Characterization of the response of a PHDs GeGI to enable quantitative assay of nuclear materials*

Passive nondestructive assay of the presence and/or amount of nuclear or radiological materials present in an item is a key technique for nuclear safeguards, forensics, and consequence management. Portable gamma-ray detector systems, like PHD’s GeGi High-Purity Germanium (HPGe) gamma-ray imager are used for onsite screening for some of these missions but are not well understood when analyzing complex nuclear materials quantitatively. Characterizing the full energy photo-peak (FEP) efficiency curve and the cascade summing corrections are key to accurate quantitative analysis when using portable spectrometers. This is typically done in concert with energy calibration across a wide range using standardized radioactive sources.

Monte Carlo simulations were performed using G4ARES, a Geant4 framework for nuclear particle transport, to model FEP efficiency curves with key parameters of the HPGe crystal and surrounding materials. A variety of experimental data gathered from these Geant4 simulations was compared to the models utilizing a data processing framework developed using CERN’s ROOT toolset, and the detector's parameters were optimized until the efficiency curves agreed within uncertainties.