230 Th- 234 U model ages of some uranium standard reference materials

Abstract

The “age” of a sample of uranium is an important aspect of a nuclear forensic investigation and of the attribution of the material to a source. The 230 Th- 234 U chronometer can be used to determine the production date of even very recently-produced material ( 234 U half life = 245250 ± 490 years; 230 Th half life = 75690 ± 230 years [1]), provided that the 230 Th/ 234 U at the time of formation is known, and that there has been no Th-U fractionation in the sample since production. For most samples of uranium, ages determined with this chronometer are “model ages”, because they are based on the assumptions of a) some initial amount of 230 Th in the sample, and b) closed-system behavior of the sample since production. 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 materials where these assumptions may be tested. The U isotopic standards have known purification and production dates and closed-system behavior in the solid form (U 3 O 8 ) may be reliably assumed. In addition, these materials are widely available and can serve as informal round-robin inter-laboratory comparison samples. We determined 230 Th- 234 U model ages for seven of these isotopic standards by isotope dilution mass spectrometry using a multi-collector ICP-MS. The standards dated for this study are U005-A, U010, U030-A, U100, U850, U900 and U970. Model ages obtained range from ∼ 30 to ∼ 52 years ago (reference date: 5-May-2009). The model age of U100 is the same as the purification date, within uncertainty. The other six standards analyzed all give model ages older than the purification dates of record. The magnitude of the discrepancy between model age and purification date does not correlate with the model age or the amount of 232 Th in the samples. This indicates that excess 230 Th in these six standards results from incomplete purification during production.