Ion sources for earth and planetary sciences

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Accelerator Mass Spectrometry (AMS) has many applications in the Earth and Planetary Sciences especially radiocarbon dating. The milligram sample sizes and the short counting time of the rare long-lived radioactive atoms by mass spectrometry require that both atomic and molecular isobars must be removed. Negative ions eliminate the 14N isobar at the ion source and the tandem accelerator destroys the molecular isobars. 14C dates relevant to climate change and archaeology can then be obtained. The measurement of 14C in natural hydrocarbons is relevant to the construction of kiloton scintillation counters for solar neutrino and dark matter studies. Many other long-lived isotopes are present naturally or as a result of the human use of uranium. Research into the AMS method is being carried out at the IsoTrace Laboratory with the aim of generalizing AMS to all long-lived isotopes. Long-lived 129I is used to trace oceanic circulation and to trace underground water flow for Nuclear Waste Storage sites studies, such as Yucca Mountain (USA). The detection of 10Be and 7Be from the stratosphere, to study ground level ozone, uses the anion of the molecule BeF as BF does not form stable anions. This research, sponsored by Health Canada, led to the discovery of the value of the large electron affinity super-halide anions for AMS. The BeF3 anion is a dominant ion from a sputter ion source but the BF3 anion is at most metastable leading to another possibility for even better measurements of 10Be and incidentally 7Be. During this work the smallest dianion, LiF3, was discovered. A currently funded proposal uses quadrupole reaction cells for a generalised isobar separator for anions prior to tandem acceleration. This development, in collaboration with industry, promises to make isobar selective ion sources commercially available, starting with 36Cl, as well as provide much important information on the properties of the super-anions in general. In the long run the elimination of the tandem accelerator is a possibility for some isotopes.