235U–231Pa age dating of uranium materials for nuclear forensic investigations

Abstract

Age dating of nuclear material can provide insight into source and suspected use in nuclear forensic investigations. We report here a method for the determination of the date of most recent chemical purification for uranium materials using the ²³⁵U-²³¹Pa chronometer. Protactinium is separated from uranium and neptunium matrices using anion exchange resin, followed by sorption of Pa to an SiO₂ medium. The concentration of ²³¹Pa is measured by isotope dilution mass spectrometry using ²³³Pa spikes prepared from an aliquot of ²³⁷Np and calibrated in-house using the rock standard Table Mountain Latite and the uranium isotopic standard U100. Combined uncertainties of age dates using this method are 1.5 to 3.5 %, an improvement over alpha spectrometry measurement methods. Model ages of five uranium standard reference materials are presented; all standards have concordant ²³⁵U-²³¹Pa and ²³⁴U-²³⁰Th model ages.