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The first technical exposition of a practical nuclear weapon

The Frisch–Peierls memorandum was the first technical exposition of a practical nuclear weapon. It was written by expatriate German Jewish physicists Otto Frisch and Rudolf Peierls in March 1940 while they were both working for Mark Oliphant at the University of Birmingham in Britain during World War II. The memorandum contained the first calculations about the size of the critical mass of fissile material needed for an atomic bomb. It revealed that the amount required might be small enough to incorporate into a bomb that could be delivered by air. It also anticipated the strategic and moral implications of nuclear weapons. It helped send both Britain and America down a path which led to the MAUD Committee, the Tube Alloys project, the Manhattan Project, and ultimately the atomic bombings of Hiroshima and Nagasaki.

Rudolf Peierls was born in Berlin in 1907. He studied physics at the University of Berlin, at the University of Munich under Arnold Sommerfeld, the University of Leipzig under Werner Heisenberg, and ETH Zurich under Wolfgang Pauli. After receiving his DPhil from Leipzig in 1929, he became an assistant to Pauli in Zurich. In 1932, he was awarded a Rockefeller Fellowship, which he used to study in Rome under Enrico Fermi, and then at the Cavendish Laboratory at the University of Cambridge under Ralph H. Fowler. Due to Adolf Hitler's rise to power in Germany, he elected to not return home in 1933, but to remain in Britain. He worked with Hans Bethe at the University of Manchester, then at the Mond Laboratory at Cambridge. In 1937, Mark Oliphant, an Australian and newly appointed professor of physics at the University of Birmingham , recruited him for a new chair there in applied mathematics.

Otto Robert Frisch was born in Vienna in 1904. He studied physics at the University of Vienna, from which he received his DPhil in 1926. He worked at the Physikalisch Technische Reichsanstalt in Berlin until 1930, when he obtained a position at the University of Hamburg under the Nobel Prize winning scientist Otto Stern. As non Aryans, Stern and Frisch were dismissed after Hitler's accession. Stern found Frisch a position in Britain with Patrick Blackett at Birkbeck College at the University of London, and a grant from the Academic Assistance Council. He followed this with a five year stint at the Niels Bohr Institute in Copenhagen with Niels Bohr where he increasingly specialised in nuclear physics, particularly the physics of neutrons, Meitner and Frisch's paper, dated 16 January 1939, explained the physics behind the phenomenon. Frisch went back to Copenhagen, where he was able to isolate the fragments produced by fission reactions. Frisch later recalled that:
The news of the discovery of fission was brought to America by Bohr in January 1939. Bohr and John A. Wheeler set to work applying the liquid drop model developed by Bohr and Fritz Kalckar to explain the mechanism of nuclear fission. George Placzek, who was skeptical about the whole idea of fission, challenged Bohr to explain why uranium seemed to fission with both very fast and very slow neutrons. Bohr had an epiphany that the fission at low energies was due to the uranium 235 isotope, while at high energies it was due mainly to the more abundant uranium 238 isotope. On 16 April, Bohr, Placzek, Wheeler, Eugene Wigner and Leon Rosenfeld discussed whether it would be possible to use a nuclear chain reaction to make an atomic bomb, and concluded that it was not. Bohr observed that "It would take the entire efforts of a country to make a bomb." Peierls also attempted to simplify the problem by using the fast neutrons produced by fission, thus omitting consideration of moderator. He then calculated the critical mass of a sphere of uranium metal in a theoretical paper written in 1939. He later recalled that the size of the critical mass "was of the order of tons. It therefore appeared to me that the paper had no relevance to a nuclear weapon." However, Bohr had argued that the uranium 235 isotope was far more likely to capture neutrons, so fissile even using neutrons of low energy. Frisch wondered what would happen if he were able to produce a sphere of pure uranium 235. When he used Peierls' formula to calculate this, he received a startling answer. Peierls later observed that:

Realising the sensitive nature of the document, Peierls typed it himself. One carbon copy was made. Today the original is in the Bodleian Library at Oxford University.

The memorandum was written in two parts. The first was an elegant and comprehensive outline of the implications of their calculations. It included a proposal that the best defence against such a weapon would be to develop one before Germany did so. In a few short pages these two scientists anticipated the policies of deterrence which would shape Cold War geopolitics. The second was an explanation of the science supporting their conclusions.

The memorandum was given to Oliphant, who passed it on to Tizard in his capacity as the chairman of the Committee for the Scientific Survey of Air Warfare (CSSAW). He in turn passed it to Thomson, the chairman of the committee to which the CSSAW had delegated responsibility for uranium research. Thomson's committee was about to disband. It had studied nuclear reactions in uranium, and the use of graphite as a neutron moderator in a nuclear reactor, but its results had been negative, and it had concluded that the rate of capture of neutrons by the graphite was too great to make such a reactor a practical proposition. The Frisch–Peierls memorandum caused Thomson to reconsider. After discussions between Cockcroft, Oliphant and Thomson, CSSAW created the MAUD Committee to investigate further. As enemy aliens, Peierls and Frisch were initially excluded from its deliberations, but they were later added to its technical subcommittee. The research from the MAUD committee was compiled in two reports, commonly known as the MAUD reports in July 1941. The first report, "Use of Uranium for a Bomb", discussed the feasibility of creating a super bomb from uranium, which they now thought to be real. The second, "Use of Uranium as a Source of Power" discussed the idea of using uranium as a source of power, not just a bomb. The MAUD Committee and report helped bring about the British nuclear program, the Tube Alloys project. Not only did it help start a nuclear project in Britain but it helped jump start the American project. Without the help of the MAUD Committee the American program, the Manhattan Project, would have started months behind. Instead they were able to begin thinking about how to create a bomb, not whether it was possible. Historian Margaret Gowing noted that "events that change a time scale by only a few months can nevertheless change history." In August 1941, Oliphant was sent to the US to assist the Americans with microwave radar. He took the initiative to enlighten the scientific community there of the ground breaking discoveries of the MAUD Committee. He travelled to Berkeley to meet with his friend Ernest Lawrence, who soon caught his enthusiasm. Oliphant convinced the Americans to move forward with nuclear weapons, and his lobbying resulted in Vannevar Bush taking the report directly to the president. Leo Szilard later wrote: "if Congress knew the true history of the atomic energy project, I have no doubt but that it would create a special medal to be given to meddling foreigners for distinguished services, and that Dr Oliphant would be the first to receive one."