Potential Benefits from Detection of Neutrons via Gamma Spectra of RIIDs

One of the primary goals of our multi-year test and evaluation (T&E) program for radioisotope identifiers (RIIDs), conducted in support of DTRA, is assisting manufacturers in improving their instruments. As part of this program, we routinely examine spectral data from testing for insight into unexpected features as well as for clues to RIID performance deficiencies. The presence of occasional spectral lines not attributable to either test sources or NORM backgrounds was observed during early testing phases and these are often a nuisance since they can interfere with analysis of the detected spectra. These lines turned out to arise from neutron interactions with nearby materials (such as shielding or support structures) and with the RIIDs themselves (gamma sensors and housing materials). Examples of these lines are the 847 and 1248 keV from 56Fe(n,n’), 58 and 203 keV from 127I(n,n’) in NaI, 139 keV from 72Ge(n,?) in HPGe, and 478 keV from 10B(n,a). Nuclear data for these reactions are well-known and show that their cross sections are neutron-energy sensitive. We presented our earlier observations in the 2011 INMM conference and the present interpretations agree with other published reports. In this report we seek to take advantage of this “nuisance phenomenon” by exploring its potential for indicating the presence of neutrons and providing other information of possible interest to users, such as indications of materials in the vicinity of the neutron-emitting source. Most of the tested RIIDs have an option to include dedicated neutron detectors and associated electronics. The RIIDs that include a dedicated neutron channel typically outperform the conversion approach in neutron detection. However, even when available as an option, some RIIDs may not include a neutron channel due to the additional cost, size or weight, and the conversion technique may be appropriate to determine the presence of neutrons. We present results on potential configurations that may provide a neutron-detection capability via the gamma spectrum. In addition to reducing cost, size and weight of an instrument, this approach can also help characterize a neutron- emitting source and its surroundings.