Spatially-resolved HPGe Gamma-ray Spectroscopy for Nuclear Forensics

The Germanium Gamma-ray Imager (GeGI) is a planar high purity germanium (HPGe) imaging detector developed by PHDS Co for far-field imaging. This research investigates the GeGI’s ability to measure heterogeneous sources in the near field, placed directly on the detector’s faceplate, to perform isotopic mapping useful to nuclear forensic missions. The intrinsic efficiency is strongly dependent on where the photons interact within the germanium. The efficiency was mapped using a collimated beam of 154Eu photons measured at 108 locations spanning the detector’s face. This data set was then fit with a univariate quadratic function to interpolate the efficiency at any point on the detector’s face. This allows the efficiency at any location to be calculated, which is paramount for making measurements with the most accuracy possible. The position and energy dependence are uncorrelated and thus the absolute efficiency at any position and for any gamma-ray energy can be calculated by the convolution of the spatial and energy efficiencies. Also, after an initial in-laboratory calibration, the field calibration can be reduced to a single measurement. The efficiency varies by 6-20% within the sensitive volume of the detector. To better understand experimental results, the GeGI was simulated with ANSYS Maxwell R18.2 to model the electric fields and a custom charged particle transport code to determine the charge collection as a function of gamma-ray interaction location. The simulations resulted in the creation of a dimensionless number, ψ, that is linear and unique as a function of the induced currents on neighboring electrodes. Using ψ, the location of the event at a sub-strip level can be calculated with greater precision than is possible with the GeGI’s base software. This feeds into the known spatially dependent intrinsic efficiency previously measured within the GeGI. By improving the sub-strip event localization, and using the correct positon dependent efficiency, the ability to quantify a source is improved by up to 20%