Impact of non-uniform dead layer distribution on efficiency calibration for volumetric sources in HPGe detectors

Accurate characterization of the HPGe detector dead layer is crucial for reliable Monte Carlo simulations in gamma-ray spectrometry. This study investigates the impact of source geometry on the determination of the effective dead layer thickness. Experimental measurements and MCNP6 simulations were conducted using a p-type coaxial HPGe detector with three geometries: a cylindrical source (S1), a 3π source (S2), and a Marinelli beaker (S3). Results show that while the physical dead layer distribution is intrinsic to the crystal, the determined effective dead layer parameter is strongly dependent on the irradiation geometry. The on-axis source (S1) indicated a stable frontal dead layer of approximately 0.60 mm. Conversely, volumetric sources (S2 and S3) revealed a thicker lateral dead layer (∼1.10–1.21 mm) with a significant transition zone at low energies due to the geometric weighting of photon interactions. Crucially, the study demonstrates that applying a uniform dead layer thickness derived from point-source calibration (S1) to volumetric geometries (S3) results in a systematic overestimation of efficiency across the energy range. These findings highlight the inadequacy of uniform dead layer models for complex geometries and the necessity of multi-region characterization for high-accuracy environmental monitoring.