Rousing the dragon: Polonium production for neutron generators in the Manhattan Project

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. (C) 2019 American Association of Physics Teachers.