Uranium age-dating using in-situ isotope ratio for nuclear forensics

Okubo, Ayako; Shinohara, Nobuo ; Magara, Masaaki  

As an effort of strengthening nuclear security in the world, capacity building of nuclear forensics in each state have been required. The age-dating of nuclear material is to estimate an elapsed time since the final purified day of the nuclear material. Because age-dating result could enable us to better constrain the origin of unidentified material, which is recognized as an important parameter for nuclear forensics. The isotope dilution mass spectrometry (ID-MS) method is recognized as the most reliable method to estimate U/Th ratio for uranium age-dating. Uranium-234 and Th amounts in a uranium material were respectively measured by ID method using spikes (U and Th). The isotopic equilibrium between the isotopes in-situ the sample and added spikes is assumed in ID method. In order to obtain the accurate concentrations of U and Th, precise determination of sample weight and control of spike concentration are required. We have developed a new method to estimate U/Th ratio without spike addition for uranium age-dating. In this method, using secular equilibrium between U and Th (Th/U = 1.45 E-11), U/Th ratio is calculated from measured isotope ratios of U and Th. This new method does not require precise sample weighing and control spike concentration, and is suitable for routine analysis in developing of nuclear forensics database.