Neutron-Activation Analysis of Actinides for Nuclear Forensics

Cyclic neutron activation analysis (CNAA) of fissile and fissionable actinides is being explored as a potential nuclear forensics tool. A sample of high-purity thorium-oxide powder was studied using an accelerator-based fusion neutron generator and a high-purity germanium detector. The goal of this work was to use the time-varying delayed gamma-ray intensities to measure the half-lives, and main gamma-ray-line ratios of observed short-lived fission fragments. With these parameters measured, a future objective will be to compare these measurements between different fissile and fissionable actinides to examine an alternative material-assay technique based on peak ratios. Simulations have shown that the large variations in fission yields of short-lived fission products can be magnified using short irradiation times (on the order of seconds) and counting of the irradiated targets within a few seconds after irradiation. This alternative assay technique is theoretically capable of characterizing the isotopic concentrations of small samples of actinide mixtures. The ultimate goal is an active non-destructive assay method capable of characterizing pre/postdetonation samples within a couple of hours. The data were acquired using signal digitization for time-differential spectral analysis. Irradiation timing control with a resolution of 100 milliseconds was used for each cycle to match the time-differential spectra and sum the data to improve the statistical certainty of short-lived fission product gamma-ray peaks. The data acquired from these experiments have been compared with published results to find that additional work is needed to measure the half-lives of short-lived fission products accurately, especially along the more complicated decay-chain isobars. In addition to these findings, a fast isotopically sensitive actinide-assay technique has been identified with applications in pre/post-detonation nuclear weapon and spent-fuel characterization. The paper discusses the sensitivity and potential of the CNAA technique for nuclear forensics.