|Fig. 1: Uranium gun-type weapon and plutonium implosion weapon designs. The Thin Man was designed to be a plutonium gun-type assembly weapon. (Source: Wikimedia Commons)|
In the context of nuclear weaponry, "Thin Man" refers to the plutonium, gun-type nuclear fission bomb design that was first proposed in the United States as an initiative of the Manhattan Project.  The design was proposed in competition with the alternative "Fat Man" design that relied on implosion.  After over a year of design and testing from 1942 to 1944, the development of the Thin Man nuclear bomb was ultimately abandoned due to multiple design flaws that made further testing and development unfeasible. 
The gun-style design of the Thin Man bomb relied upon two subcritical masses of plutonium coming into contact with each other at very high speeds. One component would serve as a "bullet" and the other as a "target". The specifications of such a design were initially discussed over the course of two conferences in the summer of 1942 organized by Robert Oppenheimer in Chicago, Illinois and Berkeley, California.  A uranium based gun-type weapon design and the alternative implosion weapon design can be seen in Fig. 1.
The two subcritical bullet and target components of the Thin Man were made of Pu-239. The bullet would have to be launched at 3,000 feet per second into the target in order to avoid predetonation.  The bomb was designed to be 17 feet long in order to allow for such high velocities. An initiator was added to the design of the Thin Man in order to ensure detonation would occur by releasing a burst of neutrons to trigger the chain reaction.  Po-210-Berylllium was eventually chosen as the initiator for the Thin Man. 
The design of the Thin Man bomb posed two primary challenges. First, the immense length of the bomb necessary to allow the gun-type mechanism to function properly made it difficult for any aircraft to carry.  As a result, modified B-29 aircrafts with oxygen tanks set between the bomb bays removed and a portion of the bulkhead removed were used for testing of the device at full scale.  Second, and perhaps more significantly, it was discovered in the spring of 1944 that Pu-239 "had an unusually high spontaneous fission rate."  The unexpectedly high rate "increased the neutron background enough to make it highly probable for a gun-assembled gadget to predetonate and thus undermined the plutonium gun program."  The speeds necessary to avoid predetonation were such that the gun barrel would need to be too long for any current or future bomber to be able to carry the bomb. As a result, the Thin Man design was abandoned.
The challenges facing the design of the plutonium, gun-style Thin Man bomb ultimately proved too much, as the development of the weapon was abandoned on July 14, 1944.  Researchers then turned their efforts towards developing the implosion bomb.
© Samuel Premutico. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 L. Hoddeson et al., Critical Assembly: A Technical History of Los Alamos During the Oppenheimer Years, 1943-1945 (Cambridge University Press, 1993).
 J. Xu, "The Atomic Bomb," Physics 241, Stanford University, Winter 2016.
 C. Hansen, Swords of Armageddon (Chukelea Publications, 1996).