Year
2008
Abstract
We have developed a Modified Hydrogen Correction (MHC) based on the ratio of the measured Triples to Doubles rates as a complementary technique to the Add-A-Source (AAS) method of matrix compensation applied to coincident neutron counting. The approach is an evolution Los Alamos’s Hydrogen Correction Factor approach, which also exploits the Triples to Double ratio as a an internal diagnostic, but the algebraic form has been altered to better reflect the trends observed in experimental calibration data taken with the Box Neutron Assay System (BNAS). In this work we extend the method from the case of 200 drums to larger containers and comment on the potential role of the MHC in the routine application of the BNAS. The BNAS is a large volume, transportable, passive neutron coincidence counter intended to assay transuranic waste in the DOE complex. Its mission will begin at Savannah River Site (SRS) where it will form part of an integrated suite of instrumentation. The system was designed and factory calibrated to assay a variety of container types including 200 drums, the standard waste box (SWB), the standard large box (SLB-2) and the ten drum over-pack (TDOP). Factory calibration data was acquired by mapping the spatial response in the various containers systematically as a function of matrix severity using point-like 252Cf sources. The principle objective of the mapping procedure was to construct Volume Weight Average (VWA) responses so as to calibrate the multiposition AAS and also to formulate the associated allowance to the Total Measurement Uncertainty (TMU) coming from the (unknown) distribution of activity in the items being assayed. The mapping data was therefore optimized (in terms of source activity and count times) for the AAS calibration and this resulted in the observed Triples rates having poorer precision than would be desired for a study optimized to MHC. Non-the-less the data proved invaluable in examining the important trends and is unique for a system of this size. The advantage of using 252Cf, which we were able to make use of, lies in the fixed properties it offers in particular; no self-multiplication effects, a negligible (a, n) reaction rate and a definite spontaneously fissioning system. In the most challenging cases it was therefore possible to reasonably infer the Triples/Doubles ratio (T/D) from the Doubles/Singles (D/S) ratio with greatly improved measurement precision. This allowed for a more detailed examination of the MHC and its applicability. We present our results along with the engines designed for implementation.