Ionization Chambers to Determine Neutron and Gamma-Ray Kerma in a Research Reactor

Ionization chambers were designed and constructed to determine the kerma rates in various materials within several centimeters of a Training, Research, Isotopes, General Atomics (TRIGA) reactor core operating at 1 MW. The primary aim of this article was to compare kerma measurements with the advanced Monte Carlo code calculations of nuclear heating. Wall thickness, collection gap, and fill gas pressure were chosen to satisfy Bragg-Gray criteria, so that the measured ionization current was related to the kerma rate in the wall material. Chamber wall materials composed of low mass number elements, including hydrogen-rich C552 air-equivalent plastic and beryllium, were selected to measure the kerma due to fast neutron elastic scattering. By operating these neutron sensitive chambers coincidentally with relatively neutron insensitive chambers composed of aluminum and Zircaloy-4, we were able to measure the total heating due to fast neutrons and gamma rays in a material and to differentiate these heating components. A chamber composed of borated stainless steel was used in a similar fashion to measure the thermal neutron flux. The total kerma rate was also measured in various materials typically found in a reactor core. Chamber collection volumes were initially determined using ambient air fill gas and National Institute of Standards and Technology (NIST)-traceable air-kerma rates from a
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Co source. All chambers were sealed with argon gas to provide thermal and compositional stability. Chamber properties, including stability, saturation, and gas-phase mass subject to charge collection, were determined using the
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Co source. Chambers were operated for approximately 30 min adjacent to the reactor core, and the integrity of gas seals was subsequently verified by repeating the measurement with the
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Co source.