Development of uranium age-dating using Th/Th ratios

Okubo, Ayako; Shinohara, Nobuo ; Magara, Masaaki  

The uranium age-dating is a nuclear forensics analysis techniques, by measuring the Th / U isotope ratio in uranium sample, to estimate the elapsed time from being separated and purified. We have determined Th and U using isotope dilution mass spectrometry method, it has conducted for uranium age-dating. Act to perform the isotope analysis after the addition a known quantity of the isotope (spike) to the sample, after spiking, analytical sample is assumed to be in the state of isotopic equilibrium. However, the samples which have complex speciation of uranium and thorium are analyzed, may not sufficiently reach to the state of isotopic equilibrium has been suggested from the experimental results. In this study, without the addition of a spike, from the measurement results of the uranium isotope ratios and thorium isotope ratio in the sample, it was investigated a method for calculating the Th / U isotope ratio. The state of radioactive equilibrium between U and Th in the sample was utilized in the Th / Th method. The Th / U ratio was calculated using measured Th / Th isotope ratio, U / U isotope ratio and Th / U ratio in the radioactive equilibrium.