Improved Performance of Energy Window Criteria Using Multiple Windows at Radiation Portal Monitoring Sites

Radiation portal monitors are being used to detect radioactive target materials in vehicles transporting cargo. As vehicles pass through the portal monitors, they generate count profiles over time that can be compared to the average panel background counts obtained just prior to the time the vehicle entered the monitor. Pacific Northwest National Laboratory, in support of U.S. Customs and Border Protection (CBP) and U.S. Domestic Nuclear Detection Office (DNDO) under the U.S. Department of Homeland Security (DHS), has accumulated considerable data regarding such background radiation and vehicle profiles from portal installations. Energy window criteria have been shown to increase sensitivity to certain types of target radioactive sources while also controlling to a manageable level the rate of false or nuisance alarms. First generation equipment had only two-window capability, and while energy windowing for such systems was shown to be useful for detecting certain types of sources, it was subsequently found that improved performance could be obtained with more windows. Second generation equipment instead has more windows and can thus support additional energy window criteria which can be shown to be sensitive to a wider set of target sources. Detection likelihoods are generated for various sources and energy window criteria, as well as for gross count decision criteria, based on computer simulated injections of sources into archived vehicle profiles.