Cyclic Neutron-Activation Analysis of Actinides for Nuclear Forensics

摘要

Cyclic neutron activation analysis (CNAA) of fissile and fissionable actinides is being exploredas a potential nuclear forensics tool. A sample of high-purity thorium-oxide powder was studiedusing an accelerator-based fusion neutron generator and a high-purity germanium detector. The goalof 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 parametersmeasured, a future objective will be to compare these measurements between different fissile andfissionable 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 canbe magnified using short irradiation times (on the order of seconds) and counting of the irradiatedtargets within a few seconds after irradiation. This alternative assay technique is theoreticallycapable of characterizing the isotopic concentrations of small samples of actinide mixtures. Theultimate goal is an active non-destructive assay method capable of characterizing pre/postdetonationsamples 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 matchthe time-differential spectra and sum the data to improve the statistical certainty of short-livedfission product gamma-ray peaks. The data acquired from these experiments have been comparedwith published results to find that additional work is needed to measure the half-lives of short-livedfission products accurately, especially along the more complicated decay-chain isobars. In additionto these findings, a fast isotopically sensitive actinide-assay technique has been identified withapplications in pre/post-detonation nuclear weapon and spent-fuel characterization. The paperdiscusses the sensitivity and potential of the CNAA technique for nuclear forensics.