Accurate geometry-dependent efficiency calibration of High-Purity Germanium (HPGe) detectors is critical for the applications in low radiation level \(\gamma\) -spectrometry, rare decay studies, and assessments of environmental radioactivity. At IIT Ropar, a low-background facility, ILMI (IIT Ropar Low-background Measurement Infrastructure) is being developed to feature an array of coaxial p-type HPGe detectors with 40% relative efficiency. These detectors can operate in coincidence and are shielded with radiopure lead to minimize environmental background interference. This work presents a comprehensive spectrometric characterization and dead layer assessment of an HPGe detector, GEM40P4-83-RB/ORTEC, which is housed in a radiopure carbon fiber. The detailed Monte Carlo simulation in GEANT4 was used to model the detector response across a wide energy range (80.99-2614.50 keV). The methodology was validated by comparing the simulated results with experimental data using various gamma sources, including \(^{109}\) Cd, \(^{137}\) Cs, and \(^{60}\) Co, over a range of source-to-detector distances. Excellent agreement has been observed between simulated and experimental efficiencies, with a deviation of 2.5% for close distances (less than 5 cm) and 3.8% for farther distances (5 cm or greater). These minor discrepancies may be attributed to excluding the electronic acquisition chain effects in the simulation or photon scattering from the environment in actual experiments. The validated simulation model demonstrates high reliability and predictive accuracy, establishing a robust methodology for determining efficiency calibration in complex detector geometries.
