Otto Frisch and Rudolph Peierls - German/English Physicists
(1904-1979)(1907-1995)
March 1940
Working in Birmingham, Frisch (on left) and Peierls consider fast neutron fission of Uranium-235. They calculate that a critical mass of only about one kg is needed and that the resulting explosion would be tremendous.

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Critical Mass
The amount of fissionable material
needed to support a chain reaction.
Frisch's and Peierl's estimate turned out to be a bit optimistic. The actual values of critical masses were determined latter at Los Alamos and are listed below. Scientists discovered that the critical mass could be reduced by the use of a thick casing of natural uranium which surrounded the critical mass, or core. This tamper, as it was called, reflected neutrons back into the core and helped to hold the core together for a fraction of a second while the chain reaction progressed.

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