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

Ionization chambers were designed and constructed to measure the kerma rates (heating) in various materials within several centimeters of a TRIGA reactor core operating at 1 MW. Wall thickness, collection gap and fill gas pressure were chosen to satisfy Bragg-Gray criteria, so that measured ionization current across the collection gap was proportional to the kerma in the wall material. Chamber wall materials comprised 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 relative heating due to neutrons and gamma rays in a material. Chamber collection volumes were initially determined using NIST-traceable kerma rates and a cobalt-60 source. All chambers were sealed with argon gas to provide thermal and compositional stability. Chamber collection gas mass, stability and saturation curves were determined using the
⁶⁰
Co source. Chambers were operated for approximately 30 minutes and the integrity of gas seals was subsequently verified by repeating the measurement with the
⁶⁰
Co source.