Estimating the Singles Background Rate During a Passive Neutron Correlation Counter Assay

Passive neutron time-correlation counting makes use of various auto-correlation counting rates, for example, in the case of multiplicity counting these are the singles, doubles and triples rates, derived from the neutron detector pulse train. In an operating nuclear facility the singles background rate at the time of an assay performed with a passive neutron correlation counter may differ markedly from that at the time of system set-up. The magnitude of the difference will depend on the amount and location of special nuclear material in storage nearby and possibly also on the prevailing cosmic ray intensity which can influence ambient neutron production in and around the detector. The background corrected singles rate could consequently be biased and this will impact the quality of the assay result for those analysis modes (known-α and multiplicity) that rely on using the singles rate in the inversion of the point-model equations. A means to estimate the singles background rate during the assay is therefore desirable especially for weak sources involving long counts where a background performed before and after would considerably reduce item throughput. We derive an expression to predict the singles background count rate at the time of a passive neutron time-correlation count performed with a multi-ring neutron detector. The method makes use of the inner-to-outer ring ratio and requires a background measurement during a quiescent period to be recorded as well as a reference measurement using a representative item. The approach will be most useful when low mass items requiring a long assay time are needed.