Improving Calibration of HPGe Detectors for In Situ Measurements – a Comparison of Semi-empirical and Monte Carlo Methods

Calibrations of an HPGe detector were made with respect to photon energy and angle of incidence for the use in field gamma spectrometry. The same detector was modelled in MCNP4C and simulated. This revealed some significant differences between the theoretical (simulated) efficiency and the measured efficiency of the detector, especially at lower energies. In order to investigate the reason for this several modifications to the model were tested, e.g. increasing the dead layer of the crystal or reducing its active length. None of these alterations however proved entirely satisfactory. The detector was therefore scanned along its side and front with collimated Am and Eu sources to check its response. The count rates of the 59.54 keV and 244.67 keV peaks were recorded at 2 mm intervals, and compared with results from MCNP simulations of the setup. It was found that the active volume of the detector appears to be somewhat smaller than its physical dimensions, and most likely the dead layer is thicker than what is stated by the manufacturer.