Monte Carlo simulation and experimental measurement of a nonspectroscopic radiation portal monitor for photon detection efficiencies of internally deposited radionuclides

Particle transport of radionuclide photons using the Monte Carlo N-Particle computer code can be used to determine a portal monitor's photon detection efficiency, in units of counts per photon, for internally deposited radionuclides. Good agreement has been found with experimental results for radionuclides that emit higher energy photons, such as Cs-137 and Co-60. Detection efficiency for radionuclides that emit lower energy photons, such as Am-241, greatly depend on the effective discriminator energy level of the portal monitor as well as any attenuating material between the source and detectors. This evaluation uses a chi-square approach to determine the best fit discriminator level of a non-spectroscopic portal monitor when the effective discriminator level, in units of energy, is not known. Internal detection efficiencies were evaluated experimentally using an anthropomorphic phantom with NIST traceable sources at various internal locations, and by simulation using MCNP5. The results of this research find that MCNP5 can be an effective tool for simulation of photon detection efficiencies, given a known discriminator level, for internally and externally deposited radionuclides. In addition, MCNP5 can be used for bounding personnel doses from either internally or externally deposited mixtures of radionuclides.