T. Jacomb-Hood, “Spatially-Resolved HPGE Gamma-Ray Spectroscopy for Nuclear Forensics”, M.S. Thesis, Nuclear Engineering, Texas A&M University, College Station, TX (2020).
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 detectors ability for measuring heterogeneous sources in the near field, placed directly on the detector’s faceplate, to perform isotopic mapping for nuclear forensic missions. The intrinsic efficiency is strongly dependent on where the photons interact within the germanium. The efficiency varies by up to 20% within the sensitive volume of the detector. The efficiency was mapped using a collimated beam of 154Eu photons measured at 108 locations in order to interpolate the efficiency at any point on the detector’s face. 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. To better understand experimental results, the detector has been simulated with ANSYS Maxwell R18.2 to model the electric fields and a custom charge particle transport code to determine the charge collection as a function of gamma-ray interaction location. The simulations have 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. This feeds into the known spatially dependent intrinsic efficiency previously measured within the GeGI detector. By improving the sub-strip event localization, the ability to quantify a source is directly improved.