Uranium age determination by measuring the ²³⁰Th/²³⁴U ratio

Summary

A radiochemical isotope dilution mass spectrometry method has been developed to determine the age of uranium materials. The amount of ²³⁰ Th activity, the first progeny of ²³⁴U, that had grown into a small uranium metal sample was used to determine the elapsed time since the material was last radiochemically purified. To preserve the sample, only a small amount of oxidized uranium was removed from the surface of the sample and dissolved. Aliquots of the dissolved sample were spiked with ²³³U tracer and radiochemically purified by anion-exchange chromatography. The ²³⁴U isotopic concentration was then determined by thermal ionization mass spectrometry (TIMS). Additional aliquots of the sample were spiked with ²²⁹ Th tracer, and the thorium was purified using two sequential anion-exchange chromatography separations. The isotopic concentrations of ²³⁰ Th and ²³² Th were determined by TIMS. The lack of any ²³² Th confirmed the assumption that all thorium was removed from the uranium sample at the time of purification. The ²³⁰ Th and ²³⁴U mass concentrations were converted to activities and the ²³⁰ Th/²³⁴U ratio for the sample was calculated. The experimental ²³⁰ Th/²³⁴U ratio showed the uranium in this sample was radiochemically purified in about 1945.