Optimisation of the dead layer of an aged p-type HPGe detector used in environmental measurements with computational modelling

The gamma spectrometric laboratory of Environmental Assessment division, IGCAR, Kalpakkam, uses P-type high-purity germanium (HPGe) detector for the assessment of environmental radioisotopes due to their high resolution. The Monte Carlo (MC) simulations are used to determine the full energy peak (FEP) efficiency of the detector. But studies reveal that the simulated results are not completely in line with the experimental data. An exact knowledge of the detector geometry is required to eradicate the erroneous results of FEP efficiency calculated by the MC simulations. The manufacturer can provide the exact geometry of the detector that has been procured recently. But the detector that is being used in our laboratory had been procured in the year 2007. In such case of aged detectors, the dimension of the dead layer (DL) at the germanium surface does not match with the manufacturer's information. An initial MC detector model has been developed with the DL value mentioned by the manufacturer (1 mm) for the determination of FEP efficiency. It has been observed that the computed FEP efficiency has been decreased to a maximum of 17%. From the simulations, it is evident that the reduction in FEP efficiency is due to the increase of the thickness of the dead layer of the detector that is not wrapped up by the manufacturer. With the progressive increase of the germanium dead layer thicknesses in the detector model, a good conformity has been obtained between the simulated and measured efficiencies for 2 mm thickness, in comparison to 1 mm provided by the detector manufacturer. The variation in the relative deviation between the experimental and the simulation for 2 mm DL thickness lies within + 6% for the wide range of gamma energies in the case of natural uranium and thorium series. (author)