Uranium age determination by measuring the 230Th/234U ratio
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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.
KeywordsUranium Thorium 234U Thermal Ionization Mass Spectrometry Uranium Metal
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