Preparation and calibration of a .sup.231Pa reference material

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