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Seth Henry Neddermeyer (September 16, 1907 – January 29, 1988) was an American physicist who co-discovered the muon, and later championed the implosion-type nuclear weapon while working on the Manhattan Project at the Los Alamos Laboratory during World War II.
Neddermeyer contributed to the research which led to the 1932 discovery of the positron, for which Anderson was awarded the Nobel Prize in Physics in 1936. That year, Neddermeyer and Anderson discovered the muon, using cloud chamber measurements of cosmic rays. Their discovery predated Hideki Yukawa's 1935 theory of that postulated the particle as mediating the nuclear force. Anderson and Neddermeyer collaborated with Millikan in high altitude studies of cosmic rays, which confirmed Robert Oppenheimer's theory that the air showers produced in the atmosphere by cosmic rays contained electrons. They also obtained the first evidence that gamma rays can generate positrons.
Though many remained unimpressed, Oppenheimer appointed Neddermeyer the head of a new group to test implosion. His group became the E-5 (Implosion) Group, which was part of Captain William S. Parsons' E Division. A gun-type nuclear weapon was the preferred method, but implosion research constituted a backup. Neddermeyer embarked on an intensive series of experiments testing cylindrical implosions. The result was a series of distorted shapes. Progress was made; Neddermeyer and a member of his team, Hugh Bradner, along with James L. Tuck from the British Mission, conceived the idea of , in which shaped charges are used to focus the force of an explosion. Nevertheless, seemingly unsolvable problems with shock wave uniformity brought progress on implosion to a crawl.
By September 1943, Neddermeyer's team had grown from five people to fifty. That month, John von Neumann came to Los Alamos at Oppenheimer's request. Von Neumann was impressed by the implosion concept and, working with Edward Teller, an old friend, made a series of suggestions. Von Neumann was able to create a sound mathematical model of implosion, enabling Neddermeyer to present a proposal for a greatly expanded research program. Edwin McMillan and Isidor Isaac Rabi recommended that George Kistiakowsky, who had a specialized knowledge in the precision use of explosives, be brought in to help the program. In February 1944, Kistiakowsky became Parsons' deputy for implosion.
In April 1944, tests on the first sample of plutonium that had been produced with neutrons in a nuclear reactor revealed that reactor-bred plutonium contained five times more plutonium-240 than that hitherto produced in . This unwanted isotope that spontaneously decayed and produced neutrons promised to cause a predetonation without sufficiently quick critical mass assembly. It now became apparent that only implosion would work for practical plutonium bombs; a powerful enough gun could not be constructed small enough to be carried in an aircraft, and plutonium-240 was even more difficult to separate from plutonium-239 than the isotopes of uranium that were giving the rest of the Manhattan Project such difficulties. Plutonium was unusable unless implosion worked, but only plutonium could be produced in quantities that would allow regular production of atomic bombs. Thus, the implosion technique now suddenly stood as the key to production of nuclear weapons.
In mid-June 1944, a report from Kistiakowsky to Oppenheimer detailing dysfunctionality within the implosion team led to the ousting of Neddermeyer. He was replaced as the head of the E-5 Group by Kistiakowsky on June 15, 1944, but remained a technical adviser to the implosion program, with group leader status. Neddermeyer was said to have been much embittered by this episode. In Oppenheimer's August 1944 reorganization of the Los Alamos Laboratory, Neddermeyer's group was renamed X-1, with Norris Bradbury as group leader. The implosion method championed by Neddermeyer was used in the first atom bomb exploded (in the Trinity test), the Fat Man bomb dropped on Nagasaki, and almost all modern nuclear weapons. Kistiakowsky later insisted that "the real invention should be given full credit to Seth Neddermeyer" (sic).
In 1982, he was presented with the Department of Energy's Enrico Fermi award. His citation read:
In later life, Neddermeyer was sometimes troubled by the nuclear weapons he had helped to invent. He told an interviewer in 1983:
Neddermeyer died in Seattle on January 29, 1988, from complications of Parkinson's disease.
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