Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

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. The long-lived ¹³⁵Cs is also of interest in stellar nucleosynthesis models, where ¹³⁴Cs is an important s-process branching point, defining the ¹³⁴Ba/¹³⁶Ba ratio as both those nuclides are shielded from the r-process. ¹³⁵Cs is a pure beta-emitter with a low end-point energy and a long but not very well known half-life around 2.3 Ma. Therefore, ¹³⁵Cs is hard to detect via radiometric methods. Mass spectrometry on the other side has to deal with the isobaric interferences 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 affinities of CsF₂⁻ and BaF₂⁻. There, the ion beam is cooled and overlapped with a 532 nm laser beam of 10W power in a He buffer gas filled radiofrequency quadrupole. Ions like BaF₂⁻ with a detachment energy lower than the photon energy of 2.33 eV are efficiently suppressed by photodetachment, while CsF₂⁻ ions with a detachment energy higher than the photon energy remain unaffected. With this approach an isobar suppression of more than 10⁶ was achieved for BaF₂⁻, while reaching a CsF₂⁻ transmission of 40% through the RFQ ion cooler. A ¹³³CsF₂⁻ current on the order of 50 nA from a mixed Cs₂SO₄ and PbF₂ – matrix is extracted from the MC-SNICS ion source and measured in the 3+ charge state on the high-energy side with an accelerator transmission of 30%. In order to improve the yield for CsF₂⁻ and keep cross-contamination in the ion source between samples low, we investigated and present the results of two sputtering processes: Rubidium sputtering and negative ion production without external sputter agent.
We achieved reproducible detection of ¹³⁵Cs and ¹³⁷Cs in an in-house reference material with an isotopic ratio of ¹³⁵Cs/¹³⁷Cs ≈ 2.5. Further, first environmental samples showing the ¹³⁵Cs/¹³⁷Cs signature of the nuclear accidents in Chernobyl and Fukushima were measured at VERA and compared to values obtained by ICP-QQQ-MS by Zok et al. [1] at the Leibniz University of Hannover. The ILIAMS assisted AMS measurements at VERA reach blank levels of ¹³⁵Cs/¹³³Cs ≈ 6∙10⁻¹² and ¹³⁷Cs/¹³³Cs ≈ 3∙10⁻¹² .
Monitoring mass 136 throughout a measurement, where only stable barium and cerium are present, shows that at these levels there is no contribution to the background from insufficient isobar suppression so that the limitation for the AMS blank level is cross contamination in the ion source. We aim to reduce this blank value by at least two orders of magnitude to perform measurements of environmental samples also far from directly contaminated sites.

[1] Zok et al., Determination of Characteristic vs Anomalous ¹³⁵Cs/¹³⁷Cs Isotopic Ratios in Radioactively Contaminated Environmental Samples, Environ. Sci. Technol. 2021, 55, 8, 4984–4991