Year
2004
Abstract
The High Efficiency Neutron Counter (HENC) has been employed by the Los Alamos National Laboratory (LANL) to assay plutonium waste using passive neutron methods since 1997. Over the last several years, measurement data have been acquired using three different methods: the add-asource (AAS) corrected neutron coincidence counting method that corrects the mass via a calibration constant and a matrix correction; the multiplicity analysis method that relates the singles, doubles, and triples neutron rates to mass; and the truncated multiplicity method that performs the same analysis as multiplicity but eliminates the higher multiplicity events. The AAS method requires a calibration constant to relate the count rate to mass, but both multiplicity and truncated methods use analytical solutions to solve for mass without the need for a calibration constant. However, their solutions suffer in that they require several assumptions regarding detector efficiency and neutron multiplication; a system of four unknowns must be solved with only three relationships to solve for mass. Since May 2002, matrix correction checks have been performed to assess the long-term stability of the Non-destructive assay (NDA) instrument’s matrix correction. At LANL, 208-L drum matrices of combustible, metal, and glass have been used with Pu sources in randomly selected positions. The sources spanned the mass range of 3 to 200 g. This has permitted the evaluation of bias and precision of the NDA instruments. This study will present the results of the matrix correction checks for all three modes of the HENC assay and summarize the waste assays performed since 2000. The AAS method is generally the preferred method of operation but multiplicity can provide information when AAS fails and truncated multiplicity can be combined with AAS to permit better assays at the lower mass range.