Precise and accurate isotopic analysis of uranium and thorium in uranium ore concentrates using ICP-MS and their age dating for nuclear forensic analysis

Abstract

Isotope ratios and age dating of uranium ore concentrates (UOCs) are of great significance in nuclear forensic analysis. This paper presents accurate and precise isotopic analysis of uranium and thorium in 15 different UOCs using two types of ICP-MS instruments. ²³⁵U/²³⁸U, ²³⁴U/²³⁸U and ²³⁰Th/²³²Th isotope ratios were determined by Multi-Collector Inductively Coupled-Plasma Mass Spectrometry (MC-ICP-MS), while ²³⁰Th/²³⁴U and ²³⁴U/²³²Th atom ratios were determined by triple quadruple Inductively Coupled-Plasma Mass Spectrometry (ICP-MS/MS). Results show that the relative uncertainties in the determined values of ²³⁵U/²³⁸U, ²³⁴U/²³⁸U and ²³⁰Th/²³⁴U ratios for CRM124-1 are 0.0014%, 0.02% and 3.2%, respectively. It has been proved that an aliquot containing 1.12 μg of ²³⁸U was the minimum sample amount to obtain the representative ²³⁵U/²³⁸U ratios of the UOC samples for bulk analysis by MC-ICP-MS, especially when the sample of UOCs was inhomogeneous or a mixture of several UOCs. No correlation between the measured ²³⁸U/²³⁵U and ²³⁵U/²³⁴U ratios is observed in the UOC samples. It is possible to distinguish a single UOC sample from others by synthesizing ²³⁵U/²³⁸U ratio, ²³⁴U/²³⁸U ratio and ²³⁰Th/²³⁴U ratio simultaneously. The ²³⁰Th–²³⁴U model ages estimated using a ²³⁰Th–²³⁴U chronometer lie between 3.5 a and 4435 a. However, it is usually not reliable to determine the age over 100 a of UOCs, mainly because the material was not completely purified from relevant decay products during production. To overcome this problem, an alternative approach for age dating is proposed using multiple measurements of the ratios of ²³⁰Th/²³²Th and ²³²Th/²³⁴U at an interval of about 2 a to 5 a. This work demonstrates that multiple parameters obtained from a combination of multiple analytical techniques, including bulk analysis, imaging and microanalysis techniques, can be used to narrow the range of potential origins for nuclear forensic purpose.