Performance of an HPGe-based and NaI-based Radionuclide Identifier (RIID) in the Identification of Radioactive Materials

The process for detection and interdiction of illicit trafficking of radioactive and nuclear materials relies on Portal Monitors or hand-held instruments to first detect, then identify any radioactive material located in the shipment. The high number of medical, industrial and Naturally Occurring Radioactive Materials (NORM) in legitimate transit gives rise to frequent innocent alarms, that is, alarms on non-threatening nuclides. In order to allow these shipments to pass and the illicit material to be stopped, the nuclides must be accurately and rapidly identified. The RIID (Radionuclide Identifier) is used to identify the nuclides in the shipment as the second step in the interdiction process. Recent emphasis in the design and testing of RIIDs has been on accurate identification of nuclides in order to reduce the false positive rate and the innocent alarm rate. In previous work, the search mode of an HPGe-based RIID was compared with a NaI-based RIID for neutron and gamma-ray sources. In this work, the performance of the same two instruments in the identification of radionuclides is compared. The ANSI N42.34 requirements are the basis for the tests. Previous work also showed the performance of a HPGebased RIID with mixtures of nuclides. Some of the single nuclides and certain mixtures of nuclides for both shielded and unshielded sources in the standard were used. Due to the short halflives of the medical isotopes, they were not used in these tests. Results comparing the performance of both systems, with the standard and with each other will be given. The results show the HPGe system provides the correct identification more rapidly and on less spectral data than the NaI system and that it easily exceeds the ANSI N42.34 requirement to identify the nuclides in the shipment accurately and quickly.