A gas ion source for radiocarbon measurement at SUERC

Abstract

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 12μA can be achieved for an optimal CO2 gas flow rate of 1.5±0.5μlmin−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 >200μg C and 10–200μg 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. 1pMC.Large-sized samples (usually >200μgC) can readily be run as pure CO2 while samples as small as 40μgC can be measured at an optimal flow rate using helium carrier gas. Even smaller samples (>5μgC) have been analysed at a reduced current. At present, source poisoning effects at helium flow rates above 130μlmin−1 limit the further reduction in measurable sample size. A manifold redesign is expected to address this problem.