The influence of dead layer thickness increase on efficiency decrease for a coaxial HPGe p-type detector

The present work aims at studying the decrease in an HPGe p-type detector efficiency over its operation period and the influence of the dead layer thickness increase on this efficiency decreasing process. The study was conducted with the Canberra coaxial GC1518 p-type detector using MCNP5 code. It is found that the efficiency of this detector has reduced by 18.5% over its 13 years of operation, corresponding to the increase of dead layer thickness from 0.35 to 1.46 mm. The relative reduction in efficiency linearly depends on the dead layer thickness and both these parameters are linearly grown versus operation time. The increase of the dead layer thickness causes two effects influencing the decrease in efficiency. They are the additional shielding effect of increase in dead layer thickness (shielding effect) and the effect of decrease in germanium active volume (volume effect). It is found that for 59 keV gamma energy, the efficiency decrease is determined only by the shielding effect whilst the volume effect can be negligible. As for 511–1332 keV gamma energies, the efficiency decrease is determined by both the volume and the shielding effects.