Nuclear Forensics Analysis: Reference Materials For Age Determinations

Several Standard Reference Materials (SRMs) of uranium from the National Bureau of Standards (NBS) made more than 60 years ago are in use even today, for calibration of instruments, as quality assurance/quality control standards for isotopic composition measurements, and as test material in performance evaluation programs. 235U abundances in these SRMs vary over a wide range, from 0.5% to 93%. These SRMs appear to be good candidates to serve as (working) reference materials for age determinations in nuclear forensics, if the production dates of these materials could be established. Towards this end, we studied the reports from the Union Carbide Nuclear Company on the production and purification of the material in the SRMs and can assign the production dates. The production consisted of withdrawing uranium hexafluoride at various levels of enrichment, from the cascades at the Oak Ridge Gaseous Diffusion Plant, purification of the hydrolyzed fractions, and final conversion to U3O8 - all accomplished during 1957-1959. The uranium isotopic composition and trace elements were also characterized during this period. The highly pure U3O8, is assumed to be free from daughter products from the decay of 238U, 235U, and 234U isotopes, on the production dates. Now, more than 60 years later, these SRMs contain known and measurable amounts of 230Th and 231Pa from the decay of 234U and 235U, respectively. Age determinations are therefore possible based on measurements of amounts of the parents (234U and 235U) and daughter isotopes (230Th and 231Pa). The NBS SRMs with assigned production dates and known amounts of parent and daughter isotopes (as a function of time) are therefore suitable to serve as (working) reference materials for age determinations of interdicted/illicit materials in nuclear forensic analysis. Model ages determined for some of the SRMs in this set, based on measurements of amounts of parents and daughters, agree with the ages reckoned from the assigned production dates. Details pertaining to the production, purification, and characterization of the material in the NBS SRMs will be presented.