Larger Volume Detector Efficiency Parameterization with the MGA Code

The Multiple Group Analysis (MGA) code is widely used for the non-destructive determination of the relative isotopic composition of plutonium items. The MGA method is based solely on the analysis of the gamma-ray spectrum recorded using a high resolution gamma spectrometer. The form and function of the code has evolved greatly since its origins in the 1980’s. Over the lifetime of the code there have been tremendous developments in detector technology. The small volume planar Low Energy Germanium (LEGe) detectors available for safeguards work in the early years have given rise to the use of large volume Broad Energy Germanium (BEGe) detectors today in many applications - including the challenging arena of nuclear waste assay. Certain algorithms within MGA have been hardened to meet this challenge and the code is now more robust to poor counting statistics and poorer pulse height resolution as a consequence. The universe of applications to which MGA is being applied is constantly growing. These applications frequently step outside the initial expectations of the code and/or of the technical boundaries of the day. In particular we address here the functional form used within MGA to describe the energy dependence of the relative full energy peak efficiency curve of the Ge detector. The relative efficiency curve is represented by the product of three terms. The filter attenuation factor and self-attenuation factors together modify the basic response function of the Ge crystal. This in turn is parameterized in terms of the active volume of the crystal. At the time MGA was conceived it was envisioned as a safeguards tool to be used exclusively with LEGe detectors. The parameterization was therefore based on this assumption and the maximum detector volume that could be entered was 20 cm3. In waste assay applications it is now routine to use large BEGe detectors for reasons of higher sensitivity. BEGe detectors have volumes up to about 150 cm3. We have reviewed the database of BEGe detectors characterized at our facility to derive an appropriate parametric form for this class of large volume detector. The impact on MGA performance is considered.