U (T = 0.159 My) has aroused much interest in the last few years because of the potential of U/U to inform about anthropogenic U sources. The reported U/U atom ratios in general environmental samples are in the 10–10 range. We demonstrate that U/U abundance sensitivities at the 5 × 10 level can be achieved at the 1 MV Accelerator Mass Spectrometry system at the Centro Nacional de Aceleradores (CNA, Seville, Spain). Unresolved U, U and Th molecular fragments are identified as the main U background contributors and procedures to subtract them are proposed. U/U and U/U atom ratios were analysed in the IAEA reference materials coded Soil-6, 300, 385, 410, and 412 at the CNA and at the 600 kV ETH AMS system for intercomparison. U/U atom ratios for samples 412 and 385 were only reported by the ETH.
This work was partly financed through the project PGC2018-094546-B-I00 provided by the Spanish Government (Ministerio de Ciencia, Innovación y Universidades).