ILIAMS assisted AMS of ⁹⁰Sr at VERA

The long-lived fission product ⁹⁰Sr is produced in the nuclear fuel cycle or in nuclear weapon tests with a high yield of 4%. It is very mobile in the environment and due to its chemical similarities to calcium is easily incorporated in the body, e.g. in bones or in teeth, following ingestion or inhalation. With a half-life of T1⁄2=28.90 years [1] its uptake and retention in the human body poses potential health risks. ⁹⁰Sr also has significant potential as an environmental tracer.

The established method to measure ⁹⁰Sr is decay counting. However, this is time consuming, as the ingrowth of ⁹⁰Y over a period of two weeks is needed due to ⁹⁰Sr being a pure and low-energy beta emitter. The main challenge in ⁹⁰Sr detection with mass spectrometric methods is the interference of isobars, i.e. ⁹⁰Zr and ⁹⁰Y. Limits of detection of mass spectrometric methods such as ICP-MS, RIMS and conventional AMS are all above or close to the radiometric detection limit of 3 mBq.

The new Ion Laser InterAction Mass Spectrometry (ILIAMS) technique [2,3] at the Vienna Environmental Research Accelerator (VERA) overcomes the isobaric problem. ILIAMS achieves near complete suppression of isobars via element selective laser photodetachment in a gas-filled radio frequency quadrupole ion guide. The technique exploits differences in the electron affinities (EA) between the isotope of interest and its isobars by neutralizing anions with EAs below the photon energy while leaving anions with EAs above the photon energy unaffected. Additionally, chemical reactions with the buffer gas may enhance separation.

The Sr samples are produced as SrF₂ and mixed with PbF₂, which allows the extraction of an intense SrF₃⁻ ion beam [4]. Using a 532 nm continuous wave laser at 10 W and a He+O₂ mixture, with an oxygen content of 3%, as buffer gas, ILIAMS achieves a suppression of ZrF₃⁻ and YF₃⁻ vs. SrF₃⁻ of >10⁷ at 35% ⁹⁰Sr transmission through ILIAMS. Extraction of SrF₃⁻ from the ion source and elemental separation in an ionization chamber leads to an additional ⁹⁰Zr suppression of 10⁵. The overall ⁹⁰Sr detection efficiency is 0.4‰; a blank level of ⁹⁰Sr/Sr = (4.5±3.2)×10⁻¹⁵ is reached. This corresponds to a more than tenfold improved detection limit of <0.1 mBq, i.e. 10⁵ atoms of ⁹⁰Sr in a 1 mg Sr sample. Measurements of samples from an in-house dilution series with ⁹⁰Sr/Sr ratios ranging from 10⁻¹¹ to 10⁻¹⁴ prove the linearity of this technique. First tests on different environmental samples – from bone to soils – were successful, showing no influence on the detection limit on “real” samples.

[1] R.R. Kinsey et.al., The NUDAT/PCNUDAT Program for Nuclear Data, Data extracted from the NUDAT database, version 2.8
[2] M. Martschini et.al., The ILIAMS project – An RFQ ion beam cooler for selective laser photodetachment at VERA, Nucl. Instrum. Methods. Phys. Res. B, 456 (2019) 213-217
[3] M.Martschini, this meeting
[4] X.-L. Zhao. et. al., Studies of anions from sputtering I: Survey of MFn⁻, Nucl. Instrum. Methods. Phys. Res. B, 268 (2010) 807-811