Terrestrial background reduction in RPM systems by direct internal shielding

Radiation detectors are often placed in positions difficult to shield from the effects of terrestrial background. This is particularly true in the case of radiation portal monitor (RPM) systems, as their wide viewing angle and outdoor installations make them susceptible to terrestrial background from the surrounding area. This issue will become even more important as the next generation of spectroscopic-capable RPMs is deployed. A low background is desired in most cases, especially when the background noise is of comparable strength to the signal of interest. Previous modeling work by Pacific Northwest National Laboratory has shown the enhanced signal-to-noise ratio (SNR) and isotope identification sensitivity possible through direct internal shielding, as well as some benefits of collimation and direct ground shielding. The gross-count sensitivity of a polyvinyl toluene (PVT)-based RPM detector or the isotope identification capability of a sodium iodide (NaI)-based detector will be significantly enhanced through the use of dense background shielding, as close as possible to the detector elements. In this work, the problem of shielding a generalized RPM from terrestrial background is considered. Detector and shielding scenarios are modeled with the Monte Carlo N-Particle (MCNP) computer code. Amounts of nominal-density shielding needed to attenuate the terrestrial background to varying degrees are given, along with optimal shielding geometry to be used in areas where natural shielding is limited, and where radiation detection must occur in the presence of natural background. Guidance is provided as to the enhancement possible to detection with realistic field modifications. Common shielding solutions such as steel plating are evaluated based on the SNR, and benefits are weighed against feasibility.