ARGONNE NATIONAL LABORATORY / SCIENCE PHOTO LIBRARY ARGONNE NATIONAL LABORATORY / SCIENCE PHOTO LIBRARY
Record of the first nuclear chain reaction, which occurred on 2 December 1942 in the atomic pile built by Enrico Fermi & colleagues in a University of Chicago squash court. The plot records neutron intensity as a function of time. Just left of centre are 3 successive increases in neutron intensity, caused by progressive removal of the main control rod. Each rise levels off because the pile is still sub-critical. At far right is an exponential peak, with no levelling off. It marks the onset of criticality - the first chain reaction - at 3.36 pm. The peak declines sharply after insertion of the safety control rod.
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