A long-term project in our research on nuclear forensics is focused on developing techniques to discriminate the reactor origin and irradiation history of chemically separated plutonium. Ideally, these forensic techniques may be applied in the investigation of trafficked or proliferated special nuclear material to help determine its origin. One key to this research has been our collaborations with the Center for Nuclear Security Science and Policy Initiatives in the Department of Nuclear Engineering at Texas A&M, especially former Prof. Sunil S. Chirayath and then Prof. Craig M. Marianno.
Over a period of years, we have developed a maximum likelihood methodology which compares simulated fission product isotope ratios (modeled under a variety of irradiation conditions) to a set of measured fission product isotope ratios in actual irradiated samples. The application of this forensic methodology requires the modeling and simulation of nuclear reactors using a Monte Carlo radiation transport code (MCNP6), as well as various radiochemical dissolutions, separations, and assays by radiation detection and mass spectrometry. This work culminated in the analysis of a sample that has multiple signatures consistent with being part of the Manhattan Project! This is sample is likely one of the oldest samples of reactor-produced plutonium yet identified.
More recently, our group has been working on applying these techniques to new challenges. We have begun to study how to apply nuclear forensics to the element radium, which was used for luminescent watch dials and other purposes in the early 20th century. The radium was mixed with zinc sulfide, which caused the glow when struck by an alpha particle. This is a particularly difficult matrix, and special techniques are required for sample dissolution and assay of the radium and its radioactive decay products. We have also begun to study whether fission products can be used to used to determine the irradiation history of americium. In addition, we are studying whether high-resolution mass spectrometry can be used to speed up the assay of irradiated samples. There is significant processing required with any sample, and a major goal of modern nuclear forensics is to accelerate these analyses. This work is benefiting from our participation in the Nuclear Science and Security Consortium based at UC Berkeley and the Consortium for Nuclear Forensics based at the University of Florida.
Students working on these projects would likely find themselves handling samples of radioactive fission products and actinides. Hands-on work is performed with irradiated uranium dioxide as well as plutonium. Analytical separations including liquid-liquid extractions and multiple forms of chromatography are performed frequently. Mass spectrometry and gamma spectrometry are typically favored as analytical tools to assay metal content on the order of 0.01-1 ppb.