Validation of a Monte Carlo model for a GMX detector used for measurements of environmental radioactivity

In an Environmental Radioactivity Laboratory, samples from several products are analyzed, in order to determine the amount of radioactive products they contain. A usual method is the gamma activity measurement of these samples, which typically requires the use of High Purity Germanium Detectors (HPGe). GMX (n-type) detectors can be found among this group of detectors. They have a high efficiency for low energy emissions. As any detector, it must be calibrated, in energy, efficiency and resolution (FWHM). To do this calibration, a gamma standard solution is used, whose composition and activity are certified by a reference laboratory. This source contains several radionuclides, providing a wide energy spectrum. The simulation of the detection process with MCNP5, a code based on the Monte Carlo method, is a useful tool in an Environmental Radioactivity Laboratory, since it can reproduce the experimental conditions of the essay, without manipulating radioactive sources, and consequently reducing radioactive wastes. On the other hand, the simulation of the detector calibration permits to analyze the influence of different variables on detector efficiency. In this paper, the simulation of the calibration of the GMX detector used in the Environmental Radioactivity Laboratory of the Polytechnic University of Valencia (UPV) is presented. Results obtained with this simulation are compared with laboratory measurements, in order to validate the model.