NEUTRON EFFECTS ON RADIOISOTOPE IDENTIFIERS (RIIDs)

摘要

We are engaged in a multi-year program to evaluate detection and identification (ID) capabilities of COTS RIID instruments. The framework of our testing program, conducted for DTRA, is briefly described in a companion paper in this conference. In the course of evaluating the performance of RIIDs, we sometimes subject them to unusual test conditions, either to simulate a possible application scenario, or simply for the sake of completeness. Neutrons are occasionally present in our test scenarios. In this report we focus on a single phenomenon that is visible under certain test conditions-gamma rays arising from neutron interactions with the test environment and with the RIIDs themselves. Depending on the test conditions, these neutron-induced gammas can add unexpected lines to the spectral signature of the test source. One such example is the 847-keV line generated from neutron interactions with steel, a shielding material frequently used in our testing. This photopeak occurs in a region of the test spectrum where the underlying background is relatively low. Thus it is visible, and at times even prominent. Recently, we have identified spectral lines that arise from neutron interactions with three detecting materials-HPGe, Nal, and LaBr_3. The lines of primary interest occur in the 50 - 300 keV range, and have led to incorrect identification reports from RIIDs. For example, a line at 139.9 keV from ~(75m)Ge arises from neutron interaction with ~(74)Ge (as reported by others earlier), and has been incorrectly identified by HPGe-based RIIDs as due to ~(99m)Tc, which has a primary line at 140.5 keV. Similarly, a line at 57.6 keV arises from neutron inelastic scattering by ~(127)I (per earlier reports by others), and has been frequently confused by Nal-based RIIDs with the 59.5-keV line of ~(241)Am. Neutron interactions with LaBr3-based RIIDs produce a complex spectral structure, but to date only a line at about 166 keV has led to incorrect reports (of ~(201)Tl). Our application-oriented examination of neutron effects on RIIDs has focused on understanding the origin of such extraneous lines in instrument test spectra, and not on the associated nuclear structure phenomenology, which others have examined. Our goal is to reduce incorrect RIID identifications and to facilitate appropriate instrument advances by the respective manufacturers.