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Fission-Neutron Age Measurements in Aluminum-Water Mixtures

J. D. Spencer, T. G. Williamson

Nuclear Science and Engineering

Volume 27 / Number 3 / March 1986 / Pages 568-572


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The age of fission neutrons was measured to the indium resonance energy for light water and three metal-to-water volume ratios (1/4, 1/2, 2/3) in aluminum- water mixtures. A large plane highly enriched 235U fission plate provided the source of neutrons. Aluminum plates (0.127 × 122 × 122 cm) oriented parallel to the source plane were used for the metal. Indium foil activities were taken in planes parallel to the source and integrated over these planes to obtain the equivalent activities that would result from an infinite source. From these data, the ages for the four cases were: M/W = 0, (26.24 ± 0.33 cm2); M/W = 1/4, (32.28 ± 0.50 cm2); M/W = 1/2, (39.96 ± 0.50 cm2); M/W = 2/3, (44.88 ± 0.59 cm2). To investigate heterogeneous effects, the plates were lumped to simulate 0.635-cm-thick plates and the measurements repeated for the 2/3 metal-to-water ratio. The measured age was 44.50 ± 0.49 cm2, indicating no lumping effect in this measurement. The data for the pure water measurement were also analyzed by applying geometric corrections for the finite size of the source plate. This resulted in a measured age of 27.82 ± 0.66 cm2, which agrees with previous measurements utilizing this method of data analysis.

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