Integrated monte carlo simulation and experimental approach for efficiency calibration of an HPGe detector using bulk environmental samples

Calibration of high-purity germanium (HPGe) detectors for bulk environmental samples is particularly challenging due to complex geometries, heterogeneous matrices, and coincidence summing effects. In this study, we address these issues by validating a detailed MCNPX model of an N-type HPGe detector against experimental measurements performed with a certified multi-gamma source (12ML01) encapsulated in a 0.45 L Marinelli beaker. The comparison between experimental full-energy peak efficiencies and simulations shows strong agreement, with deviations within 1.2% across the energy range of 59 to 1836 keV. These discrepancies are mainly linked to coincidence summing and uncertainties in dead-layer characterization. The validated model provides a reliable and flexible calibration strategy that extends beyond the limited lifetime of physical standards, offering a practical and cost-effective solution for routine environmental radioactivity monitoring.