Improved methods to age-date uranium certified reference materials for nuclear forensics using the 231 Pa/ 235 U radiochronometer

Improved methods employed at Los Alamos National Laboratory for 231 Pa/ 235 U radiochronometry are outlined. We present elution curves obtained during 233 Pa tracer preparation from 237 Np. Additionally, we report model ages for uranium certified reference materials (CRMs) exhibiting a range of 235 U enrichments including the first 231 Pa/ 235 U model ages for CRMs U200 and U900. Our results enable these CRMs to be used, with increased confidence, as quality control materials during nuclear forensics investigations.