recent discussion about the necessity and morality of dropping the atomic bomb on Japan, this may be a good time to publish my notes on Richard Rhodes’
. After I finished reading the book in 2001, I realized it contained such a wealth of fascinating and important information that I needed to have my own summary of it for quick reference. So I went through the book a second time taking extensive notes. The result is below.
Discoveries in Twentieth Century Nuclear Physics
Leading up to Atomic Bomb
Auster’s notes on The Making of the Atomic Bomb, by Richard Rhodes
This is basically a chronology and description of the basic discoveries, cumula-tively leading, step by step, to the possibility of making an atomic bomb. The number and types of different things that had to be true for the bomb to be possible—things that no one could have imagined beforehand and yet that all turned out to be true over the course of the 1930s—is astounding. It’s one of the most interesting books I ever read, though marred by the author’s periodic intrusions of liberal globaloney derived from Neils Bohr—a giant in physics but a fuzzy thinker in politics.
1894 Wilhelm Röntgen discovers X-rays by holding his hand to a beam emanating from fluorescing gases in cathode-ray tube.
1896 Henri Becquerel in Paris discovers that uranium emits radiation that creates image on photographic film.
1897 J.J. Thompson at Cambridge discovers the electron. The stream passing through a cathode ray vacuum tube consists of particles that (1) are much lighter than hydrogen atoms and (2) are the same regardless of the gas used to generate them. Therefore these particles must be a constituent of the atom, common to all atoms. He envisioned the atom as a pudding with electrons embedded evenly in it. This was the beginning of the modern understand-ing of the atom.
1898 Marie and Pierre Curie discover radium. Marie Curie coins term radioactivity.
1998-1910 Ernest Rutherford (1871-1937) at McGill and Cambridge discovers that uranium emits alpha and beta rays. With Frederick Soddy he discovers radioactive decay after noticing inert gases emitted from thorium and uranium, understanding that radioactivity is a result of the decay of the atom. Then they discover half-life, the time it takes the intensity of radiation to decline in half, which corresponds with the decay of a radioactive element to some other element. (It’s useful to remember those distinctions: first they discovered the rays, then they discovered that this was associated with a chemical breakdown of the element itself).
1905 Albert Einstein writes Special Theory of Relativity including E=MC2.
1911 Rutherford proposes solar system-like structure of atom, with a dense compact nucleus (the nucleus being one ten-thousandth the size of the entire atom) and electrons circling around it as planets revolve around the sun, and the atom consisting mostly of empty space. His discovery of the nucleus is the result of an experiment in which he fires alpha particles (helium nuclei consisting of two protons) into gold foil at an angle. Most of the alpha particles passed right through as expected, but some bounced off and were picked up by a scintillation screen.
He said afterward: “It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.” Such bouncing could only be explained by the positively charged alpha particle hitting a positively charged mass.
However, Rhodes, as usual, does not clearly explain why the bouncing of the particles off the nucleus would lead Rutherford to the idea that atom was mostly empty space; wouldn’t it lead to the opposite conclusion? I had to look this up on the Web to get a better explanation. The “plum pudding” model of the atom up to that time was that the atom was uniform, with electrons evenly distributed; apparently they did not yet know of the positively charged aspect of the atom, nor of a massive center of the atom. (Rutherford discovered the proton in 1919.) So when some alpha particles bounced off (though most were passing through, which Rhodes does not mention), that led to the idea of a massive, positively charged nucleus that some of the alpha particles were hitting and bouncing off of. [Note: as Paul Nachman said in response to a later question, the pudding model said that matter was positively charged throughout, with electrons scattered in this positively charged environment. Instead what he found was that the positive charge was compact.]
1913 Neils Bohr (1885-1962) figures out structure of electron shells. Rutherford’s model was impossible in terms of known laws. Since the electrons having the same electrical charge would repel each other, there seemed to be no way they could be stable in their orbits. Also, in giving up energy they should fall out of their orbits. James Clerk Maxwell had predicted that an electron revolving around a nucleus will continuously radiate electromagnetic energy until it has lost all its energy, and eventually will fall into the nucleus. Yet atoms were the most stable things there were. Around this time Bohr shows up at Rutherford’s lab and applies himself to this problem. He combines the quantum (discovered by Max Planck and then applied by Einstein to light to discover the photon) to the atom. Electrons have discrete levels of energy. With each new higher energy level, they jump to a higher orbit around the nucleus. With lower level, they jump to lower level. But within these levels or shells, they are stable. The way I put this to myself is that the laws governing the electron shells supercede the laws governing electric repulsion between the electrons.
Also, there is no in-between state; nothing can be known about the electrons as they move from one shell to another. Bohr said there was no way to predict what level or when the electron would change, that the electron had a kind of “free will.” This was strangely in keeping with Bohr’s earlier obsessions with his multiple “I’s” involved in his own thinking processes, his preoccupation with how thought endlessly reflects on itself, the impossibility of untangling how a thought occurs. This was a major obsession of his during his student years. (My feeling on going back and reading this stuff about his earlier life, which I had earlier skipped over, was that Bohr was destined to make the discoveries he made, in which the non-determinacy of the electron exactly reflected his life-long obsession with the non-determinacy of his own thought-process; that in the field of physics he was a man of destiny, in the same way that in the world of politics Washington and Churchill were men of destiny.)
Rhodes writes: “The parallels between his early psychological concerns and his interpretations of atomic processes are uncanny, so much so that without the great predictive ability of the paper its assumptions would seem totally arbitrary…. To identify a kind of freedom of choice within the atom itself was a triumph for his carefully assembled structure of beliefs…. as Kierkegaard’s stages are discontinuous, negotiable only by leaps of faith, so do Bohr’s electrons leap discontinuously from orbit to orbit. Bohr insisted as one of the two “principal assumptions” of his paper that the electron’s whereabouts between orbits cannot be calculated or even visualized. Before and after are completely discontinuous.”
1919 Rutherford discovers the proton. He fires alpha particles at a variety of gases including nitrogen, and gets hydrogen nuclei. Realizes that the hydrogen nuclei, which he later names protons, are being knocked free from the nitrogen atom, changing the nitrogen to an oxygen isotope. This is first instance of artificial transmutation of an element, and it also opens a way to explore the atom by firing protons at it, but this it very limited because the larger the atom gets, the more its positively charged nucleus repels any proton.
February 1932 James Chadwick at Cambridge discovers the neutron after years of thinking there was a neutron but failing to turn it up. Alpha rays fired at berylium released some kind of radiation which was then directed at hydrogenous substances such as paraffin knocking protons loose. It had been thought the radiation from the berylium had been photons, but they have same mass as electrons, which are something like a thousand times lighter than a proton, so they could not dislodge protons. The particle had to be equal in mass to a proton yet lacking an electric charge, thus allowing it to get close to the target nucleus and knock the proton free. “More than any other development, Chadwick’s neutron made practical the detailed examination of the nucleus. Hans Bethe once remarked that he considered everything before 1932 ‘the prehistory of nuclear physics, and from 1932 on the history of nuclear physics.’ The difference, he said, was the the discovery of the neutron.”
February 1932 Ernest Lawrence’s cyclotron begins operation at Berkeley.
September, 1933 Leo Szilard, a Hungarian Jewish refugee recently arrived in London, thinking about the neutron discovery and reading a newspaper article in which Rutherford at a physics conference said, in the words of the newspaper article, that anyone who looked for a source of power in the transformation of the atoms was talking “moonshine.” “This sort of set me pondering as I was walking on the streets of London, and I remember that I stopped for a red light at the intersection of Southampton Row…. I was pondering whether Lord Rutherford might not prove to be wrong.” “Szilard was not the first to realize that the neutron might slip past the positive electrical barrier of the nucleus; that realization had come to other physicists as well. But he was the first to imagine a mechanism whereby more energy might be released in the neutron’s bombardment of the nucleus than the neutron itself supplied.” “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 absorbs 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.”
Question: where could Szilard have gotten this notion, since no nucleus had ever been split releasing energy and no one (at least apart from Szilard) had imagined that such a thing was possible until it actually happened five years later? The discovery of the neutron by itself does not necessarily lead to the conclusion that nucleus can be split. Correction: it does lead to it. Alpha particles had previously been used for test bombardment of substances to discover their properties. But because alpha particles, which are positively charged, are repelled by the positively charged nucleus, they cannot be used to bombard and split the nucleus. The neutron, which has no charge, and thus is not repelled by the positively charged nucleus, can get into the nucleus and split it. So the discovery of the neutron made the splitting of the nucleus possible.
1933 Frédéric and Irène Joliot-Curie, the daughter and son-in-law of Marie Curie, discover that alpha rays can induce artificial radioactivity in various substances. Joliot: “I irradiate this target [aluminum] with alpha rays from my source; you can hear the Geiger counter crackling. I remove the source: the crackling ought to stop, but in fact it continues.” The strange activity declined to half its initial intensity in about three minutes. This indeed was the half-life of the artificial radioactivity. Marie Curie, who was then dying of cancer, was delighted with their discovery, “Marie Curie saw our research work and I will never forget the expression of intense joy which came over her when Irène and I showed her the first artificially radioactive element in a little glass tube…. This was doubtless the last great satisfaction of her life. ” The Joliot-Curies win the 1935 Nobel prize for this discovery, which serves as the basis for all the later work particularly Fermi’s in bombarding substances with particles
1933 Enrico Fermi in Rome, bombarding various elements with neutrons, finds that silver radiated on a wooden table gained much more activity than when it was irradiated on a marble table in the same room. The answer to this puzzle, he discovers, is that that hydrogen-rich substances such as paraffin, wood or water slow the neutrons, and that slow neutrons induce more radioactivity than fast neutrons because they are more likely to be captured by a nucleus and split that nucleus.
“I will tell you how I came to make the discovery which I suppose is the most important one I have made.” He comes to lab one day, planning to place a piece of lead before the radiation source. “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 do not 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.” This led to dramatic increase in the intensity of the activation. Laura Fermi says “the halls of the physics building resounded with loud exclamations: ‘Fantastic! Incredible! Black magic!’”
Fermi’s explanation of this: the neutrons were colliding with the hydrogen nuclei in the paraffin and the wood, which slowed them down. Slowing down the neutron actually gave it more time in the vicinity of the nucleus, allowing it to be captured by the nucleus and to split the nucleus. The reason is that hydrogen nuclei, being one proton, have the same weight as a neutron, therefore they bounce the most when hit by a neutron and absorb the most energy, thus slowing the neutron.
December 1938 Otto Hahn and Fritz Strassmann at Kaiser Wilhelm Institute, after working for years firing neutrons at uranium, realize that neutrons fired into uranium do not just produce an isotope or transuranic element by adding a neutron to the atom, but that they split the uranium nucleus creating elements much lower in the periodic table. There had been signs of this for a long time, but everyone had rejected it as impossible.
December 1938 Lise Meitner, an Austrian baptized Jew who had worked at the KWI with Hahn and had fled to Denmark and then Sweden after the Anschluss, and her nephew Otto Frisch, a refugee living in Copenhagen, on a cross-country skiing vacation in Sweden, sitting outdoors on a log underneath a historic Swedish castle, figure out the mechanics of the uranium fission that Hahn and Strassman have discovered and calculate the energy released by the splitting of a single uranium atom at 200 million electron volts. This is a small amount of energy in itself (Frisch described at as enough energy to make a visible grain of sand visibly jump), but for a single atom it is enormous.
January 1939 In experiment confirming Hahn and Strassmann’s experiment, Frisch fires neutrons at uranium and gets high-energy ionization bursts indicating that nucleus is splitting and releasing the expected amount of energy.
January 1939 Neils Bohr, visiting Princeton, brings news of these discoveries to the U.S., creating a sensation in physics community. Then he goes to Columbia to tell Fermi. Finding Fermi’s office empty, “Bohr took the elevator to the basement, to the cyclotron area, where he turned up [graduate student] Herbert Anderson:”
He came right over and grabbed me by the shoulder. Bohr doesn’t lecture you, he whispers in your ear. “Young man,” he said, “let me explain to you about something new and exciting in physics.” Then he told me about the splitting of the uranium nucleus and how naturally this fits in with the idea of the liquid drop. I was quite enchanted. Here was the great man himself, impressive in his bulk, sharing his excitement with me as if it were of the utmost importance for me to know what he had to say.Szilard realizes that beyond the energy created by the splitting, the fragments carry extra neutrons, making chain reaction possible (which he had first conceived of in 1933, but had given up on). Now once again Szilard is in the forefront. He wrote:
Wigner told me of Hahn’s discovery. Hahn found that uranium breaks into two parts when it absorbs a neutron… When I heard this I immediately saw that these fragments, being heavier than corresponds to their charge, must emit neutrons, and if enough neutrons are emitted … then it should be, of course, possible to sustain a chain reaction. All the things which H.G. Wells predicted appeared suddenly real to me. Szilard wrote to his patron Lewis Strauss:
I feel I ought to let you know of a very sensational new development in nuclear physics. In a paper … Hahn reports that he finds when bombarding uranium with neutrons the uranium breaking up…. This is entirely unexpected and exciting news for the average physicist. The Department of Physics at Princeton, where I spent the last few days, was like a stirred-up ant heap…. These might lead to large-scale production of energy and radioactive elements, unfortunately also perhaps to atomic bombs. This new discovery revives all the hopes and fears in this respect which I had in 1934 and 1935, and which I have as good as abandoned in the course of the last two years.March 1939 Shortly after this Szilard experiments at Columbia with Walter Zinn firing neutrons at uranium oxide that show neutrons being emitted in the fission process. Thus Hahn, Strassman, Frisch and Meitner had shown that the atom splits and releases energy. Szilard showed that this same split also releases neutrons.
Everything was ready and all we had to do was to turn a switch, lean back, and watch the screen of a television tube. If flashes of light appeared on the screen, that would mean that neutrons were emitted in the fission process of uranium and this in turn would mean that the large-scale liberation of atomic energy was just around the corner. We turned the switch and saw the flashes. We watched them for a little while and then we switched everything off and went home. They figure the number of neutrons emitted per fission to be about two. After this discovery, Wigner strongly appeals to Szilard that it is time to tell the U.S. Government about their discoveries. It was “such a serious business that we could not assume responsibility for handling it.” But the early contacts, including Fermi visiting Navy people, don’t lead to anything. (Later Alexander Sachs brings Einstein’s letter, instigated by Szilard, to FDR. But while FDR orders the government to deal with this, nothing much happens until after Frisch and Peierls calculate fast-neutron potential of U235 in early 1940 and the British government gets involved. In other words, the U.S. government doesn’t get seriously involved until all the pieces are in place that make a bomb possible, and that was not the case until after Frisch and Peierl’s discovery.)
February 1939 Bohr at Princeton in a burst of insight realizes that slow neutron fission in uranium is happening only in the uranium 235, not in the uranium 238. Six million electron volts are needed to split a uranium atom. When a neutron is captured by a nucleus, that adds 5.3 MeV. But U235, unlike U238 and thorium, gets a further boost of energy when acquiring an additional neutron because when an atom goes from an odd atomic number (235) to an even number (236) that boosts its energy by about 1 MeV which doesn’t happen when it goes from an even to an odd number. This additional energy boost means that a slow, low-energy neutron captured by a U235 atom will result in the atom splitting, whereas U238 will only split with fast neutrons (since the energy of the faster neutrons provides the needed energy for the fission).
Everyone at this point is focussed on the slow neutron fission because that is where U235 is so different from U238. No one yet thinks, what would happen if fast neutrons were fired at U235. One reason they don’t think of this is that they still assume that isotope separation is an impossibility. [expand on this and check my above figures].
Summer 1939 After working together for a while at Columbia on reactor problem, Fermi and Szilard during their summer vacations independently come up with the idea of using carbon in the form of graphite as moderator for reactor. Hydrogen is no good as a moderator because while slowing the neutrons it captures too many of them preventing them from hitting the target element.
February-March 1940 Otto Frisch and Rudolf Peierls working at University of Birmingham, for the first time focus on possibility of fast neutrons operating on U235, a combination that no one had thought of before. They assume that any neutron hitting a U235 nucleus is going to split the atom, so they simply apply the odds, or “cross-section,” of a neutron hitting a nucleus (which was an order of magnitude larger than the fission cross section for natural uranium) and make that into the cross section for the U235 fission itself. “Just sort of playfully,” Frisch writes, he plugged 10-23 cm2 into Peierls’ formula. “To my amazement [the answer] was very much smaller than I had expected; it was not a matter of tons, but something like a pound or two.” But would this be a bomb? They calculated that 80 neutron generations, taking a total of 320 millionths of a second (at four millionths of a second per generation), which would still be fast enough to precede the expansion of the bomb material and the disassembly of the critical mass, “gave temperatures as hot as the interior of the sun, pressures greater than the center of the earth where iron flows as a liquid.”
Or, in the final generation you would have:
2 ^80 = 1,208,926,000,000,000,000,000,000.00 or a trillion trillion (?) times the energy it takes to make a grain of sand jump.
(However, Encarta gives a much smaller figure for the amount of time necessary for fission explosion: “Regardless of the method used to attain a supercritical assembly, the chain reaction proceeds for about a millionth of a second, liberating vast amounts of heat energy.”)
As for separating U25 from natural uranium, “we came to the conclusion that with something like a hundred thousand similar separation tubes one might produce a pound of reasonably pure unanium-235 in a modest time, measured in weeks. At that point we stared at each other and realized that an atomic bomb might after all be possible.” This made the bomb for the first time a practical possibility. Up to this point it had been believed that critical mass would be a vast amount, too much for a practical weapon.
So, three discoveries here: First they arrived at the small critical mass of practical size for a bomb; then they figured that sufficient atom splitting would occur for an explosion before the material dispersed; then they arrived at a feasible (though as it turned out, far too optimistic) time frame for separating U235.
And this is culmination of other earlier discoveries. First, Hahn and Strassmann, Frisch and Meitner established that fission exists. Then Szilard and others established that fission releases neutrons and thus creates the possibility of a chain reaction. Then Frisch and Peierls establish that the critical mass of U235 is small enough for a bomb.
February 1940 Alfred Nier at the University of Minnesota separates a microscopic amount of U235, mails the sample pasted on the margin of a letter to John Dunning at Columbia, who bombards it with neutrons, which shows that U-235 is responsible for the slow neutron fission of uranium.
1940 Edwin McMillan and Philip Abelson at Berkeley find that fissioning uranium, in addition to the fission fragments that flew away from the point of impact, also left two radioactivities in the uranium itself. Since they don’t recoil away along with the fission fragments, this shows that they are not created by fission; rather they are created by capturing a neutron. One has a half-life of 23 minutes, the other of 2.3 days. The first one is uranium isotope 239 created through the additional neutron. The second, which is created by the beta decay of the first, is a new element with atomic number 93, ultimately named neptunium. Its half life is too brief for it to be useful, but it is a step on the way to the discovery of plutonium.
Once again Rhodes puts Szilard at the cutting edge: “Szilard saw beyond what Turner had seen. He saw that a fissile element bred in uranium could be chemically separated away: that the relatively easy and relatively inexpensive process of chemical separation could replace the horrendously difficult and expensive process of physical separation of isotopes as a way to a bomb. But unstable element 93, neptunium, was not yet that fissile element … “
During a walk along Hudson near Fermi’s New Jersey home, Segré and Fermi talk about a possible element 94 that might be a slow neutron fissioner. If this proved to be true, it could substitute for U235 as a nuclear explosive. “Furthermore, a nuclear reactor fueled with ordinary uranium would produce [the new element].” Seaborg at Berkeley begins working on identifying and isolating element 94 using neptunium. The new element should be an alpha-emitter like uranium. Seaborg’s team identified an alpha-emitting daughter of Np238 on January 20 (1941?). But to prove it was a new element they had to chemically separate it. Seaborb then wrote: “With this final separation from thorium, it has been demonstrated that our alpha activity can be separated from all known elements and thus it is now clear that our alpha activity is due to the new element with the atomic number 94.” They isolated a small amount of it and then made their first tests for its fissionability. On March 28 they fired neutrons at it and got strong indications of fission.
1940-41 Sir Henry Tizard, head of British wartime scientific research, sets up the “Maud” committee to investigate possibilities arising from Frisch and Peierl’s report on their calculations. In July 1941 their report saying a bomb with 25 pounds active material could explode with 1.8 kilotons. They said this was practicable, could lead to decisive results in the war, and should be given highest priority.
July 1941 Vannevar Bush, electrical engineer, former administrator at MIT and head of Carnegie Institute, who in Summer 1940 had persuaded FDR to let him start the National Defense Research Council to coordinate all scientific research for the war effort with Bush as head and reporting directly to the President, hears of the Maud report, urges serious government investment in uranium fission.
October 1941 Roosevelt gives Bush authority to spend whatever is necessary to find if an atomic bomb can be built. Godfrey Hodgson writes in The Colonel, The Life and Wars of Henry Stimson: “[T]he decision to build not only a bomb but the vast secret bureaucracy that would be required to create it, was taken by the President alone.” Congress, judiciary, and cabinet knew nothing about it. This was one of the decisive moments when the imperatives of world war tilted power toward the executive.
November 6, 1941 Stimson’s diary has its first mention of the bomb: “Vannevar Bush came in to convey to me an extremely secret statement from the Scientific Research and Development Office.” And then he added: “a most terrible thing.”
Early 1942 Arthur Compton at meeting at Columbia lays down this timetable: By July 1, 1942, determine whether a chain reaction was possible. By January 1943, to achieve the first chain reaction. By January 1944, to extract the first element 94 from uranium. By January 1945, to have a bomb.
November 1942 Fermi’s team commences construction of pile in Chicago, with two teams each working a 12 hour shift, one of them led by Herbert Anderson. It consists of 771,000 pounds of graphite bricks, 80,590 pounds of uranium oxide and 12,4000 pounds of uranium metal. Its only visible moving parts were the various control rods. This pile had no practical purpose beyond the physics experiment to prove the chain reaction, and Fermi intended to run it no hotter than half a watt. On December 2, 1942, it’s ready. It’s below zero outside. That morning the State Department had announced that two million Jews had perished in Europe. Fighting was going in at Guadalcanal.
When Fermi has the last cadmium rod withdrawn the pile attains criticality, the clicks of the neutron counter become so continuous that the counters are turned off, replaced by a chart recorder. Herbert Anderson wrote afterward: “But when the switch was made, everyone watched in the sudden silence the mounting deflection of the recorder’s pen. It was an awesome silence. Everyone realized the significance of that switch; we were in the high intensity regime and the counters were unable to cope with the situation anymore.” Fermi runs it for 4 1/2 minutes at one-half watt and then turns it off. Arthur Compton in Chicago telephones James Briant Conant in D.C.: “‘Jim,’ I said, ‘You’ll be interested to know that the Italian navigator has just landed in the new world.’” Szilard, watching from the stands, is sad. “There was a crowd there and then Fermi and I stayed there alone. I shook hands with Fermi and I said I thought this day would go down as a black day in the history of mankind.”
1940-42 Tremendous organizational challenge of coordinating military with civilian scientists and government. Bush’s idea, with Harvey Bundy, was to create a “board of directors” with military and civilian members supervising a new “chief executive” from Army Corps of Engineers, Leslie R. Groves, who would lead both the military and civilian people. If the new committee was like a corporate board of directors, and Groves was the chief executive, then Stimson was in effect the Chairman of the board. In Hollywood terms, Groves was the director, Bush was the producer, and Stimson was the executive producer. Stimson did not run it, but got involved when a problem came up or they needed protection for S-1 on Capitol Hill. Stimson had final choice of key personnel including Groves.
July 1945 Two nights before the Trinity test (July 16, 1945), Oppenheimer recites to Vannevar Bush his own translation from the Gita (or the Upanishads?):
In battle, in forest, at the precipice in the mountains,
Short Summary of Principal Discoveries Leading to Bomb
On the dark great sea, in the midst of javelins and arrows,
In sleep, in confusion, in the depths of shame,
The good deeds a man has done before defend him.
1932: Chadwick discovers the neutron.
1933: Szilard has the idea that if a neutron fired into a nucleus released two neutrons, that could produce a chain reaction making a bomb possible.
1933: The Joliot-Curies artificially induce radioactivity by bombarding subtances with alpha particles.
1933: Fermi, doing the same thing with neutrons, discovers that hydrogen-rich substances slow the neutrons, and that these slow neutrons induce more radioactivity than fast neutrons.
December 1938: Hahn and Strassmann realize that neutrons fired into uranium split the uranium nucleus. Meitner and Frisch calculate the energy released by a single uranium atom splitting at 200 million electron volts.
January 1939: Frisch fires neutrons at uranium and gets high-energy ionization bursts indicating that nucleus is splitting and releasing the expected amount of energy.
January-March 1939: Szilard realizes that beyond the energy created by the splitting, the fragments carry extra neutrons, making chain reaction possible. Szilard and Zinn fire neutrons at uranium oxide and find neutrons being emitted in the fission process. Thus Hahn, Strassman, Frisch and Meitner had shown that the atom splits and releases energy. Szilard showed that this same split also releases neutrons.
February 1939: Bohr realizes that slow neutron fission in uranium is happening only in the uranium 235, not in the uranium 238.
Summer 1939: After working together at Columbia on reactor problem, Fermi and Szilard independently come up with idea of using carbon in form of graphite as moderator for reactor.
February-March 1940: Frisch and Peierls focus on fast neutrons operating on U235, calculate that the critical mass of U235 is not a matter of tons, as had been thought, but a pound or two. Then they figure that sufficient atom splitting would occur to reach millions of degrees before the material dispersed. Then they arrive at a feasible time frame for separating U235. These discoveries make the bomb practically feasible.
1940-41: Sir Henry Tizard, head of British scientific research, sets up the Maud committee to investigate possibilities arising from Frisch and Peierl’s report on their calculations. In July 1941 their report saying a bomb with 25 pounds active material could explode with 1.8 kilotons.
July 1941: Vannevar Bush in Washington hears of the Maud report, urges serious government investment in uranium fission.
October 1941: Roosevelt gives Bush authority to spend whatever is necessary to find if an atomic bomb can be built.
December 2, 1942: Fermi gets self-sustained chain reaction going at U. of Chicago.
July 16, 1945: Plutonium implosion bomb is exploded in New Mexico.
Partial list of Figures who made major discoveries:
Marie and Pierre Curie
Irène and Frédéric Joliot-Curie
Note on Jewish refugees
Several of the major figures were Jews or half-Jews, people who had no Jewish identity but who suddenly found themselves targeted as Jews. Hans Bethe, whose mother was Jewish but whose father was Prussian, was Protestant and had German features and never thought of himself as Jewish, yet he suddenly found himself fired from his university position in early 1933. Lisa Meitner had been baptized in infancy and thought of herself as a Protestant with Jewish ancestors. In 1933 she had to tell her colleague Otto Hahn that she was Jewish. Otto Frisch shocks his colleague when he tells him he’s Jewish.