Thin Man (nuclear bomb)  "Thin Man" was the code name for a proposed plutonium gun-type nuclear bomb using plutonium-239 which the United States was developing during the Manhattan Project. They aborted its development when they discovered that the spontaneous fission rate of their nuclear reactor-bred plutonium was too high for use in a gun-type design, due to the high concentration of the isotope plutonium-240. In 1942, prior to the Army taking over wartime atomic research, Robert Oppenheimer held conferences in Chicago in June and Berkeley, California, in July at which various engineers and physicists discussed nuclear bomb design issues. A gun-type design was chosen, in which two sub-critical masses would be brought together by firing a "bullet" into a "target". The idea of an implosion-type nuclear weapon was suggested by Richard Tolman but attracted scant consideration. Oppenheimer, reviewing his options in early 1943, gave priority to the gun-type weapon, but as a hedge against the threat of pre-detonation, he created the E-5 Group at the Los Alamos Laboratory under Seth Neddermeyer to investigate implosion. 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. 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 more easily adapted from it. The gun-type and implosion-type designs were codenamed "Thin Man" and "Fat Man" projects 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" would be a very long device, and the name came from the Dashiell Hammett detective novel "The Thin Man" and series of movies by the same name. The "Fat Man" bomb would be round and fat and was named after Sydney Greenstreet's character in "The Maltese Falcon". "Little Boy" would come last and be named only to contrast to the "Thin Man" bomb. Los Alamos's Thin Man and Fat Man code names were adopted by the United States Army Air Force. A cover story was devised that Silverplate was about modifying a Pullman car for use by President Franklin Roosevelt (Thin Man) and United Kingdom Prime Minister Winston Churchill (Fat Man) on a secret tour of the United States.