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

Abstract

The analysis of 90Sr by AMS has so far required the use of very large tandem accelerators in order to separate the isobar 90Zr by the rate-of-energy-loss method. The analysis of 135,137Cs by AMS has never been attempted as the separation of the isobars 135,137Ba 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 NO2 gas in the cases of SrF3−–ZrF3− and CsF2−–BaF2− separations. Reduction factors of ∼4×10−6 for ZrF3−/SrF3− and ∼2×10−5 for BaF2−/CsF2− were measured. SrF3− and CsF2− are both super-halogen anions and are preferentially produced in the ion source rather than ZrF3− and BaF2− when using the PbF2 matrix-assisted method. Reduction factors for ion source production with such targets of ∼3×10−5 for ZrF3−–SrF3− and ∼5×10−4 for BaF2−–CsF2− were found. The combined methods would suggest a theoretical detection sensitivity for 90Sr/Sr ∼6×10−16, 135Cs/Cs ∼7×10−15 and 137Cs/Cs ∼1×10−14, assuming 10ppm 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.