Correcting for spatially dependent intrinsic efficiency on a germanium double-sided strip detector to improve nuclear forensics response

Purpose 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 detector’s ability for measuring heterogeneous sources in the near field, placed directly on the detector’s faceplate, to perform isotopic mapping for nuclear forensic missions.
Methods 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 eight different photons from 123 to 1274 keV emitted from a collimated ¹⁵⁴Eu photons. These were measured at 108 locations to interpolate the efficiency at any point on the detector’s face.
Results 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.
Conclusion The results on this research show that detection efficiency for a planar two-sided strip HPGe is spatially dependent and shows typical energy dependence. The spatial dependence, which does not have any additional energy dependence, can be corrected. A ¹⁵⁴Eu source was used in this research and was able to spatially calibrate for photon energies ranging from 100 to 1300 keV. This method is applicable for sources with higher gamma energies. The advantages of the method demonstrated that after initial in-laboratory calibration, a single measurement in the field can be used to efficiently calibrate the HPGe as a function of photon interaction on the crystal.