Performance of Energy Window Ratio Criteria at Radiation Portal Monitoring Sites

Radiation portal monitors are being used to detect radioactive materials in vehicles transporting cargo. As vehicles pass through the portal monitors, they generate a count profile versus time that can be compared to the average panel background counts obtained just prior to the time the vehicle entered the area of the monitor. Pacific Northwest National Laboratory has accumulated considerable data regarding such background radiation and vehicle profiles from portal installations. Various alarm criteria have been established to maintain sensitivity to targeted radioactive sources while also controlling to a manageable level the rate of false or nuisance alarms. False alarms are statistical anomalies while nuisance alarms occur due to the presence of naturally occurring radioactive material (NORM) in cargo, for example, kitty litter or ceramics. Discussion topics include: • Use of plastic scintillators to separate gamma counts into energy windows. • The utility of using ratio criteria for the energy window counts rather than simply using total window counts. • Detection likelihood for the various decision criteria based on computer simulated injection of sources into vehicle profiles. • Determination of when ratio criteria will or will not prove useful • Potential benefits of multi-window based algorithms.