Prototype neutron-capture counter for fast-coincidence assay of plutonium in residues

A new neutron coincidence counter with improved capabilities is being built as a commercial prototype. This counter uses 6LiF/ZnS(Ag) based scintillation detectors to achieve a short neutron die-away time (τ < 5 (micro)s), which increases the sensitivity of the counter to measurements of 240Pu by neutron coincidence counting. Greater sensitivity is required for measurements of residues, whose large α,n-neutron yields create a high accidental coincidence rate in a counter with a relatively long τ (for example, ones that use 3He detectors). Traditional problems of gamma-ray sensitivity in scintillation detectors have been overcome through pulse processing techniques. Intrinsic differences of the excitation processes associated with neutron and gamma-ray interactions in the 6LiF/ZnS(Ag) detector allow these particles to be separated via pulse shape analysis (PSA). Degree of neutron/gamma-ray separation, factors affecting separation, neutron detection efficiency, and the effects of pulse shape and energy thresholds in a small test detector using a combination of commercially available and custom electronics are discussed. Final custom pulse processing techniques and PSA results are also presented.