Developments towards the detection of 135Cs and 137Cs by AMS

The isotopic ratio ¹³⁵Cs/¹³⁷Cs can be used to assign sources of anthropogenic cesium input, as a geochemical tracer, or for modifying anthropogenic radionuclide dispersion models. Due to its long halflife, ¹³ ⁵Cs is hard to detect via decay counting. Mass Spectrometry has to deal with isobaric interferences, i.e. ¹³⁵Ba and ¹³⁷Ba for Cs detection. The new method of Ion Laser InterAction Mass Spectrometry (ILIAMS) at the Vienna Environmental Research Accelerator (VERA) overcomes this problem by exploiting differences in the electron affinites of CsF₂⁻ and BaF₂⁻ molecules.
A ¹³³CsF₂⁻ current on the order of 100 nA from a mixed Cs₂SO₄ and PbF₂ - matrix is extracted from the ion source. The sample material is mobilized by heating the ionizer only, so no external sputtering material is needed. First results show reproducible detection of ¹³⁵Cs and ¹³⁷Cs in in-house reference materials. With 1mg stable Cs carrier, we reach a blank level of ¹³⁷Cs/¹³³Cs = 6 · 10⁻¹², which corresponds to ≈ 30 mBq. We aim to lower this value by at least two orders of magnitude for measuring environmental samples.