A gas ion source for radiocarbon measurement at SUERC

Development of radiocarbon AMS measurement of CO2 samples using the 39-cathode NEC MC-SNICS gas/solid hybrid ion source in the SUERC AMS system is in progress. Stable and reproducible C− currents up to 12 μA can be achieved for an optimal CO2 gas flow rate of 1.5 ± 0.5 μl min−1, resulting in typical efficiency of 8% from CO2 to C− and that of 5% from CO2 to C4+. The precision is limited by the lifetime of the gas cathode, but measurements with modern materials have shown that a precision of better than 0.5% and 1% on >200 μg C and 10–200 μg C respectively is obtainable on a routine basis. The normalized 14C values for TIRI intercomparison sample A (Barley mash) agree well with the consensus value within the margin of uncertainty. The background comes from the gas cathode and probably cross contamination, which is estimated to be ca. 1 pMC.Large-sized samples (usually >200 μg C) can readily be run as pure CO2 while samples as small as 40 μg C can be measured at an optimal flow rate using helium carrier gas. Even smaller samples (>5 μg C) have been analysed at a reduced current. At present, source poisoning effects at helium flow rates above 130 μl min−1 limit the further reduction in measurable sample size. A manifold redesign is expected to address this problem.