing as the landing date. At a meeting with American and British military strategists at Potsdam, the Russians reported that their troops were moving into the Far East and could enter the war in mid-August. They would drive the Japanese out of Manchuria and withdraw at the end of hostilities. Nothing was said about the bomb. This was left for Truman, who, on the evening of July 24, approached Stalin without an interpreter to inform the Generalissimo that the United States had a new and powerful weapon. Stalin casually responded that he hoped that it would be used against Japan to good effect. The reason for Stalin's composure became clear later when it was learned that Russian intelligence had been receiving information about the S-1 project from Klaus Fuchs and other agents since summer 1942.



The Potsdam Proclamation

A directive, written by Groves and issued by Stimson and Marshall on July 25, ordered the Army Air Force's 509th Composite Group to attack Hiroshima, Kokura, Niigata, or Nagasaki "after about" August 3, or as soon as weather permitted.55 The 509th was ready. Tests with dummies had been conducted successfully, and Operation Bronx, which brought the gun and uranium-235 projectile to Tinian aboard the U.S.S. Indianapolis and the other components on three C-54s, was complete. On July 26 the United States learned of Churchill's electoral defeat and Chiang Kai-Shek's concurrence in the warning to Japan. Within hours the warning was issued in the name of the President of the United States, the president of China, and the prime minister of Great Britain (now Clement Attlee). The Russians were not informed in advance. This procedure was technically correct since the Russians were not at war with Japan, but it was another indication of the new American attitude that the Soviet Union's aid in the present conflict no longer was needed. The message called for the Japanese to surrender unconditionally or face "prompt and utter destruction."56 The Potsdam Proclamation left the emperor's status unclear by making no reference to the royal house in the section that promised the Japanese that they could design their new government as long as it was peaceful and more democratic. While anti-war sentiment was growing in Japanese decision-making circles, it could not carry the day as long as unconditional surrender left the emperor's position in jeopardy. The Japanese rejected the offer on July 29.

Intercepted messages between Tokyo and Moscow revealed that the Japanese wanted to surrender but felt they could not accept the terms offered in the Potsdam Proclamation. American policy makers, however, anxious to end the war without committing American servicemen to an invasion of the Japanese homeland, were not inclined to undertake revisions of the unconditional surrender formula and cause further delay. A Russian declaration of war might convince Japan to surrender, but it carried a potentially prohibitive price tag as Stalin would expect to share in the postwar administration of Japan, a situation that would threaten American plans in the Far East. A blockade of Japan combined with conventional bombing was rejected as too time-consuming and an invasion of the islands as too costly. And few believed that a demonstration of the atomic bomb would convince the Japanese to give up. Primarily upon these grounds, American policy makers concluded that the atomic bomb must be used. Information that Hiroshima might be the only prime target city without American prisoners in the vicinity placed it first on the list. As the final touches were put on the message Truman would issue after the attack, word came that the first bomb could be dropped as early as August 1. With the end now in sight, poor weather led to several days.

What are Bomb Effects

Nuclear weapons are fundamentally different from conventional weapons because of the vast amounts of explosive energy they can release and the kinds of effects they produce, such as high temperatures and radiation. The prompt effects of a nuclear explosion and fallout are well known through data gathered from the attacks on Hiroshima and Nagasaki, Japan; from more than 500 atmospheric and more than 1,500 underground nuclear tests conducted worldwide; and from extensive calculations and computer modeling. Longer-term effects on human health and the environment are less certain but have been extensively studied. The impacts of a nuclear explosion depend on many factors, including the design of the weapon (fission or fusion) and its yield; whether the detonation takes place in the air (and at what altitude), on the surface, underground, or underwater; the meteorological and environmental conditions; and whether the target is urban, rural, or military.
When a nuclear weapon detonates, a fireball occurs with temperatures similar to those at the center of the Sun. The energy emitted takes several forms. Approximately 85 percent of the explosive energy produces air blast (and shock) and thermal radiation (heat). The remaining 15 percent is released as initial radiation, produced within the first minute or so, and residual (or delayed) radiation, emitted over a period of time, some of which can be in the form of local fallout.

Blast

The expansion of intensely hot gases at extremely high pressures in a nuclear fireball generates a shock wave that expands outward at high velocity. The “overpressure,” or crushing pressure, at the front of the shock wave can be measured in pascals (or kilopascals; kPa) or in pounds per square inch (psi). The greater the overpressure, the more likely that a given structure will be damaged by the sudden impact of the wave front. A related destructive effect comes from the “dynamic pressure,” or high-velocity wind, that accompanies the shock wave. An ordinary two-story, wood-frame house will collapse at an overpressure of 34.5 kPa (5 psi). A one-megaton weapon exploded at an altitude of 3,000 metres (10,000 feet) will generate overpressure of this magnitude out to 7 km (about 4 miles) from the point of detonation. The winds that follow will hurl a standing person against a wall with several times the force of gravity. Within 8 km (5 miles) few people in the open or in ordinary buildings will likely be able to survive such a blast. Enormous amounts of masonry, glass, wood, metal, and other debris created by the initial shock wave will fly at velocities above 160 km (100 miles) per hour, causing further destruction.

Thermal radiation

As a rule of thumb, approximately 35 percent of the total energy yield of an airburst is emitted as thermal radiation—light and heat capable of causing skin burns and eye injuries and starting fires of combustible material at considerable distances. The shock wave, arriving later, may spread fires further. If the individual fires are extensive enough, they can coalesce into a mass fire known as a firestorm, generating a single convective column of rising hot gases that sucks in fresh air from the periphery. The inward-rushing winds and the extremely high temperatures generated in a firestorm consume virtually everything combustible.

Initial radiation

A special feature of a nuclear explosion is the emission of nuclear radiation, which may be separated into initial radiation and residual radiation. Initial radiation, also known as prompt radiation, consists of gamma rays and neutrons produced within a minute of the detonation. Beta particles  and a small proportion of alpha particles  are also produced, but these particles have short ranges and typically will not reach Earth’s surface if the weapon is detonated high enough above ground. Gamma rays and neutrons can produce harmful effects in living organisms, a hazard that persists over considerable distances because of their ability to penetrate most structures. Though their energy is only about 3 percent of the total released in a nuclear explosion, they can cause a considerable proportion of the casualties.

Residual radiation and fallout

Residual radiation is defined as radiation emitted more than one minute after the detonation. If the fission explosion is an airburst, the residual radiation will come mainly from the weapon debris. If the explosion is on or near the surface, the soil, water, and other materials in the vicinity will be sucked upward by the rising cloud, causing early (local) and delayed (worldwide) fallout. Early fallout settles to the ground during the first 24 hours; it may contaminate large areas and be an immediate and extreme biological hazard. Delayed fallout, which arrives after the first day, consists of microscopic particles that are dispersed by prevailing winds and settle in low places.

Hiroshima

In the early morning hours of August 6, 1945, a B-29 bomber attached to the 590th Composite Group took off from Tinian Island and headed north by northwest toward the Japanese Islands over 1,500 miles away. Its primary target was Hiroshima, an important military and communications center with a population of nearly 300,000 located in the deltas of southwestern Honshu Island facing the Inland Sea. The Enola Gay, piloted by Colonel Paul Tibbets, flew at low altitude on automatic pilot before climbing to 31,000 feet as it neared the target area. As the observation and photography escorts dropped back, the Enola Gay released a 9,700-pound uranium bomb, nicknamed Little Boy, at approximately 8:15 a.m. Hiroshima time. Tibbets immediately dove away to avoid the anticipated shockwaves of the blast. Forty-three seconds later a huge explosion lit the morning sky as Little Boy detonated 1900 feet above the city, directly over a parade field where the Japanese Second Army was doing calisthenics. Though already eleven and a half miles away, the Enola Gay was rocked by the blast. At first Tibbets thought he was taking flak. After a second shockwave hit the Diane, the crew looked back at Hiroshima. "The city was hidden by that awful cloud . . .boiling up, mushrooming, terrible and incredibly tall," Tibbets recalled.57 Little Boy killed 70,000 people (including about twenty American airmen being held as POWs) and injured another 70,000. By the end of 1945, the Hiroshima death toll rose to 140,000 as radiation sickness deaths mounted. Five years later the total reached 200,000. The bomb caused total devastation for five square miles, with almost all the buildings in the city either destroyed or damaged.

.Reading the Little Boy Bomb

Reading the Little Boy Bomb.

Within hours of the attack, radio stations began reading a prepared statement from President Harry Truman informing the American public that the United States had dropped an entirely new type of bomb on the Japanese city of Hiroshima-an atomic bomb with more power than 15,000 tons of TNT. Truman warned that if Japan still refused to surrender unconditionally as demanded by the Potsdam Proclamation of July 26, the United States would attack additional targets with equally devastating results. Two days later, on August 8, the Soviet Union declared war on Japan and attacked Japanese forces in Manchuria, ending American hopes that the war would
end before Russians entry into Pacific theater.

..The Atomic Bomb Dome - Hiroshima - Ground Zero

Hiroshima after the bombing

"Little Boy" Atomic Bomb

  • Type: Nuclear weapon

  • Nation: United States

  • Designer: Los Alamos Laboratory

  • Length: 10 feet

  • Weight: 9,700 pounds

  • Diameter: 28 inches

  • Filling: Uranium-235

  • Yield: 15 kilotons of TNT

It contained 64 kg (141 lb) of enriched uranium, although less than a kilogram underwent nuclear fission.

Nagasaki

Fat Man Being Readied.

Fat Man Plutonium Bomb Being Readied at Tinian.

Factional struggles and communications problems prevented Japan from meeting Allied terms in the immediate aftermath of Hiroshima. In the absence of a surrender announcement, conventional bombing raids on additional Japanese cities continued as scheduled. Then, on August 9, a second atomic attack took place. Taking off from Tinian at 3:47 a.m., Bock's Car (named after its usual pilot) headed for its primary target, Kokura Arsenal, located on the northern coast of Kyushu Island. Pilot Charles Sweeney found unacceptable weather conditions and unwelcome flak above Kokura. Sweeney made three passes over Kokura, then decided to switch to his secondary target even though he had only enough fuel remaining for a single bombing run. Clouds greeted Bock's Car as it approached Nagasaki, home to the Mitsubishi plant that had manufactured the torpedoes used at Pearl Harbor. At the last minute, a brief break in the cloud cover made possible a visual targeting at 29,000 feet and Bock's Car dropped her single payload, a plutonium bomb weighing 10,000 pounds and nicknamed Fat Man, at 11:01 a.m. The plane then veered off and headed to Okinawa for an emergency landing. Fat Man exploded 1,650 feet above the slopes of the city with a force of 21,000 tons of TNT.59 Fat Man killed 40,000 people and injured 60,000 more. Three square miles of the city were destroyed, less than Hiroshima because of the steep hills surrounding Nagasaki. By January 1946, 70,000 people had died in Nagasaki. The total eventually reached 140,000, with a death rate similar to that of Hiroshima. 

The mushroom cloud rising over Nagasaki.

The mushroom cloud rising over Nagasaki.

Fat Man Atomic Bomb

  • Weight: 10,800 lbs.

  • Length: 10 ft. 8 in.; Diameter: 60 in.

  • Fuel:  Highly enriched plutonium- 239

  • Plutonium Fuel: approx. 13.6 lbs.; approx. size of a softball

  • Plutonium core surrounded by 5,300 lbs. of high explosives; plutonium core reduced to size of tennis ball

  • Efficiency of weapon: 10 times that of Little Boy

  • Approximately 1 kilogram of plutonium fissioned

  • Explosive force: 21,000 tons of TNT equivalent

  • Use:  Dropped on Japanese city of Nagasaki; August 9, 1945

  • Type: Nuclear Weapon

Surrender

Still the Japanese leadership struggled to come to a decision, with military extremists continuing to advocate a policy of resistance to the end. Word finally reached Washington from Switzerland and Sweden early on August 10 that the Japanese, in accordance with Hirohito's wishes, would accept the surrender terms, provided the emperor retain his position. Truman held up a third atomic attack while the United States considered a response, finally taking a middle course and acknowledging the emperor by stating that his authority after the surrender would be exercised under the authority of the Supreme Commander of the Allied Powers. With British, Chinese, and Russian concurrence, the United States answered the Japanese on August 11. Japan surrendered on August 14, 1945, ending the war that began for the United States with the surprise attack at Pearl Harbor on December 7, 1941. The United States had been celebrating for almost three weeks when the formal papers were signed aboard the U.S.S. Missouri on September 2.

The Bomb Goes Public

The veil of secrecy that had hidden the atomic bomb project was lifted on August 6 when President Truman announced the Hiroshima raid to the American people. The release of the Smyth Report on August 12, which contained general technical information calculated to satisfy public curiosity without disclosing any atomic secrets, brought the Manhattan Project into fuller view. Americans were astounded to learn of the existence of a far flung, government-run, top secret operation with a physical plant, payroll, and labor force comparable in size to the American automobile industry. Approximately 130,000 people were employed by the project at its peak, among them many of the nation's leading scientists and engineers.

In retrospect, it is remarkable that the atomic bomb was built in time to be used in World War II. Most of the theoretical breakthroughs in nuclear physics dated back less than twenty-five years, and with new findings occurring faster than they could be absorbed by practitioners in the field, many fundamental concepts in nuclear physics and chemistry had yet to be confined by laboratory experimentation. Nor was there any conception initially of the design and engineering difficulties that would be involved in translating what was known theoretically into working devices capable of releasing the enormous energy of the atomic nucleus in a predictable fashion. In fact, the Manhattan Project was as much a triumph of engineering as of science. Without the innovative work of the talented Leslie Groves, as well as that of Crawford Greenewalt of DuPont and others, the revolutionary breakthroughs in nuclear science achieved by Enrico Fermi, Niels Bohr, Ernest Lawrence, and their colleagues would not have produced the atomic bomb during World War II. Despite numerous obstacles, the United States was able to combine the forces of science, government, military, and industry into an organization that took nuclear physics from the laboratory and into battle with a weapon of awesome destructive capability, making clear the importance of basic scientific research to national defense.

Nuclear Proliferation

The use of nuclear weapons at the end of World War II served as the starting point for an ongoing era of nuclear proliferation. Stockpiles rapidly grew as the United States and Soviet Union became embroiled in the Cold War, and rapid scientific advancement led to the creation of far more powerful weapons. Today, nuclear nonproliferation stands among the most pressing issues facing the international community.

Manhattan Project

Even prior to the United States' use of atomic weapons, the Soviet Union's atomic project was already underway. Using spies recruited at Manhattan Project sites - famous among which are Klaus Fuchs, George Koval, and Ted Hall, among others - the Soviet Union was able to accelerate their nuclear program by several years.

Cold War Vertical Proliferation

Following the war, the countries of the newly-founded United Nations needed to address the future of nuclear weapons. The United States proposed the Baruch Plan, calling for disarmament, a ban on production of nuclear weapons, the open exchange of scientific information required to use nuclear energy to achieve peaceful ends, and the use of nuclear power for peaceful purposes. The Soviet Union, citing issues of trust should they consent to such a plan under the western-dominated United Nations, rejected the idea.

As no firm agreement could be reached, the US and USSR plunged into a decades-long nuclear arms race. Partially as a result of successful espionage, the Soviets were quick to develop their first atom bomb. Access to uranium, which was much more common than previously thought, was initially achieved through mining in Eastern Bloc satellite states. Defying all expectations, the Soviet Union was able to detonate its first atomic bomb on August 29, 1949.

As neither country intended to cease nuclear development following the construction of the atom bomb, the United States and Soviet Union were in a race to create the world’s first hydrogen bomb – a weapon far more powerful than that dropped on Hiroshima or Nagasaki. Having had a head start, Edward Teller's team, sanctioned by the newly formed United States Department of Defense, and tested their first thermonuclear weapon on November 1, 1952. The following August, the Soviets followed suit with their own H-bomb, RDS-6 (referred to as Joe 4 by the US).

A replica of Sputnik 1 at the U.S. National Air and Space Museum

Despite the continuous technological developments surrounding nuclear armaments, the lack of a long-range delivery mechanism dampened the deterrent effect of said weapons. Originally, the only way to drop a nuclear bomb was through the use of strategic bombers. This changed with the production of intercontinental ballistic missiles (ICBM). To display advancement of ICBM technology, the US and USSR became intertwined in the Space Race. Although the official goal was to prove superiority in space exploration, the delivery mechanism for satellites was fundamentally the same as that for thermonuclear warheads. Thus, whichever state could successfully put a device in orbit would prove capable of reaching the other with a mounted warhead. The Soviet Union was the first to showcase the capability for long-range nuclear warfare when it launched the Sputnik satellite into orbit in 1957. The United States was slow to counter, taking two years to launch its first satellite.

At this time, both states had the capacity to obliterate each other’s major population centers. Although the United States and Soviet Union continued to improve and expand their arsenals, a stalemate of sorts had been reached. Mutually Assured Destruction (MAD) ensured, theoretically, that neither state would conduct a nuclear strike on the other, as nuclear retaliation would prove inevitable.

This concept, known as deterrence, did not guarantee peaceful interaction, as evidenced by the Cuban Missile Crisis. Even though both sides were armed with nuclear weapon-equipped ICBMs, the presence of Soviet missiles in Cuba almost led to the breakout of nuclear war.

Cold War Horizontal Proliferation

During and after the Cold War, several countries developed nuclear weapons programs.

HMS Victorious

TIMELINE OF FIRST NUCLEAR TESTS

16 July 1945, United States: Trinity Test, Alamogordo, NM, Plutonium Implosion Device (20 KT)

29 August 1949 Soviet Union: “Joe-1” aka First Lightning, Semipalatinsk, Kazakhstan, Plutonium Implosion Device (22 KT)

3 October 1952 United Kingdom: Operation Hurricane, Montebello Islands, Western Australia, Plutonium Implosion Device (25 KT)

13 February 1960 France: “Gerboise Bleue” or Blue Jerboa, Reggane, Algerian Sahara Desert, Plutonium Device (70KT)

16 October 1964 People’s Republic of China: “596,” Lop Nur, Xinjiang Province in North Western China, Uranium Implosion Device (22 KT)

18 May 1974 India: “Smiling Buddha” aka Pokhran I, Peaceful Nuclear Explosion (PNE), Pokhran in the Western Deserts of Rajasthan, Plutonium Implosion Device (8 KT)

28 May 1998 Pakistan: Chagai I, Balochistan Province in Southwest Pakistan, series of 5 tests using uranium implosion devices (maximum yield 40 KT)

9 October 2006 North Korea: Mt. Punggye-ri in the North Hamgyong Province, Plutonium Implosion Device (less than 1 KT)


OTHER NUCLEAR PROGRAMS

Israel: Israel maintains the position that it will not formally introduce nuclear weapons to the Middle East by testing a device. However, its nuclear reactor in the Negev desert named Dimona, constructed in early 1958, is believed to have gone critical shortly after the US discovered the facility in the early 1960s. By the end of the decade Israel most likely had assembled a bomb.

South Africa: South Africa pursued a nuclear weapons capability from the 1960s-1970s. In August 1977 the Soviet Union picked up intelligence that the country was preparing for a cold test of a nuclear device at Vastrap in the Kalahari Desert. The United States and its European allies successfully convinced the country to refrain from conducting the test. Subsequently, the country moved its nuclear program to an underground facility at Pelindaba. Later in September 1979, a United States Vela satellite detected a double flash, a common feature of a nuclear detonation, off of the country’s coast. While many officials and historians suspect that the incident was an actual test potentially involving Israel, evidence is inconclusive and the event remains controversial to this day. However, by the time Nelson Mandela became President in 1994, following the end of apartheid, South Africa had voluntarily dismantled its nuclear weapons.

PROLIFERATION COUNTRY-BY-COUNTRY

United Kingdom: Initially, the British were barred access to United States nuclear design data. The threat of espionage necessitated heightened security surrounding the American nuclear program. Regardless, the United Kingdom was able to detonate its first atom bomb on October 3, 1952, thanks to the expertise of British Manhattan Project scientists. When nuclear cooperation with the United States resumed in 1958, the British deployed submarine-based American Polaris missiles equipped with nuclear warheads. Having such weapons stationed off the coast of the small island guaranteed a deterrent effect.

France: In the late 1940s, the French were successfully operating their own nuclear reactors. It was not until the late 1950s, however, that France decided to accelerate its nuclear program to achieve nuclear deterrence. On February 13, 1960, the French detonated their first atomic bomb in Algeria.

People's Republic of China: In the 1950s, the Chinese gained the support of the Soviet Union in developing their nuclear program. Despite the ideological Sino-Soviet split in the latter part of that decade, the Chinese managed to detonate their first atomic device on October 16, 1964.

Israel: Although Israel's nuclear program is shrouded in ambiguity, it is widely believed that Israel is in possession of nuclear weapons. Illegally obtained evidence provided by a former Israeli nuclear technician, Mordechai Vanunu, illustrates that the state has nuclear capabilities. Development of this program likely began shortly after World War II. At the time, the French were in collaboration with the Israelis and provided them with a research reactor in the 1950s.

India: India became the next country to detonate its first nuclear device. Work on a nuclear weapon commenced in the 1960s, but was not considered a priority. Following the Indo-Pakistani war of 1971, however, the Indian government accelerated work on this project. In 1972, authorization was granted for the production and testing of a nuclear device. The "Smiling Buddha" was detonated on May 18, 1974.

Pakistan: In response to the nuclear threat emanating from bordering India, Pakistan began work on its nuclear program in 1972. Twenty-six years later, Pakistan tested its first five nuclear weapons on May 28, 1998, as a response to India's Pokhran II test earlier that month, making it the eighth nuclear state.

Other countries: Throughout the Cold War, several states had possession of nuclear weapons, but destroyed or surrendered their stockpiles. Belarus, Kazakhstan, and Ukraine agreed to get rid of their nuclear weapons following the collapse of the Soviet Union. The South African apartheid-era government eliminated their stockpile, worried that the weapons could fall into the hands of militants.

Non-Proliferation

Einstein-Russel notice.

Many view the proliferation of nuclear weapons as a danger to the international community. North Korea and Iran are the current epicenters of concern regarding the issue of horizontal proliferation. There are similarly fears that non-state actors - especially terrorist groups - may acquire these weapons. India and Pakistan have been on the cusp of nuclear war, showing that the logic of mutually assured destruction may not be as sound as previously thought. Thus, a number of steps toward nonproliferation have been taken.

The Pugwash Conference, organized by Joseph Rotblat, was the first major, non-governmental effort to curb the proliferation of nuclear weapons. This conference was the result of the Russell-Einstein Manifesto, and initially involved twenty-two scientists from countries on both sides of the Cold War divide. Over the years, Pugwash convened with the goal of reducing production and stockpiles of nuclear weapons. Rotblat, along with the Pugwash Conferences, was awarded the 1995 Nobel Peace Prize for these efforts.

The most crucial treaty dealing with the issue of nuclear weapons proliferation, the Nuclear Non-Proliferation Treaty (NPT), came into force on March 5, 1970. Noting that the trend of horizontal proliferation would inevitably continue, the United States and Soviet Union collaborated to prevent such an outcome. This treaty identified the United States, Soviet Union, United Kingdom, France, and China as nuclear weapons states. 

Appendices 1-10

Appendix 1: Basics of Electricity

Electricity is the set of physical phenomena associated with the presence and flow of electric charge.

Basic concept for electrical properties of matter is concept of electrical charge. It is said that two elementary particles, electrons and protons, out of three (electrons, protons and neutrons) that composed an atom, are carriers of electrical charge. Electrons carry negative charges and protons carry positive charges. It was found out that there is an interaction between the charges: like charges attract and unlike charges repel. The electrical interaction between charged objects is due to the presence of electric field (force) that surrounds any charged object – it spreads out - weakens with distance.

Charge is measured in Coulombs.
1 Coulomb =1 amp per second.
The charge of one electron is e= -1.6 X 10-19 Coulombs
1 𝐶𝑜𝑢𝑙𝑜𝑚𝑏 = 1/ −1.6 ∗ 10−19 = 6.25 ∗ 1018 𝐸𝑙𝑒𝑐𝑡𝑟𝑜ns.

When it comes to electricity there are generally two types of material: Conductors and Insulators.
A conductor is a material that has a large number of free electrons that continually jump to other atoms. Good electrical conductors are copper and aluminum. Gold, silver, and platinum are also good conductors, but are very expensive.
An insulator is a material that has only a few free electrons. In insulators, the electrons are tightly bound by the nucleus. Good electrical insulators are rubber, porcelain, glass, and dry wood.
Insulators prevent current from flowing.
Characteristic that allows to distinguish between conductors and insulators is called ,resistance, R . Obviously conductors have low resistance and insulators high resistance.

Current ,I, is the movement of charge through a conductor. Electrons carry the charge. Unit of measurement: Amperes (A). One ampere (amp) of current is one coulomb of charge passing a point on a conductor in one second. This measurement is analogous to “gallons or liters per second” when measuring the flow of water.
Direct Current (DC) flows in only one direction. Many uses including: Batteries, electronic circuits, LED lights, generator excitation systems and rotors, DC transmission lines – and much more.
Alternating Current (AC) continuously changes in magnitude and direction. AC is used by most lights, appliances and motors. It is used in the high voltage transmission system. AC enables use of transformers to change voltage from high to low and back.

Voltage ,V, is the force that causes electrons to move. Voltage is also referred to as potential difference or electromotive force (emf or E). Unit of measurement: Volts (V) . Similar to “pounds per square inch” when measuring water pressure.

Sample Voltage Levels

AA Battery1.5V
Car Battery12V
Household120V
Distribution Feeder Circuit12.47 kV
High Voltage Line47kV-500kV
Lightning1,000,000+ Volts

Power,P, is the rate at which work is being performed. Unit of Measurement: Watts (W) .
Power = Voltage x Current.
This means that the electrical energy is being converted into another form of energy (e.g. heat energy, light energy, mechanical energy, etc.)

Appendix 2: Range of the Electromagnetic Spectrum

The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The electromagnetic spectrum of an object has a different meaning: it is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object.


The wavelengths of various regions of the electromagnetic spectrum are shown alongside an approximate proxy for size of the wavelength.

The electromagnetic spectrum extends from below the low frequencies used for modern radio communication to gamma radiation at the short-wavelength (high-frequency) end, covering wavelengths from thousands of kilometers down to a fraction of the size of an atom. The limit for long wavelengths is the size of the universe itself, while it is thought that the short wavelength limit is in the vicinity of the Planck length (1.6 x 10-35 m), although in principle the spectrum is infinite and continuous.
Most parts of the electromagnetic spectrum are used in science for spectroscopic and other probing interactions, as ways to study and characterize matter. In general, if the wavelength of electromagnetic radiation is of a similar size to that of a particular object (atom, electron, etc.), then it is possible to probe that object with that frequency of light. In addition, radiation from various parts of the spectrum has been found to have many other uses in communications and manufacturing.

Energy of Photon
Electromagnetic waves are typically described by any of the following three physical properties: the frequency (f) (also sometimes represented by the Greek letter nu, ν), wavelength (λ), or photon energy (E). Frequencies observed in astronomy range from 2.4×1023 Hz (1 GeV gamma rays) down to the local plasma frequency of the ionized interstellar medium (~1 kHz). Wavelength is inversely proportional to wave frequency; hence, gamma rays have very short wavelengths that are a fraction of the size of atoms, whereas other wavelengths can be as long as the universe. Photon energy is directly proportional to the wave frequency, so gamma ray photons have the highest energy (around a billion electron volts), while radio wave photons have very low energy (around a femto-electron volt). These relations are illustrated by the following equations:
f= c/λ, or f= E/h, or E=hc/λ,
where c = 299,792,458 m/s is the speed of light in vacuum, h = 6.62x 10−34 J s - Planck’s constant.

Whenever electromagnetic waves exist in a medium with matter, their wavelength is decreased. Wavelengths of electromagnetic radiation, no matter what medium they are traveling through, are usually quoted in terms of the vacuum wavelength, although this is not always explicitly stated. Generally, electromagnetic radiation is classified by wavelength into radio wave, microwave, terahertz (or sub-millimeter) radiation, infrared, the visible region we perceive as light, ultraviolet, X-rays, and gamma rays. The behavior of electromagnetic radiation depends on its wavelength. When electromagnetic radiation interacts with single atoms and molecules, its behavior also depends on the amount of energy per quantum (photon) it carries.

Interaction of Electromagnetic Radiation with Matter
Electromagnetic radiation interacts with matter in different ways in different parts of the spectrum. The types of interaction can be so different that it seems justified to refer to different types of radiation. At the same time, there is a continuum containing all these different kinds of electromagnetic radiation. Thus, we refer to a spectrum, but divide it up based on the different interactions with matter. Below are the regions of the spectrum and their main interactions with matter:

  • Radio: Collective oscillation of charge carriers in bulk material (plasma oscillation). An example would be the oscillation of the electrons in an antenna.

  • Microwave through far infrared: Plasma oscillation, molecular rotation.

  • Near infrared: Molecular vibration, plasma oscillation (in metals only).

  • Visible: Molecular electron excitation (including pigment molecules found in the human retina), plasma oscillations (in metals only).

  • Ultraviolet: Excitation of molecular and atomic valence electrons, including ejection of the electrons (photoelectric effect).

  • X-rays: Excitation and ejection of core atomic electrons, Compton scattering (for low atomic numbers).

  • Gamma rays: Energetic ejection of core electrons in heavy elements, Compton scattering (for all atomic numbers), excitation of atomic nuclei, including dissociation of nuclei.

  • High-energy gamma rays: Creation of particle-antiparticle pairs. At very high energies, a single photon can create a shower of high-energy particles and antiparticles upon interaction with matter.

This classification goes in the increasing order of frequency and decreasing order of wavelength, which is characteristic of the type of radiation. While, in general, the classification scheme is accurate, in reality there is often some overlap between neighboring types of electromagnetic energy. For example, SLF radio waves at 60 Hz may be received and studied by astronomers, or may be ducted along wires as electric power, although the latter is, in the strict sense, not electromagnetic radiation at all.

Appendix 3: Units of Energy

https://home.uni-leipzig.de/energy/energy-fundamentals/img/03_joule.jpg

James Prescott Joule (1818 − 1889) was a self-educated British physicist
and brewer whose work in the midnineteenth century contributed to the establishment of the energy concept. The international unit of energy bears
his name:

1 Joule [J] = 1 Watt-second [Ws] = 1 V A s = 1 N m = 1 kg m2s−2.

It takes about 1 J to raise a 100-g-apple 1 m. Energy units can be preceded
by various factors, including the following:

kilo (k=103), Mega (M=106), Giga (G=109), Tera (T=1012), Peta (P=1015),
Exa (E=1018).

Thus, a kiloJoule (kJ) is 1000 Joules and a MegaJoule (MJ) is 1,000,000 Joules.

A related unit is the Watt, which is a unit of power (energy per unit time). Power units can be converted to energy units through multiplication by seconds [s], hours, [h], or years [yr].

For example, 1 kWh [kilowatt hour] = 3.6 MJ [MegaJoule]. With 1 kWh, about 10 liters of water can be heated from 20 ºC to the boiling point.

There are many other energy units besides the "Système International d'Unités (SI)". A "ton of coal equivalent" (tce) is frequently used in the energy business. 1 tce equals 8.141 MWh. It means that the combustion of 1 kg of coal produces the same amount of heat as electrical heating for one hour at a rate of 8.141 kW.

More Units of Energy

1 calIT = 4.1868 J, International Table calorie
1 calth = 4.184 J, thermochemical calorie
1 cal15 ≈ 4.1855 J, calories to heat from 14.5 °C to 15.5 °C
1 erg = 10−7 J, cgs [centimeter-gram-second] unit
1 eV ≈ 1.60218 × 10−19 J, electron volt
1 Eh ≈ 4.35975 × 10−18 J, Hartree, atomic energy unit
1 Btu = 1055.06 J, British thermal unit according to ISO, to heat 1 pound water from 63 °F to 64 °F
1 tce = 29.3076 × 109 J, ton of coal equivalent, 7000 kcalIT
1 toe = 41.868 × 109 J, ton of oil equivalent, 10000 kcalIT

Calories and/or kilocalories [cal and/or kcal] were historically often used to measure heat (energy) and are still used fot this sometimes today. Heating a gram of water 1 ºC requires 1 cal. Different definitions are often the result of inconsistent starting temperatures of the heating.

Multiplication Table of Units
SymbolExponentialPrefixQuantity
k103kilothousand
M106Megamillion
G109Gigabillion
T1012Teratrillion
P1015Petquadrillion
E1018Exaquintillion

The unit Megagram is not used, since there is a special
name for one million grams, one ton (t): 1 t = 1000 kg.

Multiplication of the Units of Power with Units of Time

When the Watt is multiplied by a unit of time, an energy unit is formed as follows: 1 Ws = 1 J.
The use of the kilowatt-hour is more common: 1 kWh = 3600 kWs = 3.6 MJ.
Besides the second [s] and the hour [h], the day [d] and the year [yr] are also used,
with 1 yr = 365.2425 d = 31,556,952 s.
So, for example, energy of one Megawatt-year can be written as 1 MWyr = 31.557952 TJ (TeraJoule).
The annual consumption of 1 toe/yr corresponds to the daily consumption of 31.557952 kWh/d.

Appendix 4: Energy

In physics, energy is the quantitative property that must be transferred to an object in order to perform work on, or to heat, the object. Energy is a conserved quantity; the law of conservation of energy states that energy can be converted in form, but not created or destroyed. The SI unit of energy is the joule, which is the energy transferred to an object by the work of moving it a distance of 1 meter against a force of 1 newton.

Common forms of energy include the kinetic energy of a moving object, the potential energy stored by an object's position in a force field (gravitational, electric or magnetic), the elastic energy stored by stretching solid objects, the chemical energy released when a fuel burns, the radiant energy carried by light, and the thermal energy due to an object's temperature.

Mass and energy are closely related. Due to mass–energy equivalence, any object that has mass when stationary (called rest mass) also has an equivalent amount of energy whose form is called rest energy, and any additional energy (of any form) acquired by the object above that rest energy will increase the object's total mass just as it increases its total energy. For example, after heating an object, its increase in energy could be measured as a small increase in mass, with a sensitive enough scale.

Living organisms require energy to stay alive, such as the energy humans get from food. Human civilization requires energy to function, which it gets from energy resources such as fossil fuels, nuclear fuel, or renewable energy. The processes of Earth's climate and ecosystem are driven by the radiant energy Earth receives from the sun and the geothermal energy contained within the earth.

All forms of energy are associated with motion. For example, any given body has kinetic energy if it is in motion. For estimation of kinetic energy of the moving object formula
KE=1/2(mv2), where m is mass and v is speed of the object, can be used.

A tensioned device such as a bow or spring, though at rest, has the potential for creating motion; it contains potential energy because of its configuration. Similarly, nuclear energy is potential energy because it results from the configuration of subatomic particles in the nucleus of an atom

Appendix 5: Max Plank and the idea of quantum

In the late 18th century, great progress in physics had been made. Classical Newtonian physics at the time was widely accepted in the scientific community for its ability to accurately explain and predict many phenomena. However, by the early 20th century, physicists discovered that the laws of classical mechanics are not applicable at the atomic scale, and experiments such as the photoelectric effect completely contradicted the laws of classical physics. As a result of these observations, physicists articulated a set of theories now known as quantum mechanics. In some ways, quantum mechanics completely changed the way physicists viewed the universe, and it also marked the end of the idea of a clockwork universe (the idea that universe was predictable).

Electromagnetic radiation
Electromagnetic (EM) radiation is a form of energy with both wave-like and particle-like properties; visible light being a well-known example. From the wave perspective, all forms of EM radiation may be described in terms of their wavelength and frequency. Wavelength is the distance from one wave peak to the next, which can be measured in meters. Frequency is the number of waves that pass by a given point each second. While the wavelength and frequency of EM radiation may vary, its speed in a vacuum remains constant at 3.0 x 108 m/sec, the speed of light. The wavelength or frequency of any specific occurrence of EM radiation determine its position on the electromagnetic spectrum and can be calculated from the following equation:
c= λυ

where c is the constant 3.0 x 108 m/sec (the speed of light in a vacuum), λ- wavelength in meters, and υ -frequency in hertz (1/s). It is important to note that by using this equation, one can determine the wavelength of light from a given frequency and vice versa..

The Discovery of the Quantum
The wave model cannot account for something known as the photoelectric effect. This effect is observed when light focused on certain metals emits electrons. For each metal, there is a minimum threshold frequency of EM radiation at which the effect will occur. Replacement of light with twice the intensity and half the frequency will not produce the same outcome, contrary to what would be expected if light acted strictly as a wave. In that case, the effect of light would be cumulative—the light should add up, little by little, until it caused electrons to be emitted. Instead, there is a clear-cut minimum frequency of light that triggers electron ejection. The implication was that frequency is directly proportional to energy, with the higher light frequencies having more energy. This observation led to the discovery of the minimum amount of energy that could be gained or lost by an atom. Max Planck named this minimum amount the “quantum,” plural “quanta,” meaning “how much.” One photon of light carries exactly one quantum of energy. Planck is considered the father of the Quantum Theory. According to Planck:
E =hυ,
where h is Planck’s constant = (6.62 x 10-34 J s), υ is the frequency, and E is energy of an electromagnetic wave
.
Planck (cautiously) insisted that this was simply an aspect of the processes of absorption and emission of radiation and had nothing to do with the physical reality of the radiation itself. However, in 1905, Albert Einstein reinterpreted Planck’s quantum hypothesis and used it to explain the photoelectric effect, in which shining light on certain materials can eject electrons from the material.

More Evidence for a Particle Theory of Energy


When an electric current is passed through a gas, some of the electrons in the gas molecules move from their ground energy state to an excited state that is further away from their nuclei. When the electrons return to the ground state, they emit energy of various wavelengths. A prism can be used to separate the wavelengths, making them easy to identify. If light acted only as a wave, then there should be a continuous rainbow created by the prism. Instead, there are discrete lines created by different wavelengths. This is because electrons release specific wavelengths of light when moving from an excited state to the ground state.

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Wavelength of EM radiation
The distance used to determine the wavelength is shown. Light has many properties associated with its wave nature, and the wavelength in part determines these properties.

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Emission spectrum of nitrogen gas
Each wavelength of light emitted (each colored line) corresponds to a transition of an electron from one energy level to another, releasing a quantum of light with defined energy (color).

Appendix 6: Periodic Table of the Elements

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Appendix 7: Metric (SI) Prefixes

Metric Units of Measurement

In the metric system of measurement, designations of multiples and subdivision of any unit may be arrived at by combining with the name of the unit the prefixes deka, hecto, and kilo meaning, respectively, 10, 100, and 1000, and deci, centi, and milli, meaning, respectively, one-tenth, one-hundredth, and one-thousandth. In some of the following metric tables, some such multiple and subdivisions have not been included for the reason that these have little, if any currency in actual usage.

In certain cases, particularly in scientific usage, it becomes convenient to provide for multiples larger than 1000 and for subdivisions smaller than one-thousandth. Accordingly, the following prefixes have been introduced and these are now generally recognized.

PurposePrefix NamePrefix SymbolValue
larger quantities
or whole units
yottaY1024Septillion
zettaZ1021Sextillion
exaE1018Quintillion
petaP1015Quadrillion
teraT1012Trillion
gigaG109Billion
megaM106Million
kilok103Thousand
hectoh102Hundred
dekada101Ten
100One
smaller quantities
or sub units

decid10-1Tenth
centic10-2Hundredth
millim10-3Thousandth
microμ10-6Millionth
nanon10-9Billionth
picop10-12Trillionth
femtof10-15Quadrillionth
attoa10-18Quintillionth
zeptoz10-21Sextillionth
yoctoy10-24Septillionth

.’

 Whole UnitsDecimal Units
thousandshundredstensbasic unittenthshundredths thousandths
10001001010.10.010.001
kilo-hecto-deka-meter
gram
liter
deci-centi-milli

Appendix 8: Scientific method

Scientists search for answers to questions and solutions to problems by using a procedure called the scientific method. This procedure consists of making observations, formulating hypotheses, and designing experiments, which in turn lead to additional observations, hypotheses, and experiments in repeated cycles.

 The Steps in the Scientific Method.

Step 1: Make observations

Observations can be qualitative or quantitative. Qualitative observations describe properties or occurrences in ways that do not rely on numbers. Examples of qualitative observations include the following: the outside air temperature is cooler during the winter season, table salt is a crystalline solid, sulfur crystals are yellow, and dissolving a penny in dilute nitric acid forms a blue solution and a brown gas. Quantitative observations are measurements, which by definition consist of both a number and a unit. Examples of quantitative observations include the following: the melting point of crystalline sulfur is 115.21° Celsius, and 35.9 grams of table salt—whose chemical name is sodium chloride—dissolve in 100 grams of water at 20° Celsius. For the question of the dinosaurs’ extinction, the initial observation was quantitative: iridium concentrations in sediments dating to 66 million years ago were 20–160 times higher than normal.

Step 2: Formulate a hypothesis

After deciding to learn more about an observation or a set of observations, scientists generally begin an investigation by forming a hypothesis, a tentative explanation for the observation(s). The hypothesis may not be correct, but it puts the scientist’s understanding of the system being studied into a form that can be tested. For example, the observation that we experience alternating periods of light and darkness corresponding to observed movements of the sun, moon, clouds, and shadows is consistent with either of two hypotheses:

  1. Earth rotates on its axis every 24 hours, alternately exposing one side to the sun, or

  2. the sun revolves around Earth every 24 hours.

Suitable experiments can be designed to choose between these two alternatives. For the disappearance of the dinosaurs, the hypothesis was that the impact of a large extraterrestrial object caused their extinction. Unfortunately (or perhaps fortunately), this hypothesis does not lend itself to direct testing by any obvious experiment, but scientists can collect additional data that either support or refute it.

Step 3: Design and perform experiments

After a hypothesis has been formed, scientists conduct experiments to test its validity. Experiments are systematic observations or measurements, preferably made under controlled conditions—that is, under conditions in which a single variable changes.

Step 4: Accept or modify the hypothesis

A properly designed and executed experiment enables a scientist to determine whether the original hypothesis is valid. In which case he can proceed to step 5. In other cases, experiments often demonstrate that the hypothesis is incorrect or that it must be modified thus requiring further experimentation.

Step 5: Development into a law and/or theory

More experimental data are then collected and analyzed, at which point a scientist may begin to think that the results are sufficiently reproducible (i.e., dependable) to merit being summarized in a law, a verbal or mathematical description of a phenomenon that allows for general predictions. A law simply says what happens; it does not address the question of why.

One example of a law, the law of definite proportions, which was discovered by the French scientist Joseph Proust (1754–1826), states that a chemical substance always contains the same proportions of elements by mass. Thus, sodium chloride (table salt) always contains the same proportion by mass of sodium to chlorine, in this case 39.34% sodium and 60.66% chlorine by mass, and sucrose (table sugar) is always 42.11% carbon, 6.48% hydrogen, and 51.41% oxygen by mass.

Whereas a law states only what happens, a theory attempts to explain why nature behaves as it does. Laws are unlikely to change greatly over time unless a major experimental error is discovered. In contrast, a theory, by definition, is incomplete and imperfect, evolving with time to explain new facts as they are discovered.

Because scientists can enter the cycle shown in Figure at any point, the actual application of the scientific method to different topics can take many different forms. For example, a scientist may start with a hypothesis formed by reading about work done by others in the

Summary

The scientific method is a method of investigation involving experimentation and observation to acquire new knowledge, solve problems, and answer questions. The key steps in the scientific method include the following:

  • Step 1: Make observations.

  • Step 2: Formulate a hypothesis.

  • Step 3: Test the hypothesis through experimentation.

  • Step 4: Accept or modify the hypothesis .

  • Step 5: Development into a law and/or a theory

Appendix 9: International System of Units

The SI system, also called the metric system, is used around the world. There are seven basic units in the SI system: the meter (m), the kilogram (kg), the second (s), the kelvin (K), the ampere (A), the mole (mol), and the candela (cd).

Related image

This

Appendix 10: Introduction to Wave Motion

Wave motion arises when a periodic disturbance of some kind is propagated through a medium. Pressure variations through air, transverse motions along a guitar string, or variations in the intensities of the local electric and magnetic fields in space, which constitute electromagnetic radiation, are all typical examples of wave motion. For each medium, there is a characteristic velocity at which the disturbance travels.

https://s3-us-west-2.amazonaws.com/courses-images/wp-content/uploads/sites/752/2016/09/26194415/v3920lmfqiwlvu8zge8o.pngSinusoidal wave
This image shows the anatomy of a sine curve: the crest is the peak of each wave, and the trough is the valley; the amplitude is the distance between the crest and the x-axis; and the wavelength is the distance between two crests (or two troughs).

There are three measurable properties of wave motion: amplitude, wavelength, and frequency (the number of vibrations per second). The relation between the wavelength λ (Greek lambda) and frequency of a wave ν (Greek nu) is determined by the propagation velocity v, such that
v= λ/υ

For light, this equation becomes c= λ/υ,
where c is the speed of light, 2.998 x 108 m/s.
When utilizing these equations to determine wavelength, frequency, or velocity by manipulation of the equation, it is important to note that wavelengths are expressed in units of length, such as meters, centimeters, nanometers, etc; and frequency is typically expressed as megahertz or hertz (s–1).

Young’s Double-Slit Experiment


In the early 19th century, English scientist Thomas Young carried out the famous double-slit experiment (also known as Young’s experiment), which demonstrated that a beam of light, when split into two beams and then recombined, will show interference effects that can only be explained by assuming that light is a wavelike disturbance. If light consisted strictly of ordinary or classical particles, and these particles were fired in a straight line through a slit and allowed to strike a screen on the other side, we would expect to see a pattern corresponding to the size and shape of the slit. However, when this single-slit experiment is actually performed, the pattern on the screen is a diffraction pattern in which the light is spread out. The smaller the slit, the greater the angle of spread. If light were purely a particle, it would not exhibit the interference.
Similarly, if light consisted strictly of classical particles and we illuminated two parallel slits, the expected pattern on the screen would simply be the sum of the two single-slit patterns. In actuality, however, the pattern changes to one with a series of alternating light and dark bands. When Thomas Young first demonstrated this phenomenon, it indicated that light consists of waves, as the distribution of brightness can be explained by the alternately additive and subtractive interference of wave fronts. Young’s experiment, performed in the early 1800’s, played a vital part in the acceptance of the wave theory of light, superseding the corpuscular theory of light proposed by Isaac Newton, which had been the accepted model of light propagation in the 17th and 18th centuries. Almost a century later, in 1905, Albert Einstein’s Nobel-Prize winning research into the photoelectric effect demonstrated that light can behave as if it is composed of discrete particles under certain conditions. These seemingly contradictory discoveries made it necessary to go beyond classical physics and take the quantum nature of light into account.

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