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Measurement of sulphur isotope ratio (34S/32S) in uranium ore concentrates (yellow cakes) for origin assessment

A novel method has been developed for the measurement of the 34S/32S isotope ratio in uranium ore concentrate (yellow cake) samples for the origin assessment in nuclear forensics. The leachable sulphate is separated and pre-concentrated by anion exchange separation followed by the 34S/32S ratio measurement using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). The effect of sample composition on the accuracy was investigated and optimized. Matching of the sample to the bracketing standard was necessary to obtain accurate results. The method was validated by the measurement of standard reference materials (IAEA-S-2, IAEA-S-3 and IAEA-S-4) and the d34S value could be determined with an uncertainty between 0.45‰ and 1.9‰ expressed with a coverage factor of 2. The method was then applied for the analysis of uranium ore concentrates of world-wide origin. In the studied materials distinct 34S/32S isotope ratios could be observed, which can be a promising signature for the nuclear forensic investigations to identify the source of unknown nuclear materials.