During the Manhattan Project, the rare element polonium (Po) was synthesized by neutron bombardment of bismuth in nuclear reactors for use in neutron-generating triggers that were used to initiate the fission chain reactions in the Hiroshima and Nagasaki bombs. In this paper, I explore the physics of why polonium was used, why the triggers contained the amount of polonium that they did, and for how long a given mass of bismuth had to be irradiated in the Oak Ridge X-10 pile or a Hanford plutonium-production pile to generate a given amount of Po. A simple neutron-flux model gives results in reasonable accord with estimates recorded in a 1943 letter written by Robert Oppenheimer.

1.

Oppenheimer's letter to Groves can be found in National Archives and Records Administration, Bush-Conant File Relating to the Development of the Atomic Bomb, 1940–1945, Records Group 227. The microfilm publication number is M1392, and the letter appears on Roll 9. The files can be purchased from the NARA on DVD; on the Roll 9 disk acquired by this author, the letter comprises images 992 and 993.

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