Frontiers of challenging studies utilizing accelerator mass spectrometry in geoscience

The fission-product 90Sr (half-life 28.9 years) and 135Cs (half-life 2.3×106 years) are present in the environment. Strontium-90 is one of the most concerning nuclides in the assessment of internal exposure of residents because it can accumulate in bones and cause health problems. Therefore, it is essential to study the distribution of 90Sr in the environment and its temporal variation (90Sr enrichment in organisms and plants). On the other hand, 135Cs, which has a longer physical half-life than 137Cs (30.1 years), is expected to be utilized as a tracer to follow the long-term environmental fate of 137Cs, which is difficult to measure due to decay. These studies require high-throughput multi-sample analysis. However, as 90Sr and 135Cs are pure β-emitters, other β-emitters in the environmental samples (e.g., Ra isotopes, 137Cs, and 210Pb) must be entirely removed, which would interfere with β-ray detection. While this is impossible for the conventional β-ray detection method of 135Cs, it does work for 90Sr. Still, it requires a large sample volume due to the low concentrations of 90Sr in general environmental samples1). Therefore, the chemical separation of the target nuclide is very time-consuming and challenging for reliable quantification.
This study addressed solving these problems using accelerator mass spectrometry (AMS) for sensitive analysis of 90Sr and 135Cs in environmental samples2,3). AMS has the advantage of allowing more precise analysis of small sample volumes. However, the most concerning aspect of AMS for 90Sr and 135Cs is the interference of the isobars 90Zr and 135Ba. Therefore, the measurements of 90Sr and 135Cs were carried out at the University of Vienna (VERA). The AMS system is equipped with an "Ion cooler" that can effectively separate the isobars. For 90Sr, various molecular ions such as SrFn− and ZrFn− (n ≥ 1) were extracted from the target (a mixture of SrF2 and PbF2 in a weight ratio of 1:8) by Cs sputtering, and then the molecular ions with an m/q of 147 (90SrF3− and 90ZrF3−) were selectively passed through a 90° bending magnet. The ion beam (200-300 nA) was decelerated to ~30 eV and injected into the Ion cooler, an isobaric separation system with built-in radio-frequency quadrupole (RFQ). Collisions with a buffer gas mixture of He and O2 gas inside the RFQ reduced the ion energy to <1 eV. In addition, the O2 gas produces oxide ions and separated Zr. Here, a 12 W laser (532 nm) further suppressed Zr (neutralized 90ZrF3− by photo-detachment). In this isobaric separation system, Zr was suppressed by >107 for Sr (current ratio), and the overall Zr suppression is >1012 (ion source >105). The overall transmission efficiency of Sr was 0.4‰. Meanwhile, a simple chemical separation scheme was developed which efficiently separates Zr to maximize the isobaric separation performance of AMS: acid leaching → two-step chromatography with crown ether and anion exchange → SrF2 precipitation (2 days for the precipitation) to environmental reference materials (soil, beef bones, fish meat) with known 90Sr concentrations. The results showed that the 90Sr concentrations quantified by the AMS method agreed with the nominal values (quantified by the β-ray detection method) within a margin of error. Furthermore, based on the measurements of 1 mg of Sr carrier treated in the same manner as environmental samples, the detection limit of 90Sr by the AMS method achieved 1/10 (< 0.1 mBq, 90Sr/88Sr = 2.5×10-15) of the general detection limit of the β-ray detection method4). The highly sensitive analysis of 90Sr by AMS is promising for studies on the detailed distribution of 90Sr in individual (and even site-specific) corals and fishes with limited sample volumes. As for 135Cs, significant issues remain, such as efficient Ba separation in chemical separation, cross-contamination between samples in AMS ion sources, and preparing 135Cs reference materials. However, the detection limit for 135C was 0.3 µBq (135Cs/Cs atomic ratio was 7×10-12), demonstrating excellent results. Therefore, this study showed that the 135Cs AMS has the potential to apply geoscience.