On-line ion chemistry for the AMS analysis of 90Sr and 135,137Cs

Abstract The analysis of 90 Sr by AMS has so far required the use of very large tandem accelerators in order to separate the isobar 90 Zr by the rate-of-energy-loss method. The analysis of 135,137 Cs by AMS has never been attempted as the separation of the isobars 135,137 Ba by the traditional method requires even higher energies, so that this approach would become prohibitively expensive for routine analysis. Following the successful demonstration of Cl − –S − separation by the Isobar Separator, the same apparatus was used to test the separation of other pairs of isobars. Surprisingly effective results were obtained with NO 2 gas in the cases of SrF 3 − –ZrF 3 − and CsF 2 − –BaF 2 − separations. Reduction factors of ∼4 × 10 −6 for ZrF 3 − /SrF 3 − and ∼2 × 10 −5 for BaF 2 − /CsF 2 − were measured. SrF 3 − and CsF 2 − are both super-halogen anions and are preferentially produced in the ion source rather than ZrF 3 − and BaF 2 − when using the PbF 2 matrix-assisted method. Reduction factors for ion source production with such targets of ∼3 × 10 −5 for ZrF 3 − –SrF 3 − and ∼5 × 10 −4 for BaF 2 − –CsF 2 − were found. The combined methods would suggest a theoretical detection sensitivity for 90 Sr/Sr ∼6 × 10 −16 , 135 Cs/Cs ∼7 × 10 −15 and 137 Cs/Cs ∼1 × 10 −14 , assuming 10 ppm Zr and Ba contamination in the AMS targets. In addition to the earlier Cl − –S − separation work, these measurements further illustrate the potential of on-line ion chemical methods for broadening the analytical scope of small AMS systems.