Baseline suppression of vehicle portal monitor gamma count profiles: A characterization study

Radiation portal monitor (RPM) systems based upon polyvinyl toluene scintillator (PVT) gamma-ray detectors have been deployed to detect illicit trafficking in radioactive materials at border crossings. This report sets forth a characterization of the baseline suppression effect in gross-count gamma-ray profiles due to shadow shielding by vehicles entering RPMs. Shadow shielding is of interest because it reduces the alarm sensitivity of RPMs. This observational study investigated three types of PVT-based commercial RPM systems currently deployed at selected ports of entry in terms of spatial effects relative to detector panel positioning. Radiation portal monitor sites were characterized by driver versus passenger side, top versus bottom panel, and narrow lanes versus wide lanes as observed for a large number of vehicles. Each portal site appears to have a distinctive baseline suppression signature, based on percent maximum suppression relative to measured background. Results suggest that alarm algorithms based on gross-counts may be further refined through attention to individual site characteristics. In addition, longer vehicle transit times were often correlated with stronger baseline suppression, suggesting that baseline suppression studies should take transit time into account.