Evaluating cosmic-ray backgrounds in HPGe gamma spectrometry: Experimental observations and GEANT4 simulations

This study demonstrates the efficacy of employing the GEANT4 simulation toolkit for investigating the background spectra of High Purity Germanium (HPGe) detector induced by atmospheric cosmic rays. GEANT4 is utilized to accurately replicate the experimental setup, detector geometry, and the gamma background spectrum for the HPGe detector. The EXPAC model is used to generate above-ground-level cosmic-ray particles for the terrain. Various techniques, such as the use of low-activity lead and oxygen-free copper as shielding materials, along with an external plastic scintillator to veto cosmic rays from the atmosphere, are employed to improve the signal-to-background ratio of HPGe detector systems. Photopeaks associated with ⁴⁰K, ⁶⁰Co, ²³²Th and ²³⁸U decay chain were observed in the measured background gamma spectrum. The integral count rates of the experimental and simulated background spectrum in the 20-1750 keV energy range were 1.92 ± 0.06 s^-1 and 1.84 ± 0.14 s^-1, respectively. The experimental and simulated background spectra match well, both qualitatively and quantitatively, indicating that GEANT4 is an effective simulation tool for predicting gamma spectra of HPGe detectors accurately.