Design of a Fiber/Scintillator Neutron-Capture Counter to Assay Impure Plutonium

Computational and experimental evaluations of two new, commercially manufactured neutron detector elements are being performed. The purpose of this assessment is to compare calculated and experimental results for a detector concept that uses stacks of thin layers of neutron-capture scintillator with plastic fibers to achieve a high-efficiency (e), short die-away time (t) neutron coincidence counter with gamma-ray discrimination. Ribbons of 1.) a variablediameter 10B-loaded scintillating plastic fibers and 2.) a 6Li/scintillator wavelength shifting plastic fibers were the detector elements for well-counter design studies. These computational studies were used to explore whether assemblies of these elements could achieve the performance required for a new generation of neutron counters that assay plutonium in residues with high, uncorrelated neutron yields that destroy the detection sensitivity of conventional coincidence counters. The conventional technology uses polyethylene-moderated 3He detectors with physically distinct gaseous neutron-capture and solid polyethylene-moderator media. Shorter die-away times, achieved by homogenizing neutron-capture targets (10B or 6Li) with the moderating media, permit shorter coincidence-gate widths reducing accidental coincidence rates proportionally. Agreement between experimental and computational determinations of fissionneutron detection efficiency is demonstrated. Practical assemblies of both types of experimental detector elements achieve the required e (comparable to 3He counters) and t (5%-20% that of 3He counters). A prototype well counter using the most promising elements is being built commercially.