INITIATORS AND POLONIUM

In both the implosion and gun-type bomb designs, the basic idea was to take a stable, subcritical assembly of fissile material, and then use conventional explosives to rapidly create a critical assembly, at which point a neutron would trigger a chain reaction. However, if a neutron triggered a reaction before the entire core was brought together, the bomb would fizzle, and ideally the chain reaction would start with a burst of many neutrons rather than just one, so as to speed up the chain reaction and thereby increase its power. An initiator is a devise designed to produce this burst of neutrons on demand, and designs for including an initiator in atomic bombs began early in the Manhattan Project.

In April 1943, Robert Serber proposed incorporating an initiator into the gun-type bomb as "extra insurance" of a critical reaction. The design of the implosion bomb initiator was largely the work of Hans Bethe. Ultimately his design used a combination of polonium, a strong alpha particle emitter, and beryllium, which absorbed alpha particles and emitted neutrons. The polonium and beryllium would come into contact during implosion and would then trigger a fission chain reaction.

Even once designed, producing a functioning initiator required a great deal of research, experimentation, and work to scale up production to necessary levels. Small amounts of polonium were available and had been a popular neutron source in scientific laboratories before the war, but nowhere near enough polonium of necessary purity was available for initiator production. Oppenheimer identified two possible sources for polonium: separation from lead residues and production in nuclear piles. Pile production meant bombarding large quantities of bismuth, which in turn had to be pure enough to not contain radioactive antimony and thereby become a health hazard for workers. Work began on this process at the Clinton pile. Meanwhile, lead dioxide at the Port Hope refinery in Ontario, Canada provided a second source for polonium after a chemical separation process.

Charles Allen Thomas, chemist and research director of the Monsanto Chemical Company, took over polonium production responsibilities in July 1943. Monsanto facilities in Dayton, Ohio served as the initial location for the difficult (and at that point largely unknown) chemical processes necessary to separate, purify, and make use of polonium. This Dayton Project quickly expanded to other facilities. Working with polonium proved difficult, but the Dayton team managed to provide sufficient polonium for initiator production throughout the war. As the conflict wound down, initiator production itself was shifted from Los Alamos to a new, underground facility near Dayton, where it would remain for decades to come.

Back in Los Alamos, Kenneth Bainbridge headed an Initiator Committee in charge of designing and building initiators capable of withstanding rough handling, accidental drops, airplane vibrations, and the shock of firing. Bainbridge, working in the Ordnance and Engineering Division, agreed with Oppenheimer that design should focus on designs at the 20-mm scale, as this was the scale of readily available anti-aircraft guns. Charles Critchfield took over theoretical design, making enough progress in the late winter and early spring of 1944 for the committee to settle on a design in June. Real-world experiments began as soon as construction finished on a 20-mm lab attached to Building B at Los Alamos, and on March 15, 1945, Oppenheimer officially agreed to incorporate the initiators into the gun gadget.

Initiator design for the plutonium-based implosion device required redesign and new experimentation. The theoretical division under Hans Bethe joined with the initiator group under the direction of Critchfield to tackle this problem in January 1945. Realistic experiments were impossible, as neither the precision explosives nor active plutonium were available. Small-scale models did allow some testing, however, if under considerably less violent circumstances than would be the case in an actual nuclear detonation. The final design depended on polonium and beryllium which, upon being hit by the explosive shock wave, would mix and emit a burst of neutrons. Much more polonium was necessary for these tests and for the final design than was expected, and Oppenheimer had to send an emergency order to Thomas in Dayton for increased shipments in February 1945. The Dayton Project was able to meet this increased order, and the final initiator design and components were ready for the assembly of the implosion device for the Trinity test by March 1945.

• Gun-type Design
• Implosion Design
• Initiators and Polonium
• High Explosives
• Fuzes and Detonators
• Hydrogen Bomb

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