New Shift-register Electronics for Improved Precision of Neutron Coincidence and Multiplicity Assays of Plutonium and Uranium Mass

Active and passive neutron-coincidence counting (NCC) is used routinely for nondestructive assay (NDA) of uranium and plutonium, in many forms. Passive neutron multiplicity counting (PNMC) is used routinely for NDA of plutonium scrap and waste. We’ve developed and extensively tested new electronics that significantly improves precision of double and triple coincidences, and multiplicity assays. Using both a time-correlated pulser and neutron sources, we’ve simulated PNMC assays (PNMAs) for a wide variety of plutonium samples. Plutonium-240-effective masses ranged from 0.1 to 200 g. The important parameter α [ratio of (α,n) to spontaneous-fission neutrons] varied from 0 to 10. We’ve also tested one circuit (Fast Accidentals Sampling or FA) on Pu-oxide standards. For sources and standards measurements, representative the majority of NCC and PNMC applications, FA precision reductions relative to conventional multiplicity shift-register (CMSR) circuits, are 20-29% for doubles, and 24–31% for multiplicity assays. For Pu-effective masses of 50–100 g, FA gains are roughly independent of α. For α =0, FA gains are roughly independent of Pu-effective mass. FA sampling has been implemented in the Advanced Multiplicity Shift Register (AMSR). The AMSR, relative to CMSRs, reduces doubles measurement times by factors of 1.6 to 2.0. The reduction for PNMC assays is by factors of 1.7 to 2.1. Testing of FA sampling on plutonium was done using an integrated system: the Epithermal Neutron Multiplicity Counter (ENMC), the commercial AMSR-150, and the general-purpose international NCC software package, INCC ver. 4.00. FA sampling also significantly improves measurement precision for active NCC assays of uranium. The new electronics reduces the need for high efficiency, and therefore cost, of neutron coincidence and multiplicity counters.