We have developed a high-efficiency neutron detector for passive neutron coincidence and multiplicity counting of spontaneous fission neutrons. The detector uses 10-atm ~3He tubes to increase the efficiency and to shorten the neutron die-away time. We have achieved an efficiency of 80percent and a die-away time of 18.8 mus by inserting a small sample coincidence counter (INVS) in the sample cavity of the Epithermal Neutron Multiplicity Counter (ENMC). The ENMC/INVS is designed for high-precision measurements of small inventory samples with diameters less than 50 mm. To make accurate background corrections for room neutrons, we use the ratio of the counts in the outer ring of detectors to the inner ring to give a real-time measure of the room background neutrons.
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