Atomic History Timeline 1900-1942
Chronology For The Origin Of Atomic Weapons
Courtesy of Carey Sublette
I have included here a chronological listing of events and milestones leading up to the use of atomic weapons against Japan. Brief explanatory notes are inserted to provide some context and interpretation. The interested reader is directed to several excellent books available (see bibliography), particularly the Pulitzer Prize winning book by Rhodes, and Critical Assembly: A Technical History of Los Alamos During the Oppenheimer Years 1943-1945, from whom the bulk of the material for this timeline was extracted.
The timeline is divided into several epochs which seem to me to be naturally separated by critical events. Each epoch begins with a short summary of the key themes that characterize it. Although this is a strict chronology which list events that are more or less datable, occasional paragraphs are interspersed summarizing the thrust of events.
Early History of Nuclear Weapons
*** From 1920 To December 1938 ***
During this epoch the basic physics upon which the discovery of fission would be based were worked out.
June 3, 1920 - Ernest Rutherford speculates on the possible existence and properties of the neutron in his second Bakerian Lecture, London.
December 28, 1931 - Irene Joliot-Curie reports studying penetrating particles produced by beryllium when bombarded by alpha rays. She believes the particles, which are actually neutrons, to be energetic gamma rays.
February 7 to 17, 1932 - In a series of experiments James Chadwick demonstrates the existence of the neutron.
September 12, 1932 - Leo Szilard conceives the idea of using a chain reaction of neutron collisions with atomic nuclei to release energy. He also considers the possibility of using this to make bombs. This predates the discovery of fission by more than six years.
May 10, 1934 - Enrico Fermi's research group publishes a report on experiments with neutron bombardment of uranium. Several radioactive products are detected.
July 4, 1934 - Leo Szilard files patent application describing the use of neutron induced chain reactions to create explosions, and the concept of the critical mass.
September, 1934 - Ida Noddack publishes a paper in Zeitshrift fur Angewandte Chemie arguing that the anomalous radioactivities produced by neutron bombardment of uranium may be due to the atom splitting into smaller pieces.
October 22, 1934 - Enrico Fermi discovers the principle of neutron moderation, and the enhanced capture of slow neutrons.
October 8, 1935 - The British War Office rejects Szilard's offer to turn over to them his patents of nuclear energy for free, an offer made to bring them under British secrecy laws.
December, 1935 - Chadwick wins Nobel Prize for discovery of the neutron.
February, 1936 - The British Admiralty accepts Szilard's offer to turn over his patents.
The Discovery of Fission and Its Properties
*** From December 1938 To September 1939 ***
This period, initiated by the discovery of fission by Hahn, was marked by preliminary investigation the properties and principles of fission. There was also substantial speculation about the possible uses of fission, but without firm experimental support for making projections.
December 21, 1938 - Otto Hahn submits paper to Naturwissenschaften showing conclusive evidence of the production of radioactive barium from neutron irradiated uranium, i.e. evidence of fission.
January 13, 1939 - Otto Frisch observes fission directly by detecting fission fragments in an ionization chamber. With the assistance of William Arnold, he coins the term "fission".
Mid January, 1939 - Leo Szilard hears about the discovery of fission from Eugene Wigner. He immediately realizes that the fission fragments, due to their lower atomic weights, would have excess neutrons which would have to be shed.
January 26, 1939 - Niels Bohr publicly announces the discovery of fission at an annual theoretical physics conference at George Washington University in Washington, DC.
January 29, 1939 - Robert Oppenheimer hears about the discovery of fission, within a few minutes he realizes that excess neutrons must be emitted, and that it might be possible to build a bomb.
February 5, 1939 - Niels Bohr realizes that U-235 and U-238 must have different fission properties, that U-238 could be fissioned by fast neutrons but not slow ones, and that U-235 accounted for observed slow fission in uranium.
At this point there were too many uncertainties about fission to see clearly whether or how self-sustaining chain reactions could arise. Key uncertainties were 1) the number of neutrons emitted per fission, and 2) the cross sections for fission and absorption at different energies for the uranium isotopes. For a chain reaction there would need to be both a sufficient excess of neutrons produced, and the ratio between fission to absorption averaged over the neutron energies present would need to be sufficiently large.
The different properties of U-235 and U-238 were essential to understand in determining the feasibility of an atomic bomb, or of any atomic power at all. The only uranium available for study was the isotope mixture of natural uranium, in which U-235 comprised only 0.71%.
March, 1939 - Fermi and Herbert Anderson find that there are about two neutrons produced for every one consumed in fission.
June, 1939 - Fermi and Szilard submit paper to Physical Review describing sub-critical neutron multiplication in a lattice of uranium oxide in water, but it is clear that natural uranium and water cannot make a self-sustaining reaction.
July 3, 1939 - Szilard writes to Fermi describing the idea of using a uranium lattice in carbon (graphite) to create a chain reaction.
August 31, 1939 - Bohr and John A. Wheeler publish a theoretical analysis of fission. This theory implies U-235 is more fissile than U-238, and that the undiscovered element 94-239 is also very fissile. These implications are not immediately recognized.
September 1, 1939 - Germany invades Poland, beginning World War 2.
Organizing to Investigate Atomic Weapons
*** From September 1939 To September 1941 ***
The preliminary research into fission indicated that it was probable that power could be produced from fission. Two general approaches both seemed viable, the uranium-graphite and uranium-heavy water reactor. The possibility of a bomb was still controversial, but it hadn't been ruled out by experiments to date. With the growth in scale of the experiments additional sources of funds were increasingly necessary to continue work. The outbreak of war in Europe also created pressure on the scientists to resolve the bomb question quickly. Attempts to gain governmental attention and support became increasingly strident.
During this phase efforts to investigate the possibility of atomic bombs, and to support basic research, were pressed on the governments of both Britain and the US Considerably more success in this was made in Britain, although the larger research establishment in the US which was still at peace made more of the fundamental discoveries. The favorable results of Britain's MAUD committee in investigating the feasibility of atomic bombs was instrumental in eventually spurring the US to action.
October 11, 1939 - At Szilard's urging Alexander Sachs presents Pres. Franklin D. Roosevelt with the "Einstein Letter". The letter, signed by Einstein but drafted by Szilard in consultation with Einstein, warns the President of the possibility of nuclear weapons and urging him take action to prevent Germany from gaining an advantage with them.
October 21, 1939 - First meeting of the Advisory Committee on Uranium (the "Briggs Uranium Committee") in Washington, DC, created at Pres. Roosevelt's order. Lyman Briggs of the Bureau of Standards presides, attendees include Szilard, Wigner, Sachs, Edward Teller, Army Lt. Col. Adamson, and Navy Cmdr. Hoover. Physicists argue for urgent government attention, Adamson is hostile. Teller requests $6000 for research on preliminary uranium-graphite slow neutron experiments, which is grudgingly approved. A report of the meeting is sent to FDR on Nov. 1, but no action results.
From the outset it was clear to all of the physicists who thought about the problem seriously that fast fission was necessary to construct a bomb. Rapid multiplication is essential to develop significant explosive force, and the process of slowing down neutrons takes too long. But it was known that the average cross section for fast fission in U-238 was too small to support such a reaction. Up to this point U-235 had been considered only for its slow fission potential - leading to power plants, not bombs. No one had yet developed a plausible approach for building a bomb. The fact that a large cross section for slow fission implied a large fast fission cross section as well was not realized.
February 1940 - Frisch and Rudolf Peierls, living in the UK, consider the possibility of fast fission in U-235. Based on a theoretical estimate of the fast fission cross section they estimate the critical mass of pure U-235 at "a pound or two", and that a large percentage could be fissioned before explosive disassembly. They also estimate the likely effects of the bomb, and possible assembly methods, as well as estimates of the feasibility of isotope separation. After preparing a memorandum on this discovery, they give a copy to Mark Oliphant, who passes it along to Henry T. Tizard, chairman of the Committee on the Scientific Survey of Air Defense. At this point the "Tizard Committee" is the most important scientific committee for defense in Britain.
March, 1940 - After much prodding by Szilard, Briggs finally releases the promised $6000.
March 2, 1940 - The first direct measurements of the enormous slow fission cross section of U-235 are made by John Dunning at Columbia University.
April 9, 1940 - Germany invades Denmark and Norway.
April 10, 1940 - First meeting of the UK committee (later code-named the MAUD Committee) organized by Tizard to consider Britain's actions regarding the "uranium problem". Research into isotope separation and fast fission is agreed upon.
April 27, 1940 - Second meeting of the Briggs Uranium Committee. Briggs' decision is that neither research on fast fission, nor work on building a critical uranium-graphite assembly, should begin until the small scale lab experiments, just getting underway, are finished.
May, 1940 - George Kistiakowsky suggests gaseous diffusion as a possible means for producing U-235 to Vannevar Bush during a meeting at Carnegie Institution.
May 10, 1940 - Germany launches its assault on Western Europe, attacking Holland, Belgium and France.
May 27, 1940 - Louis Turner mails Szilard a manuscript arguing that element 94-239 (not yet discovered) should be highly fissionable like U-235, and could be manufactured by bombarding U-238 with neutrons, to form U-239, which would undergo two beta-decays to form elements 93-239 and 94-239 in succession.
May 27, 1940 - Edwin McMillan and Philip Abelson submit a report "Radioactive Element 93" to Physical Review describing their discovery of neptunium produced by bombarding uranium with neutrons. Britain subsequently protests the publication as a violation of wartime secrecy.
June, 1940 - The MAUD Committee acquires its name. Franz Simon begins research on isotope separation through gaseous diffusion.
July 1, 1940 - The newly founded National Defense Research Council (NDRC), headed by Vannevar Bush, takes over responsibility for uranium research. In his final report Briggs requests $140,000 for further work: $40,000 for lab measurements, and $100,000 for large scale uranium-graphite studies. Bush approves only $40,000.
November, 1940 - Dunning and Nobel prize winner Harold Urey begin investigating isotope separation techniques without US government support.
November 1, 1940 - The $40,000 contract from the NDRC finally comes through and work begins at Columbia University to assemble a large sub-critical pile made of graphite and uranium oxide.
December, 1940 - The MAUD Committee issues report on isotope separation authored by Simon. Report concludes manufacturing U-235 by gaseous diffusion is feasible on a scale suitable for weapons production.
February, 1941 - Philip Abelson begins working on uranium enrichment at the Naval Research Laboratory. He selects liquid thermal diffusion as the technique to pursue.
February 26, 1941 - Glenn Seaborg and Arthur Wahl conclusively demonstrate the presence of element 94, which they later name plutonium.
March, 1941 - Department of Terrestrial Magnetism (DTM) at the Carnegie Institution measures the fast cross section of U-235. Using it Peierls, on the MAUD Committee, calculates a new critical mass for U-235 at 18 LB as a bare sphere, or 9-10 lb. when surrounded by a reflector. A memorandum is prepared by the MAUD Committee describing the importance of fast fission for bomb design and transmits it the US - Lyman Briggs locks up the document on arrival and shows it to no one.
March 6, 1941 - Seaborg and Wahl isolate the first pure neptunium-239 (0.25 micrograms), in a matter of days it decays into a (barely) visible speck of pure plutonium.
March 28, 1941 - Joseph Kennedy, Seaborg and Emilio Segre show that the plutonium sample undergoes slow fission, which implies it is a potential bomb material.
May, 1941 - After months of growing pressure from scientists in Britain and the US (particularly Berkeley's Ernest Lawrence), Bush at the NDRC decides to review the prospects of nuclear energy further and engages Arthur Compton and the National Academy of Sciences for the task. The report is issued May 17 and treats military prospects favorably for power production, but does not address the design or manufacture of a bomb in any detail.
At this same time, Bush creates the larger and more powerful Office of Scientific Research and Development (OSRD), which is empowered to engage in large engineering projects in addition to research, and becomes its director.
Also during this month Tokutaro Hagiwara at the University of Kyoto delivers a speech in which he discusses the possibility of a fusion explosion being ignited by an atomic bomb, apparently the first such mention.
May 18, 1941 - Segre and Seaborg determine that the slow cross section of Pu-239 is 170% of that of U-235, proving it to be an even better prospective nuclear explosive.
July 1941 - Segre and Seaborg measure the fast fission cross section of Pu-239, finding a high value.
July 15, 1941 - The MAUD Committee approves its final report and disbands. The report describes atomic bombs in some technical detail, provides specific proposals for developing them, and includes cost estimates.
Although the contents of the MAUD report reach Vannevar Bush at the OSRD immediately, he decides to wait for the report to be transmitted officially before taking any further action on fission development.
August-September, 1941 - Fermi and his team at Columbia begin assembling a sub-critical experimental pile containing 30 tons of graphite and 8 tons of uranium oxide. It gives a projected k value 0f 0.83, indicating purer materials are needed.
September, 1941 - Fermi muses to Teller ("out of the blue") whether a fission explosion could ignite a fusion reaction in deuterium. After some study Teller concludes that it is impossible.
Organizing to Develop Atomic Weapons
*** From September 1941 To September 1942 ***
Programs to conduct research and development of atomic bombs actually begin in Britain and the US The funding during this period is modest, much of the basic science remains sketchy. Split between the OSRD and the Army, the US program remains disorganized, bureaucratic and, under Compton, weakly lead. Theoretical work becomes more detailed, and large scale experiments leading toward self-supporting chain reactions begin. Efforts at developing the infrastructure to produce atomic weapons (buying materials, buying property, assembling a staff with the necessary skills, preparing preliminary engineering designs) make little headway.
September 3, 1941 - With PM Winston Churchill's endorsement, the British Chiefs of Staff agree to begin development of an atomic bomb.
October 3, 1941 - The MAUD Committee Final Report reaches the US through official channels.
October 9, 1941 - Bush brings the MAUD Report to Pres. Roosevelt. FDR approves a broader project to investigate the feasibility and to confirm the British estimates.
October 21, 1941 - Compton holds a meeting in Schenectady, NY with Lawrence, Oppenheimer, George Kistiakowsky, and James Conant (new head of the NDRC), reviewing the MAUD Committee report, and the latest US work. The meeting ends with a consensus of the likely feasibility of a bomb.
November 1, 1941 - Compton issues the final NAS report endorsing the importance of exploring the feasibility of a U-235 bomb. The report is delivered to the president by Bush on November 27.
November, 1941 - John Dunning and Eugene Booth at Columbia demonstrate the first measurable U-235 enrichment through gaseous diffusion.
December 6, 1941 - A meeting is held in Washington by Bush to organize an accelerated research project. Compton remains in charge. Urey is appointed to develop gaseous diffusion and heavy water production at Columbia; Lawrence will investigate electromagnetic separation at Berkeley; and Eger Murphree will develop centrifuge separation and oversee engineering issues. Conant advocates pursuing Pu-239, but no decision on this is made.
December 7, 1941 - Pearl Harbor is attacked by a Japanese Naval task force.
December 8, 1941 - The US declares war on Japan.
December 11, 1941 - The US declares war on Germany and Italy following their declaration of war on the US.
December 18, 1941 - The first meeting of the S-1 project is held, sponsored by the OSRD. S-1 is dedicated to the full scale research development of fission weapons.
January, 1942 - Compton creates the Metallurgical Laboratory (Met Lab) at the University of Chicago to act as a consolidated research center. He transfers work on "uranium burners" - reactors - to it. Oppenheimer organizes a program on fast neutron theoretical physics at Berkeley.
February, 1942 - Compton asks Gregory Breit to coordinate physics research on fast neutron phenomena. At this time available experimental data on all aspects of fast neutron reactions and fission is extremely limited and imprecise. Theoretical techniques are also rudimentary.
It is essential to realize that in early 1942 fission physics, and fast neutron physics in general, was a realm that had been scarcely explored. The possibility that any number of undiscovered phenomena might disrupt the development of a weapon was quite real, and extensive research would be necessary to ensure that the program was not heading into a blind alley. The scarcity and poor quality of experimental data was a major problem even if no new problems were discovered.
March 23, 1942 - S-1 program leaders discuss priorities. Conant urges proceeding with *all* options for producing fissionable material simultaneously: gaseous diffusion, centrifuges, electromagnetic separation, and plutonium breeding using both graphite and heavy water reactors. He argues that redundant development will reduce the time to successful production to the shortest possible time, regardless of cost.
April, 1942 -
- Fermi relocates to Chicago. He builds an experimental pile in the Stagg Field squash courts with a projected k value of 0.995, then begins planning the construction of the world's first man-made critical pile, to be called CP-1. Fermi's efforts now shifts from demonstrating feasibility to securing graphite and uranium of adequate purity and in sufficient quantity to build the reactor.
- Seaborg arrives in Chicago and starts work on developing an industrial-scale plutonium separation and purification process.
- Percival Keith of the Kellog Co. begins designing a gaseous diffusion pilot plant.
May 18, 1942 - Breit quits, leaving the neutron physics effort without leadership. Compton asks Oppenheimer to take over in his place.
May 19, 1942 - Oppenheimer write Lawrence that the atomic bomb problem was solved in principle and that six good physicists should have the details mostly worked out in six months. His optimism is based on the belief that gun assembly would suffice for both uranium and plutonium.
June, 1942 -
- Oppenheimer joins the Met Lab to lead an effort on fast neutron physics, and prepares an outline for the entire neutron physics program.
- Production of plutonium through marathon irradiation by cyclotron begins.
- Met Lab engineering council begins developing plans for large scale plutonium production reactors.
- Pres. Roosevelt approves a plan for spending $85 million for a weapon development program.
June 18, 1942 - Due to continuing, and increasing, organization problems Col. James Marshall is ordered by Brig. Gen. Steyr to organize an Army Corps of Engineers District to take over and consolidate atomic bomb development.
July to September, 1942 - Oppenheimer assembles theoretical study group in Berkeley to examine the principles of bomb design. Included are Oppenheimer, Hans Bethe, Teller, John Van Vleck, Felix Bloch, Robert Serber, and Emil Konopinski. During the summer the group develops the principles of atomic bomb design, and examines the feasibility of fusion bombs. Oppenheimer emerges as a natural leader. The group estimates the mass of U-235 required for a high yield detonation at 30 kg (estimated at 100 Kt), megaton range fusion bombs are also considered highly likely.
During this period Richard C. Tolman and Serber discuss the idea of using explosives to collapse a shell of fissile material in place of the gun assembly method. Serber reports that they co-authored a short paper on the subject, although this paper has not been found.
At this time Fermi and his staff are busy arranging for the materials required for CP-1.
July 27, 1942 - First shipment of irradiated uranium arrives at the Met Lab (300 lb.).
Mid August, 1942 - Fermi's group demonstrates an experimental pile with a projected k value of close to 1.04. Achieving a chain reaction is now certain.
August 20, 1942 - Seaborg isolates pure plutonium through a separation process suitable for industrial scale use.
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