to Los Alamos scientists by Robert Serber, Neddermeyer was one of the first to urge its full development. Unable to find much initial enthusiasm for the concept among his fellow Los Alamos scientists, Neddermeyer presented the first substantial technical analysis of implosion in late April 1943. Oppenheimer considered this to be the beginning of implosion research at Los Alamos. 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 explosive lenses, 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 a cyclotrons. 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