Optimized Monte Carlo modeling of HPGe detector efficiency with true coincidence summing corrections using PHITS and MCNP5.

This study presents an optimized Monte Carlo model of an ORTEC GEM-series p-type coaxial high-purity germanium detector using PHITS and MCNP5, achieving mean relative differences in full-energy peak efficiency of approximately 2% across source-to-detector distances of 0 to 9 cm and gamma-ray energies from 59 keV to 1332 keV. By iteratively refining two critical geometric parameters (front dead-layer thickness optimized to 1.3 mm and crystal-to-endcap window distance optimized to 6.5 mm), the model successfully accounts for manufacturing tolerances and long-term dead-layer growth commonly observed in operational detectors. True coincidence summing effects in cobalt-60 were accurately reproduced for point and small-disk source geometries using PHITS' native correlated-emission capability and, alternatively, by applying correction factors derived from P-TReCK-TCS Monte Carlo software to conventional MCNP5 simulations; both approaches yielded nearly identical correction factors (average discrepancy approximately 0.3%) despite differences in geometric detail. The validated model provides a flexible, cost-effective alternative to repeated experimental calibrations with physical standards, enabling reliable efficiency predictions for various sample geometries and radionuclide mixtures in environmental monitoring, nuclear safeguards, and high-precision gamma-ray spectrometry applications.