"Thin Man" was the code name for a proposed plutonium gun-type nuclear bomb that the United States was developing during the Manhattan Project. Its development was abandoned when it was discovered that the spontaneous fission rate of 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 the plutonium produced at the Clinton Engineer Works.
Early decisions
In 1942, prior to the United States Army taking over control of wartime atomic research in what became known as the Manhattan Project, Robert Oppenheimer held conferences in Chicago in June and Berkeley, California in July, at which physicists discussed nuclear bomb design issues. A gun-type design was chosen, in which two sub-critical masses of plutonium would be brought together by firing a "bullet" into a "target". The alternative idea of an implosion-type nuclear weapon was suggested by Richard Tolman, but it attracted scant consideration, being far more complex.
Oppenheimer reviewed his options in early 1943, and he gave priority to the gun-type weapon, but as a hedge against the threat of predetonation, 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. But it was decided that the plutonium gun-type bomb 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.
Naming
The gun-type and implosion-type designs were code-named "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 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 would be round and fat and was named after Sydney Greenstreet's character in The Maltese Falcon. The Little Boy uranium gun-type design came later and was named only to contrast with the Thin Man. Los Alamos's Thin Man and Fat Man code names were adopted by the United States Army Air Forces (USAAF). 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. Air Forces personnel used the code names over the phone to make it sound as though they were modifying a plane for Roosevelt and Churchill.
Development
To work on the plutonium gun design, Oppenheimer assembled a team at the Los Alamos Laboratory that included senior engineer Edwin McMillan and senior physicists Charles Critchfield and Joseph Hirschfelder. Critchfield had been working with sabots, which Oppenheimer believed would be required by the Thin Man to achieve the high muzzle velocities that critical assembly would require; Hirschfelder had been working on internal ballistics. Oppenheimer led the design effort himself until June 1943, when United States Navy Captain William Sterling Parsons arrived and took over the Ordnance and Engineering Division and direct management of the Thin Man project.
These four created and tested all the elements of the Thin Man design between April 1943 and August 1944. Parsons, who had developed the proximity fuze for the Navy, ran the division and handled liaison with other agencies. As the head of the E-6 Projectile, Target, and Source Group, Critchfield calculated critical masses, and instituted a system of live testing with scale models using 20 mm cannon and 3-inch guns. These were readily and easily obtained, while full-scale Thin Man tubes took months to produce. It was not possible to conduct tests with plutonium, as it was not yet available. Indeed, the actual physical characteristics of the metal were little more than educated guesses at this time.