Prototype Neutron-Capture Counter for Fast-Coincidence Assay of Plutonium in Residues

A new, 4-sided detector for counting neutrons in coincidence is being built as a commercial prototype. It has a short neutron die-away time (= < 5 ?s), which increases its sensitivity to measurements of 240Pu by neutron coincidence counting. Greater sensitivity is required for measurements of aged, impure oxide or certain residues with large ~,nneutron yields that create a high accidental coincidence rate in a counter with a relatively long = (for example, one that uses 3He detectors). The high ƒ-ray sensitivity of bulk scintillators (compared to 3He detectors) limits their use in neutron coincidence counting. The new detector is composed of alternating layers of a scintillator/6Li mixture and optical ribbons for light transport. This design provides limited (ƒ-ray/neutron) discrimination capability based on pulseheight analysis (PHA). However, a two-parameter analysis of pulses obtained using 252Cf and 137Cs sources with a 20- layer detector element shows that the discrimination capability of pulse-shape analysis (PSA) greatly exceeds that of PHA. The PSA is based on the decay time of the pulses from the detector. The decay time of pulses produced by charged particles from the 6Li neutron-capture reaction exciting the scintillator is more than an order of magnitude longer than that from ƒ-ray interactions in the scintillator. The PSA method measures the decay time of the pulse and produces an analog pulse whose amplitude is proportional to it. A neutron event will produce a large PSA pulse while a gamma-ray event produces a smaller PSA pulse. The height of the PSA pulse is independent of the height of the pulse produced in the detector. We are designing electronics for processing the detector pulses by PSA. A challenge in this design is the long component of the multi-component decay of neutron pulses. A high detection efficiency is required for neutron coincidence counting. Therefore, the detectors must be able to count at high rates, particularly for measurements of high-~,n materials. To achieve the required high count-rate performance, the long decay time of neutron pulses must be addressed electronically to minimize the effects of pileup in the short-= counter. Data for the 20-layer detector element will be presented.