Survey of naturally occurring 236U with direct-AMS

From a multi-isotope direct-AMS survey of 310 uranium ore concentrates (UOCs), ²³⁶U/²³⁸U ratios from ∼2 × 10⁻¹² to ∼2 × 10⁻⁷ are measured. However, a range from low-10⁻¹² to mid-10⁻¹⁰ is expected from directly mined natural uranium ore minerals. This upper-end discrepancy would normally be attributed to inputs of spent fuel materials, but the numerous occurrences and the lack of economic reasons to recycle anything in the front stage of UOC production make the abnormally high ²³⁶U/²³⁸U ratios rather perplexing. When plotted against the ¹⁸⁷Os/¹⁸⁸Os ratios, which were also obtained in the survey, the high ²³⁶U/²³⁸U results are seen to occur around the value of ¹⁸⁷Os/¹⁸⁸Os ∼10, a non-random pattern as might be anticipated from unwitting contaminations. This observed correlation favors those measured high ²³⁶U/²³⁸U ratios being natural, and as such, it refers to an overlooked natural ²³⁸U/²³⁵U isotope fraction mechanism. Therefore, this work calls for further studies of naturally occurring ²³⁶U/²³⁸U in a wider range of native uranium-bearing minerals. To secure unambiguous natural ²³⁶U surveys, an approach is described that combines UF₅⁻ production directly from powdered native samples and ion source reconfiguration to minimize background.