U-236 analyses with the prototype 300kV MILEA system at ETH Zurich

Full text: Compact, low energy accelerator mass spectrometry (AMS) has evolved over the past years as one of the most sensitive, selective, and robust techniques for the analysis of light and heavy and long lived radionuclides. Particularly the analysis of actinides in environmental samples profited a lot from the technical advances in compact AMS. In this contribution we discuss the setup and the analytical capabilities of the compact prototype AMS system MILEA for ²³⁶U and Pu-isotope analyses. The prototype MILEA system has been developed and installed at ETH Zurich in 2018/2019. Meanwhile, evolved MILEA versions have become commercially available by the ETH Zurich spin off company Ionplus. The prototype MILEA system has been optimized for low background and highly efficient analyses of light and heavy long-lived radioisotopes.A special setup for AMS of U-isotopes has been developed that allows the sequential measurement of three isotopic ratios (²³⁶U/²³⁸U, ²³³U/²³⁸U, and ²³⁵U/²³⁸U) using fast bouncing mode for ²³⁸U. The system provides an abundance sensitivity at the order of 10-13 in the mass range of the actinides and thus allows determining the ²³⁶U/²³⁸U ratio in samples with U from anthropogenic and natural sources. Efficiency tests show that the MICADAS-type ion source provides a maximum ionization efficiency of almost 3% for UO- and PuO- extracted from a sample matrix containing a mix of Fe-oxide and Nb powder. First routine analyses of ²³⁶U/²³⁸U were carried out in small volume (<2L) sea water samples containing about 5μg natural U. The samples provided average ²³⁸U3+ currents of up to 2 nA on the high energy side and ²³⁶U/²³⁸U ratios were determined with an overall uncertainty level of 2%. Our results show that the ETH Zurich MILEA system allows highly efficient and background free determination of anthropogenic ²³⁶U/²³⁸U ratios of ≥10-10 in small (1-2 L) sea water samples. Overall uncertainties of a few percent can be reached and are dominated by counting statistics. The very good abundance sensitivity further allows accessing the expected range of natural U-isotopes (²³³U/²³⁸U and ²³⁶U/²³⁸U) which are estimated to be in the 10-13 – 10-14 range with small to negligible background corrections.