Enrico Fermi - Italian Physicist
(1901-1954)
September 1941
At Columbia University, Fermi assembles his first nuclear pile in an attempt to create a slow neutron induced chain reaction in uranium. He fails, but realizes that with better materials, the chain reaction is possible.

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Fermi's Pile Design

The pile was simply a lattice of uranium or uranium oxide interspersed within graphite. The graphite slowed down neutrons which allowed the fission of U-235 to proceed quite readily. Some of the neutrons were captured by U-238 creating Pu-239. Fermi defined the reproduction factor "k" to allow him to determine whether a pile would become critical. "k" is the average number of secondary neutrons produced by one original neutron in the pile. "k" had to be greater than one for the chain reaction to proceed. The "k" value for this pile was 0.87.

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