The Use of Ring Ratios to Detect Sample Differences in Passive Neutron Counting

In passive thermal neutron counting, the detection efficiency of a neutron counter is susceptible to changes in the neutron energy spectrum emitted by a sample. Variable levels of moisture and impurities in samples can cause such changes and thus affect the accuracy of an assay. In this work, we use Monte Carlo simulations of thermal neutron measurements in the four-ring pyrochemical multiplicity counter to study the effect of sample-tosample changes in the emitted neutron spectrum on the ratios of total neutron counts from the different rings. We also present ring ratio data obtained in the pyrochemical multiplicity counter for several pure and impure plutonium bearing samples.