Re-characterization of two n-type planar HPGe detectors based on experiments and Monte Carlo modeling

Accurate gamma-ray spectrometry using High Purity Germanium (HPGe) detectors relies on precise knowledge of detector efficiency, particularly the full energy peak efficiency (FEPE), which can degrade over extended periods of operation due to the growth of the detector's inactive dead layer (DL). Despite this known issue, DL correction is rarely performed in long operating detectors, and manufacturer specifications are often taken as fixed. This study emphasizes the gap by presenting a combined experimental and Monte Carlo modeling approach to re-characterize two planar n-type HPGe detector systems - Sys1 and Sys2 - that have been in continuous operation for nearly three decades. Although the detectors share virtually identical geometrical designs, they exhibit differences in performance, attributed primarily to variations in DL thickness. Gamma-ray measurements using certified point reference sources (59.5-1332.5 keV) were conducted, and corresponding Monte Carlo N-Particle (MCNP5) simulations were performed to evaluate the effect of DL variation on FEPE and on the active detector volume. Additionally, MCNP was used to apply source activity corrections, taking into account geometric and attenuation effects. The results offer a validated framework for modeling and correcting FEPE losses in aging detectors. By optimizing the DL thickness for Sys1 and Sys2 to 2.77 and 2.82 mm, respectively, the deviation between the simulated and experimental FEPE was reduced to below 5 %. Beyond nuclear instrumentation, this work has implications for any application requiring high-accuracy spectrometry over long operational timelines, including nuclear safeguards, radioactive waste assay, environmental monitoring, radiation metrology, and nuclear forensics.