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For information on American research and development on fission after January 1942, see the page for the Manhattan Project and predecessor organizations.
When nuclear fission was discovered near ht eend of 1938, it was a totally unanticipated phenomenon. It had been known for decades that an enormous energy was bound up in the atomic nucleus, but there were no clear paths toward gaining experimental, let alone practical, access to that energy. However, the nucleus had already been under intense study throughout the 1930s, as physicist and chemists catalogued the various radioactive elements and their behaviors, came to understand the sources of stability of the nuclei of various isotopes, and transmuted elements by bombarding them with the newly-discovered neutron and with alpha particles (helium nuclei). Once the intial discovery had been made, the basic outline of the fission process was quickly established, and it did not take long to develop a substantial body of experimental measurement and theory surrounding it.
As an astonishing new development in the physics of the nucleus, fission garnered widespread attention, which was further augmented by the implication that fission might be exploited to design new weapons and new sources of power. However, the unprecedented application-oriented research program that developed a few years later can only be understood by taking into account the political context: the 1933 rise to power of Adolf Hitler and his Nazi Party in Germany, and the consequent rise of German militarism and anti-Semitic persecution. Much of the research would be done by scientists who had fled fascist Europe, and the funding, institutional support, and access to personnel that made that research possible was only acquired out of the fear that the Nazis might develop their own fission-based weapon.
All early research in fission took place in independently-directed, penuriously-funded research programs at universities and research institutes. Through the initiativ eof small groups of scientists, this research was gradually integrated into coordinated state research, development, and industrial production mechanisms, which were themselves gradually growing in size and sophistication in anticipation of the coming war. This topic guide covers the early period of fission research, when application still appeared doubtful or distant to many. It ended semi-arbitrarily with the consolidation of slow-neutron chain reaction research at the University of Chicago in January 1942. Some research, notably work on isotope separation at Columbia University, continued uniterrupted through this time.
The most complete information on early nuclear fission research can be found in the thorough journalistic account, Richard Rhodes, The Making of the Atomic Bomb (New York: Simon & Schuster, 1986). In anticipation of later events, Rhodes' book sometimes takes a jaundiced view of the pace of support for fission research, and concentrates heavily o nsome actors while neglecting others.
Rhodes' work can be supplemented with the official history, Richard G. Hewlett and Oscar E. Anderson, Jr., A History of the United States Atomic Energy Commssion, Volume I: The New World, 1939-1946 (University Park, PA: Pennsylvania State University Press, 1962). On the British project, see its official history, Margaret Gowing, Britain and Atomic Energy, 1939-1945 (London: Macmillan & Co Ltd., 1964). On early fission research in France, see Spencer R. Weart, Scientists in Power (Cambridge, Mass.: Harvard University Press, 1979).
Working at the Kaiser Wilhelm Institute for Chemistry in Berlin, nuclear chemists Otto Hahn and Fritz Strassmann find that when uranium nuclei are bombarded with neutrons, they seem to transmute into much lighter elements rather than nearby elements, as would be the case with most nuclei. They send the results on to their physicist colleague Lise Meitner, who was in Sweden having escaped Nazi persecution. She and her visiting nephew, Copenhagen physicist Otto Frisch, quickly develop a physical model of the nucleus-splitting, or "fission", process and begin measuring it. Initial results are published in January 1939. Meitner and Frisch's interpretation appears in Nature in February.
Online resource: Read Meitners and Frisch's paper at atomicarchive.com.
Online resource: Visit an online exhibit at the AIP Center for History of Physics on the discovery, prehistory, and early news of fission, with audio clips of some of the participants.
On the 16th, Niels Bohr and Léon Rosenfeld arrive in New York on a visit, having been handed the news of the fission discovery just prior to their departure. Rosenfeld reports the fission news to Princeton physicist John Wheeler, who introduces it to the journal club there; news quickly spread to others at Princeton and Columbia, including Eugene Wigner, Leo Szilard, and Enrico Fermi. Szilard, having long been interested in the possibility of chain reactions of neutrons, begins campaigning to keep the research segret. Fermi begins research on neutron production in the fission process with Columbia colleague John Dunning and graduate student Herbert Anderson.
Bohr and Fermi announce th efission discovery at the Fifth Washington Conference on Theoretical Physics to an audience that includes Harold Urey, Merle Tuve, Lawrence Hafstad, Hans Bethe, Gregory Breit, and Edward Teller. Tuve, Hafstad, Richard Roberts, and R. C. Meyer, all of the Carnegie Institution of Washington (CIW), begin to research the fission process. Meanwhile, in Paris, Frédéric Joliot-Curie detects fission fragments and he and his wife Irène establish a fission research program at the Radium Institute in Paris. Days later, at Berkeley, upon learning of th efission discovery, Luis Alvarez and his student Philip Abelson, who had been bombarding uranium with neutrons but had not identified the fission products, initiate their own research.
Online resource: Summary of the Fifth Washington Conference at the GW and Foggy Bottom Historical Encyclopedia.
In the first week of the month, in response to early experimental data from CIW showing uranium fission by "slow" neutrons, Bohr suggests th every rare uranium-235 (U235) isotope, which would fission under all circumstances, is solely responsible for slow-neutron fission, while the abundant uranium-238 (U238) isotope requires "fast" neutrons of over 1 MeV in energy in order to fission. Because U238 generally scatters incident neutrons, most neutrons will lose energy until they have an energy matching the "absorption resonance" energy of U238, at which point they will be absorbed and removed from the reaction. However, slow neutrons already below the absorption resonance will continue to scatter until they fission a U235 nucleus.
To sustain a chain reaction, a fissioning nucleus must eject, on average, at least between 1 and 2 free neutrons. In separate experiments Frèdèric Joliot-Curie, Lew Kowarski, and Hans von Halban (Paris); Fermi and Anderson (Columbia); and Szilard and Walter Zinn (Columbia), all measure appropriately high numbers. Until th eParis group publishes, ther eis discussion of keeping results secret.
At Columbia, Fermi, Szilard, and Anderson experiment with a design for a chain reaction device that would use water as a "moderator" to slow fast neutrons produced by fissioning nuclei to an energy where they would sustain a chain reaction. The experiments reveal that the hydrogen in water itself absorbs too many neutrons to be an effective moderator. They write up the results in June, and they appear in the Physical Review in August (see citation). Fermi leaves Columbia for the summer, but Szilard remains in New York working to stimulate political awareness of the fission issue, and also seeking an alternate moderator, placing his hope in graphite.
"The Mechanism of Nuclear Fission," a consolidation and extension of theoretical and experimental understanding of fission co-written by Niels Bohr and John Wheeler, appears in the Physical Review (see abstract).
Following Niels Bohr's February 1939 explanation of uranium-235 being responsible for the slow-neutron fission in natural uranium, it became important to develop a means of separating the uranium isotopes, which were chemically identical and only incrementally different in mass. This research was motivated by three problems. First, to confirm Bohr's theiry, it would be necessary to experiment with uranium samples of differing isotope composition. Second, for any chain reaction to proceed, it was thought it might be necessary to reduce substantially the amount of th eless-fissile U238 in the uranium used. Finally, even as it became increasingly clear that slow-neutron chain reactions were possible, it was equally evident that an explosive slow-neutron reaction in natural or U235-enriched uranium would be highly inefficient (i.e., producing only a small explosion) and tha tth esize of the explosive would be prohibitively massive: to build a nuclear explosive, it would be necessary to isolate pure U235 in quantity.
The Naval Research Laboratory approaches University of Virginia centrifuge expert Jesse Beams about the prospects of isotope separation.
Columbia's John Dunning and Enrico Fermi urge the University of Minnesota's Alfred Nier, an expert in mass spectroscopy, to use his instruments to separate a sample of U235 isotope to test Bohr's theory.
Having left Copenhagen for the University of Birmingham ahead of the war, Otto Frisch starts work on the thermal gaseous diffusion method of separating isotopes, which he did with the goal of testing Bohr's theory.
After struggling through late 1939 to separate different isotopes of uranium contained in the toxic, volatile, and corrosive gaseous compound uranium hexafluoride, Nier successfully separates samples using the compounds uranium tetrachloride and uranium tetrabromide. Using Nier's samples, Columbia physicists John Dunning, Eugene Booth, and Aristid von Grosse confirm that U235 has a large fission cross section and is responsible for slow-neutron fission in natural uranium. Results are published as letters in the Physical Review in March (see citation) and April (see citation).
Online Resource: See a photograph of Nier with the mass spectrometer he used to separate U235.
Amid doubts about the prospects of achieving a successful chain reaction using natural uranium, isotope separation becomes a focus at the April meeting of the American Physical Society. There Ross Gunn, Jesse Beams, Alfred Nier, Enrico Fermi, Harold Urey, and Merle Tuve agree on the significance of U235 and that the next step is to find a method of separating kilogram quantities, placing highest hopes in the centrifuge method.
Conferring with Fermi, Urey, Dunning, and George Pegram at Columbia, Lyman Briggs agrees that a concerted, multi-directional isotope-separation effort should be made beginning in June and that support should be solicited, perhaps from the Navy.
At Oxford University in England, Franz Simon (who had emigrated from Germany in 1933) begins work on gaseous barrier diffusion methods of separation.
The U. S. Navy agrees to provide $100,000 to fund isotope separation research.
Beginning in the summer of 1940 and moving into 1941 a number of isotope-separation research programs begin work in the United States. These include:
Columbia University where Harold Urey works on both centrifuge and gaseous diffusion methods. In the latter project, he was joined by John Dunning, Eugene Booth, G. B. Karelitz, and Aristid von Grosse (with assistance from student William Nierenberg), and in 1941 their work becomes the major center for the development of this method.
Carnegie Institution of Washington where Philip Abelson works on liquid thermal diffusion methods using facilities at the Natural Bureau of Standards; in the summer of 1941 he transfers to the Naval Research Laboratory.
Franz Simon reports to the British MAUD committee tha tusing gaseous barrier diffusion, a plant that would separate 1kg of U235 per day would cost £5,000,000; Simon's work would soon supercede Frisch's work on thermal diffusion, which was deemed unusable for the uranium hexafluoride gas.
Encouraged by British scientists like Ralph Fowler and Marcus Oliphant, and fostering ideas about electromagnetic isotope separation, Berkeley's Ernest Lawrence begins independently pushing for more aggressive research, emphasizing the need for more work on isotope separation. Vannevar Bush insists that if he wishes to have influence, he will have to work through the existing committee organization. Lawrence agrees, and in April he is included in the National Academy of Sciences committee to review fission-related research.
At Oxford, a 1/2-scale, single stage pilot separation plant is completed.
Metropolitan-Vickers is contracted to design a 20-stage separation plant based on the Oxford model. ICI agrees to continue to provide uranium hexafluoride for the project.
At Princeton, Robert R. Wilson invents the electromagnetic "isotron" method of uranium separation and begins a development program that lasts until 1943 (and employs Princeton student Richard Feynman, among others).
Ernest Lawrence assembles a task force at Berkeley to begin converting his 37-inch cyclotron at Berkeley into an electromagnetic isotope separator. By February 1942 his team completes an efficient device that he calls the "calutron".
John Dunningn and Eugene Booth's team at Columbia—now comprising dozens of researchers—achieve measurable U235 separation.
The S-1 Committee recommends that $400,000 be assigned to Ernest Lawrence's work on electromagnetic isotope separation.
In Paris, Frédéric Joliot-Curie's group begins sets of chain reaction experiments lasting into the next year, dominating research in this area. Initially using water as a moderator, the group also begin to consider graphite and heavy water. In March 1940, they secure the full supply of heavy water available at the Norsk Hydro plant in Norway.
Working at the University of Birmingham, émigré scientists Rudolf Peierls and Otto Frisch establish a theoretical critical mass of about 5kg for a bomb made of pure uranium-235 and employing fast neutrons, and release this value in a secret memorandum to Birmingham physicist Marcus Oliphant.
Online Resource: Read the Frisch-Peierls meorandum at atomicarchive.com.
Using the $6,000 now released by the Advisory Committee on Uranium, Leo Szilard and Enrico Fermi undertake preliminary experiments to test the absorption properties of a graphite-moderated chain reaction, with favorable results, which they report in May.
Lew Kowarski and Hans von Halban of Joliot-Curie's group escape from France to Britain with the Radium Institute's supply of heavy water; they take up their work at the Cavendish Laboratory at Cambridge University where Norman Feather and Egon Bretscher were already doing fission research.
The NDRC awards a $40,000 research contract to Fermi's group at Columbia University.
In late 1940 and 1941 theoretical and experimental research on chain reactions proceeded at a number of locations, with the objects of ascertaining fundamnetal constants such as neutron-capture and fission cross sections of uranium and other materials, and developing designs for workable reactors. Programs included:
Cambridge University, which included Lew Kowarski, Hans von Halban, Norman Feather, and Egon Bretscher.
University of Illinois; although not privy to increasingly restricted information on account of their status as aliens, fission studies by Maurice Goldhaber and his wife Gertrude Scharff Goldhaber were used in official work.
In an intermediate-sized uranium-graphite "pile" of 8'x8'x11', Fermi attains disappointing results for th eperpetuation of a neutron chain reaction, but with hope that additional improvements to pile design and material purity would result in a successful chain reaction.
Arthur Compton calls a meeting of various chain reaction research programs at the University of Chicago. It is agreed that Fermi will build a new pile at Columbia; Samuel Allison will build a new pile using a beryllium moderator at Chicago. Eugene Wigner would lead theoretical calculations of chain reacations at Princeton. J. Robert Oppenheimer would undertake theoretical studies of fast-neutron chain reactions at Berkeley. Norman Hilberry and Richard Doan would assist Compton in organizing the project.
The discovery of plutonium, which fissions as easily as uranium-235, provided additional impetus to chain reaction research because it provided a new path to the atomic bomb that did not require difficult and expensive isotope separation methods. Plutonium could be produced in a slow-neutron chain reaction in uranium containing its naturally large concentrations of uranium-238, and then chemically separated into its pure, potentially explosive form.
In an ongoing attempt to discover further chemical products of neutron bombardment and fission of uranium, Berkeley's Edwin McMillan produces substances with peculiar radioactive and chemical properties; a visiting Philip Abelson identifies it as element 93, the expected beta-decay product of uranium, which McMillan calls neptunium. The results are published in a letter to the Physical Review in June (see citation). The publication of such sensitive research in wartime incensed the British who submitted an official protest through their embassy.
Unaware of the experimental work at Berkeley, Princeton theorist Louis Turner, author of a review article on fission research (see citation), proposes to Szilard that when U238 absorbed neutrons it would transmute to higher elements; element 94 (which he referred to as "eka-osmium", but was later named plutonium) was likely to prove as fissionable as U235.
After Abelson's return to Washington, McMillan continues his research and identifies apparent products of beta decay in neptunium, presumably element 94, but leaves for service at the new MIT Radiation Laboratory in November before a positive identification can be made.
Working since December in a continuation of McMillan's research, Berkeley nuclear chemist Glenn Seaborg, Segrè and collaborators Arthur Wahl and Joseph Kennedy create and isolate element 94, which they find will fission when exposed to slow neutrons. Seaborg proposes the name plutonium.
Seaborg and Segrè's group reports that the slow-neutron fission cross section of plutonium is 1.7 times that of U235, providing further impetus to chain reaction work.
In the wake of the neutron ejection experiments at Columbia, and at the suggestion of Columbia Graduate School dean George Pegram, Enrico Fermi arranges a meeting with Adm. Stanford Hooper, the Technical Adviser to the Chief of Naval Operations, to arouse interest in fission research. The meeting secures no agreements, but Ross Gunn and other Naval Research Laboratory scientists become aware of the issue. On the 16th Adolf Hitler annexes much of Czechoslovakia.
On the 16th Leo Szilard and fellow Hungarian Eugene Wigner call on Albert Einstein, an old friend of Szilard's, to see if he can urge Belgium to secure its uranium supply in the Congo. Shortly thereafter, Szilard and another fellow Hungarian, Edward Teller, meet with Alexander Sachs, a Lehman Corporation economist who had ties to the White House and an interest in scientific issues. Sachs suggests they approach President Franklin Roosevelt directly.
Einstein signs a letter drafted by Szilard to President Roosevelt, urging him to procure uranium supplies and support fission research.
Germany and the Soviet Union sign a non-aggression pact.
Germany invades Poland, initiating World War II in Europe.
Sachs meets with Roosevelt and delivers the Einstein letter to him, which results in the creation of the ad hoc Advisory Committee on Uranium, chaired by National Bureau of Standards director Lyman Briggs, and consisting of military ordnance specialists Army Colonel Keith Adamson and Navy Commander Gilbert Hoover.
Online Resource: Einstein's letter to Roosevelt and related materials at the Roosevelt Library and Museum.
The Advisory Committee on Uranium meets for the first time, with Szilard, Teller, Wigner, Sachs, and Richard Roberts from the Carnegie Instituion of Washington as guests. At Teller's request, the committee agrees to make $6,000 available for slow-neutron chain reaction research using a graphite moderator, but the money is not made available until early the next year. Szilard estimates the graphite for the research would alone cost $33,000.
Marcus Oliphant passes the Frisch-Peierls memorandum to Henry Tizard, chair of the British Air Ministry's Committee for the Scientific Survey of Air Warfare and a prior skeptic of the practical exploitation of uranium fission. Tizard recommends a new committee be set up to decide what actions out to be taken.
The UK government establishes a committee to consider the fission bomb issue. This is the first time attention concentrates specifically on the task of possible bomb construction. In June the committee would become known by th edocename "MAUD committee". The committee is chaired by Imperial College physicist George Thomson, and comprises Oliphant, James Chadwick, John Cockcroft, Philip B. Moon, and soon also includes Patrick Blackett, Charles Ellis, and William Haworth.
Winston Churchill becomes Prime Minister of the United Kingdom.
In view of favorable research on neutron absorption in graphite and isotope separation, and at the recommendation of Ross Gunn, Naval Research Laboratory director Adm. Harold Bowen asks Harold Urey to form a scientific subcommittee to the Advisory Committee on Uranium. The committee comprises Harold Urey, Ross Gunn, George Pegram, Merle Tuve, Jesse Beams, and Gregory Breit. It meets for the first time on June 13, and recommends support for work on isotope separation and chain reactions.
France signs an armistice agreement with Germany.
Roosevelt orders the establishment of the National Defense Research Committee (NDRC), with Vannevar Bush as chair; the Advisory Committee on Uranium becomes a part of the new organization, and is redesignated the Committee on Uranium on July 2. Adamson and Hoover leave the committee, while all members of the scientific subcommittee, except Breit, become members.
Briggs reports to Bush that the Naval Research Laboratory will administer research on uranium isotope separation methods, with $100,000 in funding.
Bush informs Briggs that the NDRC, beginning November 1, will provide $40,000 for funding chain-reaction research.
At Bush's request the National Academy of Sciences (NAS) assembles a committee to review the uranium research program, which consists of Arthur Compton (chair), Ernest Lawrence, John Van Vleck, and William Coolidge. The committee delivers its first report on May 17; Bush feels it neglects discussion of remaining practical challenges.
Oliver Buckley of Bell Laboratories and L. Warrington Chubb of Westinghouse are added to the NAS review committee on account of their engineering experience. The committee issues a second report on July 11.
Germany declares war on the Soviet Union.
Roosevelt orders the establishment of the Office of Scientific Research and Development (OSRD) in the Office of Emergency Management, with Bush as director; the OSRD oversees the NDRC, which is to be chaired by Harvard president James Conant, and the Committee on Uranium is redesignated a "section" of NDRC and it is still led by Briggs.
The British MAUD Committee issues its final report indicated the likelihood that an atomic weapon can be constructed. Having completed its task, the committee disbands, and the report is taken up for consideration by the Ministry of Aircraft Production, Prime Minister Churchill, and the government's Scientific Advisory Committee.
Gunn and Tuve leave the Uranium Section; Gregory Breit, Samuel Allison, Edward Condon, Henry Smyth, and Cornell University's Lloyd Smith join. George Pegram serves as vice chair and as head of a subsection on power production. Urey is put in charge of isotope separation and heavy water groups. Enrico Fermi leads a subsection on theoretical aspects. Bush arranges for chemical engineering Warren Lewis, George Kistiakowsky, and Robert Mulliken to join the National Academy of Sciences review committee, and requests a third report.
Vannevar Bush informs President Roosevelt of British results. Roosevelt authorizes him to undertake preliminary planning for a full-scale bomb-building effort.
The United Kingdom officially establishes an atomic weaponry development project through the new codename "Directorate of Tube Alloys" in its Department of Scientific and Industrial Research.
The National Academy of Sciences review committee submits a third report detailing the feasibility, requirements, and likely characteristics of a uranium bomb, and recommending immediate scaled-up efforts at isotope separation. Bush, Conant, Compton, and Briggs begin working out the organization of the effort.
Following the December 7 surprise Japanese attack on Pearl Harbor, the United States enters World War II.
Bush finalizes plans for what will now be called the "S-1" section. Engineering tasks will be directed by a Planning Board with Standard Oil vice president and chemical engineer Eger Murphree as its chief. Research work will be divided between three "program chiefs": Harold Urey (for diffusion and centrifuge method of isotope separation and heavy-water studies), Ernest Lawrence (for small-sample preparation and electromagnetic methods of isotope separation, and some experiments related to plutonium), and Arthur Compton (for research in atomic physics and the measurement of constants). Lyman Briggs will continue to serve as chair. Although S-1 will no longer be under the NDRC, the NDRC chair James Conant will be a member of the section.
The S-1 Committee recommends that $400,000 be assigned to Ernest Lawrence's work on electromagnetic isotope separation.
Compton consolidates slow-neutron chain reaction research groups from Columbia and Princeton into the University of Chicago group.