Year
2016
Abstract
Current passive neutron multiplicity counting (PNMC) assay methodology is based on the measurement of correlated rates up to the third order (Singles, Doubles and Triples) corrected for deadtime effects using semi-empirical approaches. Recent research at Los Alamos National Laboratory (LANL) has focused on extending the PNMC methodology towards higher order correlated rates, specifically correlated Quadruples and Pentuples (Quads and Pents) to provide additional observables for use in more complex analysis algorithms to assay plutonium items. In addition, the traditional PNMC analysis based on shift register electronics was expanded to include a variety of alternative gate-sampling approaches to extract plutonium characteristics that became possible, due to the flexibility of list mode data. This allows extraction of correlated multiplet rates up to Pentuples using different gating schemes including the traditional Feynman approach. In order to use the higher order correlated rates and the additional gate-sampling approaches, an advanced deadtime correction algorithm was needed to reliably correct for the effects of system deadtime. For this purpose, a novel Dytlewski-Croft-Favalli (DCF) deadtime correction algorithm, based on Dytlewski formulation, was developed. The DCF deadtime correction extends the deadtime correction to correlated rates up to Pents and provides a consistent methodology of deadtime correction for different gate-sampling techniques. The DCF method was implemented in a dedicated software tool to assist in evaluation of its performance. Results of experimental evaluation of the DCF deadtime algorithm up to Pents are presented for typical neutron multiplicity counters used in international nuclear safeguards applications.