Development and application of in-beam delayed neutron counting technique at the NRU reactor

A delayed neutron counting (DNC) system, where the sample to be analyzed remains stationary and its exposure to the neutron beam occurs inside the counting system, has been developed at the National Research Universal (NRU) reactor of the Canadian Nuclear Laboratories (CNL) at Chalk River. It is believed that DNC using a mono-energetic thermal neutron beam is a non- destructive and rapid approach that could enable non-destructive identification and quantification of fissile isotopes within a suspect concealed and shielded sample. Despite the orders of magnitude reduction in neutron flux, the in-beam DNC can be just as informative as the usual in-core DNC for most cases while offering practical advantages and mitigated risk when dealing with large radioactive samples of unknown origin. This paper addresses the application of in-beam DNC using a mono-energetic thermal neutron beam at the NRU reactor in conjunction with a proven detector system designed originally for in-core DNC. Initial results showed that the technique could reveal presence of few milligrams of ²³⁵U in a gamma shielded sample via the assay of in-beam DNC total counts, and could differentiate easily between ²³⁵U and ²³³U isotopes by analysis of DNC count rates. The lower flux offered in the in-beam DNC at CNL does not permit trace analysis, but permits non-destructive analysis of large or highly enriched samples. (author)