a b s t r a c tWe report the performance characteristics of a water-based neutron detecting multiplicity counter for the non-destructive assay of fissile sources. This technique could replace or supplement existing 3 Hebased multiplicity counters. The counter is a 1.02 m 3 tank containing pure deionized water doped with 0.5% GdCl 3 . It has highly reflective walls and eight 10-in. PMTs mounted at the top. An unshielded source well of 19 cm diameter, mounted at the top and center, extends 73 cm down into the detector. The counter was evaluated using low intensity 252 Cf and 60 Co sources, and a fast pulsing LED to simulate higher intensity backgrounds. At low gamma ray intensities ( $ 200 kBq or less) we report an absolute neutron detection efficiency of 28% and a 60 Co rejection/suppression factor of $ 10 8 to 1. For sources with high gamma ray intensities, the neutron efficiency was 22% 71% up to a 60 Co equivalent activity of 4 MBq. The detector background event rate, primarily due to muons and other cosmogenic particles, was found to be stable over a period of almost three months. The minimum detectable neutron source intensity above background was 3.1 n/s, assuming a one-hour data acquisition.

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