Enhanced anion production and isobar separation for accelerator mass spectrometry

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Accelerator Mass Spectrometry (AMS), which developed from nuclear physics, has many applications in the Sciences. The sub-milligram samples sizes and the short counting time for the measurement of rare long-lived radioactive atoms by mass spectrometry require that stable atomic isobars must be removed from the high currents needed from the ion source. For 14C, the use of negative ions eliminates the 14N isobar at the ion source and a tandem accelerator destroys the molecular isobars, thereby facilitating 14C measurements relevant to climate change, astrophysics, medicine and archaeology. As a result of cosmic ray interactions or the human use of uranium many other long-lived isotopes are present in the environment. Examples are, 10Be, used for surface exposure dating, and long-lived 129I, used to trace oceanic circulation and to trace underground water flow for Nuclear Waste Storage site studies. Research into the AMS method is being carried out with the aim of improving and extending AMS to all long-lived isotopes, as only three can exploit negative ion instability for complete atomic isobar separation. The study of isobar enhancement at the ion source by means of superhalogen anions has proved very fruitful. During earlier work the efficacy of the super-ionic conductor PbF2 for helping to generate many of the super fluoride anions such as AlF4-, SiF5-, PF6- etc was discovered. An NSERC funded I2I proposal, which uses RF quadrupole chemical reaction cells for the generalised isobar separation of anions prior to tandem acceleration, has had successful initial tests. This has led to the detection of 36Cl and 41Ca at low levels without the need for large accelerators. In collaboration with industry, this development promises to make isobar selection near the ion source commercially available, starting with 36Cl, as well as providing much useful information on the properties of the super-anions, dianions and ion-gas chemical reactions in general.