Preparation and calibration of a 231 Pa reference material.

A 231 Pa reference material has been characterized for amount of protactinium. This reference material is primarily intended for calibration of 233 Pa tracers produced for 235 U-231 Pa model age measurements associated with nuclear forensics and nuclear safeguards. Primary measurements for characterization were made by isotope dilution mass spectrometry of a purified 231 Pa solution using a 233 Pa isotopic spike. The spike was calibrated by allowing multiple aliquots of the 233 Pa spike solution to decay to 233 U and then measuring the ingrown 233 U by isotope dilution mass spectrometry using a certified uranium assay and isotopic standard as a reverse-spike. The new 231 Pa reference material will simplify calibration of the 233 Pa isotope dilution spikes, provide metrological traceability, and potentially reduce the overall measurement uncertainty of model ages.