An Improved Technique For Passive Neutron Assay Through the Use of Extendable Dead Time and Higher Moments Analysis

The well known High Level Neutron Coincidence Counting technique for the non-destructive determination of plutonium in sealed packages has for long been plagued by complications arising from self-multiplication effects. We propose here an extension of the techniques developed recently by us for the measurement of the degree of subcriticality of highly subcritical systems in the field of reactor noise analysis, to the passive neutron assay problem. It is shown that deployment of a variable dead time filter of the extendable type rather than of the commonly adopted non-extendable type leads to a simple and practical method for measuring the mean, variance and higher moments of the multiplicity distribution of the neutron emitting source and deducing therefrom the spontaneous fission rate, the self-multiplication probability and other parameters. The applicability of the technique is demonstrated using a Cf-252 neutron source.