Year
2000
Abstract
The gamma-ray multi-group analysis code MGA developed at Lawrence Livermore National Laboratory has been widely used in the area of gamma-ray nondestructive plutonium assay. This plutonium isotopic analysis code de-convolutes the complicated, 100-keV x ray and gamma ray region to obtain the ratio of Pu isotopes. Calibration of the detector efficiency is not required, but is determined intrinsically from the measured spectra. The code can either analyze low-energy gamma-ray spectrum taken using a high-resolution HPGe detector for energies below 300 keV, or analyze the lowenergy spectrum combined with a high-energy spectrum (up to 1 MeV) in the “twodetector” analysis mode. In the latter case, the use of two detectors has been mandated by the conflicting requirements: excellent resolution at low energies (characteristic of small planar detectors) with good high-energy efficiency (characteristic of coaxial detectors). Usually, a high-energy spectrum taken using a coaxial Ge detector will not provide sufficient energy resolution for 100-keV plutonium isotopic analysis, while the small planar used at low energies has inadequate high-energy efficiency. An optimized-geometry ORTEC HPGe detector has been developed which combines good energy resolution at 100 keV combined with acceptable high-energy (~1MeV) efficiency in a single detector. It has been used to gather spectra of both lowand high-energy regions of plutonium spectra simultaneously, for analysis by MGA in the “two-detector” mode. Five Pu gamma-ray calibration standard sources were used in this study of this special detector.