Year
1999
Abstract
Thermal-Neutron Multiplicity Counters (TNMCs) assay 240Pu-effective mass, isolating spontaneous-fission (SF), induced-fission, and (a,n) neutrons emitted from plutonium metal, oxide, scrap, and residue items. Three independent parameters are measured: single, double, and triple neutron-pulse-coincidence count rates. TNMC assays can become precision limited by high (a,n) neutron rates arising from low-Z impurities, and 241Am. TNMCs capture thermal neutrons in 4-atm 3He tubes after fast-source-neutron moderation by polyethylene. TNMCs are -50% efficient with -509.~~ die-away times. Simultaneously increasing efficiency and reducing die-away time dramatically improve assay precision. Using 1 0-atm 3He tubes, we’ve developed and performancetested 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-ps dieaway time. The ENMC detects neutrons before thermalization using higher 3He 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 240Pu-effective, ENMC assay times are reduced by factors of 5 to 21, compared with prior state-of-the-art TNMCs. The largest relative gains are for the most impure items, where gains are needed most. Benefits of using lo-atm 3He tubes in conventional neutron coincidence counters, both active and passive, are described in a companion paper. 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, with performance very competitive with calorimetry. This paper will describe the ENMC, present results of characterization, calibration and verification measurements, and show the clear performance and economic advantages of implementing the ENMC for nuclear materials control and accountability.