A detailed investigation of HPGe detector response for improved Monte Carlo efficiency calculations

Monte Carlo simulations using MCNP5 software have been performed in order to estimate the full-energy peak efficiency of an HPGe detector. Since a discrepancy was found between Monte Carlo calculated values and empirical data, a thorough investigation of the detector response has been made. Radiography has been used to ascertain the physical dimensions of the actual crystal, the thickness of the “dead layer” has been determined, the detector was scanned using a collimated source to check the homogeneity of the detector response over the crystal surface, and the possible effect of a weak electrical field was tested. It was concluded that the lower efficiency of the real detector compared to the MCNP model was most likely due to a dead layer thicker than stated by the manufacturer. The dead layer was estimated to be approximately 1.5 mm, about twice the nominal value. This results in both a thicker absorbing layer as well as a decreased active crystal volume, and is sufficient to explain the observed decrease in detector efficiency.