Radiation portal monitors response to gamma radiation and to the detection capability of Orphan radioactive sources: Contribution of the Strass project

Radiation portal monitors are commonly used to detect and intercept unauthorized movement of nuclear and other radioactive materials at country borders. A total of twelve double-pillar portal monitors are present at the Greek-North Macedonian border, each containing two polystyrene scintillating detectors per pillar. Spatial and spectral response testing of the scintillating detectors to gamma radiation was performed by using different radioactive sources and comparing the measurement results with Monte Carlo simulations. A good agreement of the experimentally deduced activities of different point sources, needed for alarm triggering of the radiation portal monitors with Monte Carlo calculated values, was observed. Spectral results show no photopeaks in the spectra due to low resolution of these detectors. The broad peaks observed in the spectra correspond to the Compton edge. Measured spectra with a 137Cs source placed directly on the scintillating detector, at several positions away from the photo multiplier tube, show an energy shift of the Compton edge towards lower energies, as the source is moving away from the photo multiplier tube. The energy shift is due to light transfer mechanisms within the scintillator volume and therefore, it is only observed in optical simulations and not in gamma-ray particle simulations.