The International Atomic Energy Agency (IAEA) Safeguards Analytical Laboratory uses the modified total evaporation (MTE) method for the highly sensitive determination of minor uranium isotope ratios by thermal ionization mass spectrometry (TIMS). Precise and accurate measurements of n(U-234)/n(U-238) and n(U-236)/n(U-238) ratios help the IAEA to assess the completeness and consistency of State declarations. Uranium samples, especially non-naturally occurring ones, come in a large variety of compositions, where n(U-236)/n(U-238) can range from 10-14 in natural material to 10-2 in reprocessed material. The composition of n(U-236)/n(U238) in enriched uranium materials can provide valuable information on sample origin and process design. However, as the measured U-236 signals can be extremely low, the accurate determination of the n(U-236)/n(U-238) can be challenging. Generally, for the determination of low-level U-236 samples, the secondary electron multiplier is used and the signal at m/z 236 is corrected for peak tailing effects produced by much more intense U-235 and/or U-238 ion beams. However, one additional issue complicating the accurate determination of low-level n(U236)/n(U-238) in highly enriched uranium materials (HEU) is the potential interference of the U235H-1 hydride molecular species on the U-236 signal. Depending on several factors (e.g., lab humidity, instrument conditions, nature of the sample) the measured m/z 236 signal in HEU may be dominated by 235U1H and the n(U-235H-1)/n(U-238) may reach >10-7, obscuring the true n(U236)/n(U-238). Hence, the U-235H-1 hydride signal needs to be monitored and corrected for. Initial tests show the importance of such a correction on the accurate determination of low-level n(U-236)/n(U-238) in HEU materials. A lack of available HEU certified reference material with very low U-236 abundance (<10-7 atom%) introduces uncertainty to the verification of the correction method and highlights the need for new quality control materials. Here, we present a revised MTE-TIMS method to accurately and precisely measure n(U-236)/n(U-238) ratios in HEU materials by accurately determining the contribution from U-235H-1 hydride molecular species and applying a correction to the U-236 signal. This paper discusses potential controls of hydride formation in TIMS instruments and strategies to avoid or reduce significant corrections and consequently lower the detection limits. In addition, new quality control materials are introduced and measurements to verify the accuracy of the hydride correction in low-level U236 HEU materials is discussed.
Year
2024
Abstract