Emilio Segrè Visual Archives

More than 30,000 photos of scientists and their work

University of Chicago H23

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University of Chicago H23

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Argonne National Laboratory, courtesy of AIP Emilio Segrè Visual Archives

This is one of a series of graphite test piles constructed to furnish basic information necessary for the proper design of CP-1, the world's first nuclear reactor. This type, called a sigma pile, made it possible for scientists to determine the neutroncapture
cross sections of the different grades of graphite to be used later in CP-1.

Unlike the exponential test piles also constructed to obtain experimental data, the sigma piles contained no uranium fuel. Neutrons were furnished by radium-beryllium sources. Measurements were made on indium-foil detectors suitably placed within the structures to reveal the degree of neutron absorption of the graphite being tested.

Sigma piles were constructed in 1942 under the West Stands of Stagg Field at The University of Chicago. On December 2, 1942, a group of scientists, led by the late Enrico Fermi, successfully completed the historic reactor experiment.

Original format: 
1 photographic print (black and white; 11 x 8.5 inches)