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ABSTRACT. The suppression of interferences from atomic and molecular isobars is a key requirement for the extension of accelerator mass spectrometry (AMS) to the analysis of new cosmogenic isotopes and for increasing the range of applications for small AMS systems. In earlier work, it was shown that unwanted isobars can be eliminated by anion-gas reactions ). Recently, a prototype system in which such reactions could be applied to ions from an AMS ion source, the Isobar Separator for Anions (ISA), was described ). This system decelerates the beam of rare anions from keV to eV energies, guides them through a single radiofrequency quadrupole (RFQ) gas cell, and re-accelerates them for further analysis in a 2.5MV AMS system. Tests of this system with Cl and S anions and NO 2 gas showed a suppression of S with respect to Cl of over 6 orders of magnitude, with a transmission of ~30% for the Cl beam. In this work, results of the analysis of a range of standard reference materials are reported; these show the linearity of the system for measuring the 36 Cl/ 35 Cl ratio over a span of 2 orders of magnitude. Further tests, using the AMS system as a diagnostic tool, have provided clues about the loss of Cl at higher cell pressure and the nature of the residual low level of S transmission. These lead to the assessment of various gases for cooling the Cl -beam. Suppression measurements for 41 K in the analysis of 41 Ca, using NO 2 as a reaction gas, are also discussed. These preliminary measurements have provided data for the development of a more advanced system with separate cooling and reaction cells.

Guided by simulations using SIMION 8.1, a series of modifications were made to an experimental version of an Isobar Separator for Anions (ISA). The resulting improved version of the ISA provides a means of re-energizing the ions after they are cooled by gas collisions as they pass through the gas-filled radiofrequency quadrupoles (RFQ), and also provides higher transmission efficiencies. Reinvestigation of the separation of CaF3− and KF3− with this refined apparatus resulted in a better balance between isobar suppression and analyte transmission. KF3− was attenuated at eV energies by 4 orders of magnitude while 40% transmission of CaF3− was retained, for a 20keV CaF3− beam of Φ2mm and ±12mr. These results advance the possibility of an efficient small ISA-AMS system for both cosmogenic and medical applications of 41Ca.