Background reduction in ²³⁶U/²³⁸U measurements

The measurements of actinide isotopic ratios, in particular ²³⁶U/²³⁸U, in environmental samples requires high sensitivity. In particular, special effort must be devoted to the suppression of interfering nuclides, such as ^235,238U, when measuring ²³⁶U. At the AMS facility of CIRCE, isotopic ratios down to ∼10^-10 are currently measured using a gas E - ΔE detector. In order to push this limit lower towards natural levels, a time-of-flight system is used, featuring a micro-channel plate start detector and a Si stop detector. As the mass resolution of the latter is limited by the layout, an attempt to reduce the abundant isotope interference by other means has been undertaken. In this study, we report preliminary results on the characterization of the presence of molecular interferences when using UO^-, UC^- and UC₂^- as injected molecular ions. In particular the possibility to stabilize the current yield from carbide cathodes has been investigated: it was found that the best cathode preparation procedure is obtained pressing U salts baked at 800 °C mixed with graphite and Al powder. Moreover, the ²³⁸U background in ²³⁶U measurements is strongly reduced injecting UC^- ions, as verified using a time-of-flight detection system. On the other hand, ²³⁵U interference is larger with respect to UO^- injection, but this increase is smaller than expected on the basis of abundances of ¹³C and ¹⁷O in UC and UC₂ molecules on one side and UO, on the other.