It was the second of the only two nuclear weapons ever used in warfare, the first being Little Boy , and its detonation marked the third nuclear explosion in history. The name Fat Man refers to the early design of the bomb because it had a wide, round shape; it was also known as the Mark III. Fat Man was an implosion-type nuclear weapon with a solid plutonium core. The first of that type to be detonated was the Gadget in the Trinity nuclear test less than a month earlier on 16 July at the Alamogordo Bombing and Gunnery Range in New Mexico. Two more were detonated during the Operation Crossroads nuclear tests at Bikini Atoll in , and some were produced between and , when it was superseded by the Mark 4 nuclear bomb. The Fat Man was retired in Robert Oppenheimer held conferences in Chicago in June , prior to the Army taking over wartime atomic research, and in Berkeley, California, in July, at which various engineers and physicists discussed nuclear bomb design issues. They chose a gun-type design in which two sub-critical masses would be brought together by firing a "bullet" into a "target". Tolman suggested an implosion-type nuclear weapon , but the proposal attracted little interest. The feasibility of a plutonium bomb was questioned in Wallace Akers , the director of the British " Tube Alloys " project, told James Bryant Conant on 14 November that James Chadwick had "concluded that plutonium might not be a practical fissionable material for weapons because of impurities. Groves Jr. The committee concluded that any problems could be overcome simply by requiring higher purity. Oppenheimer reviewed his options in early and gave priority to the gun-type weapon,  but he created the E-5 Group at the Los Alamos Laboratory under Seth Neddermeyer to investigate implosion as a hedge against the threat of pre-detonation. Implosion-type bombs were determined to be significantly more efficient in terms of explosive yield per unit mass of fissile material in the bomb, because compressed fissile materials react more rapidly and therefore more completely. Nonetheless, it was decided that the plutonium gun would receive the bulk of the research effort, since it was the project with the least amount of uncertainty involved. It was assumed that the uranium gun-type bomb could be easily adapted from it. The gun-type and implosion-type designs were codenamed " Thin Man " and "Fat Man" respectively. These code names were created by Robert Serber , a former student of Oppenheimer's who worked on the Manhattan Project. He chose them based on their design shapes; the Thin Man was a very long device, and the name came from the Dashiell Hammett detective novel The Thin Man and series of movies. Little Boy came last as a variation of Thin Man. Neddermeyer discarded Serber and Tolman's initial concept of implosion as assembling a series of pieces in favor of one in which a hollow sphere was imploded by an explosive shell. Thompson was brought in as a consultant, and discussed the problem with Neddermeyer in June Thompson was skeptical that an implosion could be made sufficiently symmetric. But Neddermeyer's efforts in July and August at imploding tubes to produce cylinders tended to produce objects that resembled rocks. Neddermeyer was the only person who believed that implosion was practical, and only his enthusiasm kept the project alive. Oppenheimer brought John von Neumann to Los Alamos in September to take a fresh look at implosion. After reviewing Neddermeyer's studies, and discussing the matter with Edward Teller , von Neumann suggested the use of high explosives in shaped charges to implode a sphere, which he showed could not only result in a faster assembly of fissile material than was possible with the gun method, but which could greatly reduce the amount of material required, because of the resulting higher density. Kistiakowsky's name was immediately suggested, and Kistiakowsky was brought into the project as a consultant in October The implosion project remained a backup until April , when experiments by Emilio G.
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