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

No abstract