230Th-234U MODEL-AGES OF SOME URANIUM STANDARD REFERENCE MATERIALS.

The "age" of a sample of uranium is an important aspect of a nuclear forensic investigation and of the attribution of the material to its source. To the extent that the sample obeys the standard rules of radiochronometry, then the production ages of even very recent material can be determined using the ²³⁰Th-²³⁴U chronometer. These standard rules may be summarized as (a) the daughter/parent ratio at time=zero must be known, and (b) there has been no daughter/parent fractionation since production. For the most samples of uranium, the "ages" determined using this chronometer are semantically "model-ages" because (a) some assumption of the initial ²³⁰Th content in the sample is required and (b) closed-system behavior is assumed. The uranium standard reference materials originally prepared and distributed by the former US National Bureau of Standards and now distributed by New Brunswick Laboratory as certified reference materials (NBS SRM = NBL CRM) are good candidates for samples where both rules are met. The U isotopic standards have known purification and production dates, and closed-system behavior in the solid form (U₃O₈) may be assumed with confidence. We present here ²³⁰Th-²³⁴U model-ages for several of these standards, determined by isotope dilution mass spectrometry using a multicollector ICP-MS, and compare these ages with their known production history. [ABSTRACT FROM AUTHOR]