Checking low level wastes requires the detection of sub-milligram levels of plutonium. This is an extreme challenge for passive neutron coincidence counting as the levels of true signal are close to the background signal caused by cosmic ray interactions. The origin of cosmic-induced background is discussed. We describe the Hexagon 2000 passive neutron coincidence counter that has been designed and built for waste quality checking of low-level radioactive waste in the UK for the Environment Agency. The instrument uses computed neutron coincidence counting and improved filtering of high multiplicity cosmic-induced events. The instrument also correlates background signal with atmospheric pressure to further increase sensitivity. Preliminary measurements of background data and of test drums containing small amounts of plutonium are described.
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