The SUERC 5 MV accelerator mass spectrometer is equipped with two ion sources. Radiocarbon AMS of CO2 samples is being developed on one source and this work is described for the first time. The commercial gas-ready 39-cathode source is similar to other NEC sources for solid samples except that sample gas is delivered through a common tube to the backside of the selected hollow cathode. Several cathode designs featuring titanium stubs have been tested. There are other source differences, including the addition of a dedicated source turbo-pump which we find unnecessary. The source is complemented by a gas sample handling system for manipulating samples in synchronism with ion source operation. Up to 10 gas samples are stored in individual compressible bellows connected to the ion source via a common manifold and a 1.2 m long 0.25 mm i.d. capillary. To measure a sample the source selects the corresponding cathode while the manifold pressure can be maintained in feedback. A simple protocol ensures random and repeated sample measurement without significant cross contamination. Samples can be added to the gas handling system and measurements continued without venting the ion source provided that sufficient unused cathodes remain. In operation, stable and reproducible C- currents of up to 15 µA can be achieved for an optimal CO2 gas flow rate of 0.2 cm3 STP/h. This is also valid for CO2 /He mixtures tested down to 1:75. So, mg C-sized samples can readily be run neat while samples as small as 50 µg C can be measured at the optimal flow rate using He carrier. Even smaller samples have been analysed at a reduced current, in part due to source poisoning effects at He flow rates above 9 cm3 STP/h. A manifold redesign will reduce limiting sample size further. Negative ion production is ~5% efficient. 14C-AMS measurements of well-characterized samples have been made. Backgrounds based on CO2 from an Iceland spar calcite are about 50 ka. And the precisions on the 13C/12C ratios were better than 0.2%, suggesting that on-line δ13C determination is possible. Further performance tests such as inter-cathode reproducibility and optimal conditions for microgram samples are in progress.
See more of Ion Sourcery (optics, modeling, performance, etc)
See more of The 10th International Conference on Accelerator Mass Spectrometry (September 5-10, 2005)