Thermal neutron multiplicity counters (TNMCs) assay 240Pu-effective mass, isolating spontaneous-fission (SF),induced-fission, and (～,n) neutrons emitted from plutonium metal, oxide, scrap, and residue items. Three independentparameters are measured: single, double, and triple neutron-pulse-coincidence count rates. TNMC assays can becomeprecision limited by high (～,n) neutron rates arising from low-Z impurities and 241Am. TNMCs capture thermal neutronsin 4-atm 3He tubes after fast-source-neutron moderation by polyethylene. Conventional TNMCs are ~50% efficient with~50-←s die-away times. Simultaneously increasing efficiency and reducing die-away time dramatically improves assayprecision. Using 10-atm 3He tubes, we’ve developed and performance-tested the first of a new generation of neutronassay counters for a wide range of plutonium items. The Epithermal Neutron Multiplicity Counter (ENMC) has anefficiency of 65% and a 22-←s die-away time. The ENMC detects neutrons before thermalization using higher 3Hepressure counters and less moderator than TNMCs. A special insert raises efficiency to 80% for small samples. For fivebulk samples containing 50 to 875 g of 240Pu-effective, ENMC assay times are reduced by factors of 5 to 21, comparedwith prior state-of-the-art TNMCs. The largest relative gains are for the most impure items, where gains are neededmost. In active mode, the ENMC assay times are reduced by factors of 5 to 11, compared with the Active WellCoincidence Counter. The ENMC, with high precision and low multiplicity dead time (37 ns), can be used in standardsverification mode to precisely and accurately characterize plutonium standards and isotopic sources. The ENMC’sperformance is very competitive with calorimetry. This report describes the ENMC family; presents results ofcharacterization, calibration, and verification measurements; and shows the clear performance and economic advantagesof implementing the ENMC technology for improved nuclear materials control and accountability.
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