Gamma-ray discrimination in a proton-recoil spectrometer for a fast reactor spectrum measurement

Studies have been made on gamma-ray discrimination for a proton-recoil neutron spectrometer with pulse-shape discrimination electronics for in-core measurements. A small proportional counter filled with hydrogen, in conjunction with a pulse-shape discrimination system, was used in experiments to examine the characteristics of gamma-ray discrimination in the energy region below 100 keV. Characteristics of the discrimination by two types of systems, using the “dividing” and the “rise time” methods respectively, were compared with each other under the same conditions. Distributions of relative specific ionization or rate of rise in various energy bands and gamma elimination ratios in the energy region from 5 to 30 keV are obtained. The results of the experiments indicate that the two methods are able to distinguish completely between neutron and gamma events above 40 keV and the gamma elimination ratios decrease rapidly at energies below 10 keV. The dividing method is observed to be slightly better than the rise time method with respect to the pulse-shape discrimination.