Pulse Train Analysis Applied to the Re-Evaluation of Deadtime Correction Factors for Correlated Neutron Counting

摘要

Temporally-correlated neutron counting, including the passive neutron coincidence counting (PNCC) and passive neutron multiplicity counting (PNMC) techniques, is widely used at nuclear fuel cycle facilities for the non-destructive assay (NDA) of plutonium (Pu). Correlated event rates are used to quantify mass values of spontaneously fissile nuclides and derive total Pu mass. These methods are limited in accuracy by uncertainty in the deadtime correction. A pulse train analysis method has been developed and applied to the re-evaluation of deadtime correction factors for correlated neutron counting. The Monte-Carlo transport code MCNPXTM was used to generate a time-stamped list of neutron captures in 3He. Event times were processed in software to create neutron pulse trains akin to list mode data. The action of multiplicity shift register (MSR) electronics was modeled in software to analyse these pulse trains. Prior to MSR analysis, stored pulse trains could be perturbed in software to apply the effects of deadtime. In this work, an updating (paralyzable) deadtime model was chosen to replicate existing theoretical approaches to deadtime correction. Traditional deadtime correction methods for temporally-correlated neutron counting have been found to be accuracy limiting in cases where highly correlated rates occur over a short coincidence gate width i.e. high instantaneous rates associated with high multiplicity bursts. Here, empirical results are presented which support the development of an alternative formulism for both the traditional Singles and Doubles deadtime correction factors for PNCC. Deadtime effects are found to be dependent on the level of correlation in the pulse train yet independent of gate fraction, which is set by the shift register gate structure, for Singles deadtime correction factors. Doubles deadtime correction factors were found to have a slight dependence on gate fraction. Research work was conducted at the University of Birmingham, UK in close collaboration with Canberra Industries, Inc., USA.