Determination of virtual point for HPGe detector at various gamma rays energies by simulation and experimental methods

High Purity Germanium detectors (HPGe) are subdivisions of semiconductor detectors which are widely used in nuclear technology from space industry to nuclear medicine, due to their high resolution, low dead time, unlimited size and compatibility with a variety of environments. The( absolute and intrinsic) efficiency of the HPGe detector, which depends on the geometry of the source-detector system and also on the energy of gamma ray, is an important factor in determining the activity of the radioactive sources. A simpler way to determine the activity of radiation sources is to use the virtual point intrinsic efficiency which is independent of source-detector distance. The virtual point is a point within the detector that all interactions are assumed to take place in that point. The goal of this study is to determine the location of the virtual point of the HPGe detector of GMX 40P4-76 model and determine the virtual point intrinsic efficiency for this detector at various energies of gamma rays. To do this, we first experimentally obtained the spectrum of gamma rays emitted from a Europium source at different distances from the detector using the HPGe spectroscopy system. Similarly, using the Monte Carlo simulation method (MCNPX code) we simulated the spectroscopy system. In this study, in the first step, the virtual point distances from detector top were determined in two experimental and simulation methods for gamma ray energies from 121 keV  up to 1408 keV, the results of which were in good agreements. In the second step, using the virtual point distances, we calculated the virtual point intrinsic efficiencies at various gamma ray energies for the HPGe detector, and it was shown that this efficiency for a given energy  is almost constant with small deviations for various source-detector distances and it is therefore easier to calculate the activity of unknown sources using this efficiency, because the distance dependency is gone and we do not need to perform  measurements at different distances.