Performance of a portable HPGe-based Nuclide Identifier on Multiple Nuclide Mixtures

An important part of the Homeland Security defense against illicit nuclear trafficking is the ability to identify the specific nuclides causing a radiation portal alarm in order to differentiate between innocent radionuclides and controlled radionuclides. After a portal monitor has detected radiation emanating from a person, a package or a vehicle, the usual next step is to employ a Handheld Radio-Isotope Identifier (HHRID) to identify the source of the radiation by gammaspectroscopic means. For a HHRID to be useful, it must yield a low number of false positives (reporting a nuclide that is not present) and an even lower number of false negatives (not reporting a nuclide that is present). The ANSI N42.34 standard specifies more than 20 individual nuclides to be identified and a 133Ba with Pu mixture. In the IAEA specification1 for Hand Held Radioisotope Identifier 4 additional nuclide mixtures are listed which must be accurately identified. The Detective is a HHRID based on a HPGe detector cooled by a small mechanical cooler. The HPGe detector is comparable in efficiency to existing NaI-based instruments, but, due to its very much higher resolution, is considerably more sensitive and effective at identifying the source of the radioactivity. The spectrum analysis is performed in the internal microprocessor and reported on the built-in display. Because of the excellent resolution of the HPGe detector, good separation of the gamma-ray peaks used in the nuclide identification is achieved. The Detective has been tested with the mixtures given in the ANSI standard and IAEA specifications as well as other mixtures over a range of counting times and shielding conditions. Results will be presented that show the accuracy of the result with respect to false positives and false negatives. The real-time analysis algorithm will be shown to report results in a fraction of the time required by both the ANSI and IAEA standards.