USING THE FRAM ISOTOPIC ANALYSIS CODE IN THE REAL WORLD

The primary task of the TA-18 Early Move Project was to fully characterize over 1000 Category I and II items for custodial transfer from Los Alamos National Laboratory to the Nevada Test Site. More than 100 of these items required isotopic verification measurements in order to adequately document their isotopic mass distribution. The FRAM Isotopic Analysis Software (version 4.2) was used to analyze the spectral data that were collected with high-purity germanium detectors. The vast majority of these measurements were on pieces of highly enriched uranium (HEU) metal that had been used in criticality research at the Los Alamos Critical Experiments Facility (LACEF). This paper discusses several real world challenges that were encountered during this campaign. The project was under significant time pressure to characterize all the required materials during the 2005 fiscal year. Therefore, we were compelled to limit our count times to the bare minimum. In order to maximize throughput, we frequently employed two detectors at a time, one planar and one 50% coaxial, (on some days a 150% coaxial was substituted for the 50% coaxial). Different detectors required the analysis of different spectral regions with a variety of parameter files. The facility where the measurements were performed had a high background from both uranium isotopes and naturally occurring radionuclides. This made it essential to properly orient and shield the detectors, and also to perform an accurate background subtraction on the spectra prior to analysis. Another challenge involved the variable amounts of 232U impurities in the HEU. These impurities produce spectral photopeaks from 228Th daughters that FRAM uses to help create its relative efficiency curve (212Pb at 238.6 keV, 212Bi at 727.2 keV, and 208Tl at 583.0 and 860.3 keV). Some of the HEU items were virtually free of these impurities, thus prompting the need to modify the parameter file. Some of the 232U-free spectra also had strong interferences from 95Zr/95Nb fission products at 766 keV, thus prompting the need for further modifications to the parameter file. The many lessons learned from this project include discussions about case-by-case modifications to parameter files, determining proper count times and detector dead times, and the use of Cd filters.