An approach to multiplicity counting for a versatile new sensor for plutonium assay with a very short die-away time, and independent measurements of neutrons and gamma rays

A unique detector design incorporating a {sup 6}Li-based capture medium, ZnS scintillator, and wavelength shifting optical fibers is the basis of a new neutron coincidence counter for measurements of plutonium in highly-impure residues. The sensor elements have a high efficiency for detecting neutrons and exhibit excellent gamma-ray discrimination based on pulse-shape analysis. The short die-away time of the counter that is based on these detector elements allows coincidence-gate settings shorter than 10 microseconds. This qualifies the technology for measurements of materials with high yields of uncorrelated neutrons from {sup 241}Am(alpha,n) reactions. The characteristics of the new neutron counter will be illustrated with test data from measurements of plutonium, {sup 252}Cf, and gamma-ray sources. The integrated electronics design of the new detector also permits the simultaneous but independent measurement of both neutrons and gamma rays. Recent test results that illustrate some unique applications of the sensor's versatility will also be presented