Signatures for several types of naturally occurring radioactive materials.

Detectors to scan for illicit nuclear material began to be installed at various screening locations in 2002. On the sites considered, each vehicle drives slowly by radiation detectors that scan for neutron and gamma radiation, resulting in a time series profile. One performance limitation is that naturally occurring radioactive materials (NORM), such as cat litter, are routinely shipped across borders, leading to nuisance alarms. One strategy for nuisance alarms is to define and recognize "signatures" of certain types of NORM so that many nuisance alarms can be quickly resolved as being innocent. Here, we consider candidate profile features, such as the peak width and the maximum energy ratio, and use pattern recognition methods to illustrate the extent to which several common types of NORM can be distinguished.