Hybrid ion sources: Radiocarbon measurements from microgram to milligram

Abstract

Gas ion sources provide a very efficient way of dealing with very small samples and, by injecting samples in a stream of helium carrier gas, measurements can be obtained from as little as 1 μg carbon. This is particularly useful for measurements on high activity samples and for the study of sample contaminants in relation to high precision AMS measurements. Another benefit of gas based measurements is that they can be made in real time allowing a range of possible applications such as GC-AMS and LC-AMS which could make AMS more widely applicable in the life sciences. Graphite ion sources provide a more efficient use of accelerator time for the measurement of large samples because of the higher carbon currents attainable. The precisions attainable is this way are so good that re-evaluation of possible sources of systematic error (such as low level contamination and sample size effects) is essential. Hybrid ion sources capable of operating on either gas or graphite allow the advantages of both techniques to be exploited. With minor modifications to the gas ion source at Oxford we are now able to operate with both sample types. Currents from gas are typically 10–12 μA and those from graphite 40–50 μA while maximum currents from graphite exceed 300 μA. We discuss the implications of this for the future development of radiocarbon AMS facilities designed for a wide variety measurements and research.