Systematic BIAS Estimates of the SNAPTM Analytical Technique in Measurement Trials of Known PU Standards In Mockup Drums At Los Alamos National Laboratory

A study was conducted at the Los Alamos National Laboratory (LANL) to evaluate the performance of the SNAPTM (Spectral Nondestructive Assay Platform) analytical software for measurements of known plutonium, standards in twenty-five 55-gallon certification drums. The trials were designed to test the accuracy and precision of SNAPTM measurements as well as to probe the lower limit detection for PU-239. The overall measurement bias of the SNAP analysis process was -8.8% with standard deviation of 27.8%. As expected, the measurement precision varied inversely with total Pu-239 mass. The lower limit of detection for PU-239 was estimated as nominally 10 to 15 nCi/g in drums with approximately 35 to 65 kg matrix (15-min count time).

Two contributors to the negative bias were determined after the known values of the Pu standards were revealed to the analysis team. The wrong angular response correction for the detector-shielding configuration was used in the assay. This caused a -6.4% bias in all the results. A second source of systematic bias was caused by inadequate modeling of the attenuation losses suffered by source gamma-rays as they traveled through the source matrix. Although the source matrix is just a small amount of diatomaceous earth, the total attenuation losses appear to be 12.6% of 129.3 keV gamma-rays and 8.3% of 413.7 keV gamma-rays

Author (s): Steven C. Myers, Kathleen Gruetzmacher, Roland Bustos and Scott Ferran

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