First studies on ⁹⁹Tc detection using Ion Laser InterAction Mass Spectrometry (ILIAMS)
Johanna Pitters1,2, Fadime Gülce1, Karin Hain1, Martin Martschini1, Markus Schiffer3, Robin Golser1
1University of Vienna, Faculty of Physics, Isotope Physics, , Austria, 2Vienna Doctoral School in Physics, , , 3University of Cologne, Institute for Nuclear Physics, , Germany
Technetium-99 (t½ = 2.1·10⁵ a) is an anthropogenic isotope released by nuclear waste reprocessing, from global fallout and nuclear medicine. Owing to its high solubility, it can serve as a tracer for water transport processes. The detection of ⁹⁹Tc in environmental samples presently requires Accelerator Mass Spectrometry (AMS) at the largest AMS-facilities in order to adequately suppress the strong interference from the isobar ⁹⁹Ru. In our project, we aim at measuring ⁹⁹Tc at the 3-MV facility VERA (Vienna Environmental Research Accelerator) using laser photodetachment at our unique setup ILIAMS (Ion Laser InterAction Mass Spectrometry) for isobaric suppression (Martschini et al., this meeting). Access to ⁹⁹Tc measurements at low terminal voltage will open a whole range of applications in fields such as oceanography and nuclear waste repositories.
To make ⁹⁹Tc accessible, chemical sample preparation, ion source output and laser photodetachment all require optimization. We have already developed a chemical sample preparation method using liquid-liquid-extraction and separation with a TEVA column which is able to suppress Ru in sea water and peat-bog samples by a factor 10⁻⁴. For an effective isobaric suppression of Ru by laser photodetachment, a negative ion system had to be found, where the electron affinities favor a neutralization of Ru by the laser while Tc is unaffected. In a test series with fluoride and oxide ions, we have found that this is the case for the molecular system TcF₅⁻ / RuF₅⁻. RuF₅⁻ is suppressed by five orders of magnitude by a 532 nm laser while TcF₅⁻ is hardly affected. To ensure the formation of penta-fluorides in the ion source, a Nb₂O₅ matrix in which Tc is embedded, is mixed with PbF₂ (about 1:10 by weight) as a fluorine donor. All in all, we are optimistic about reaching a blank level of some 10⁶ atoms per sample through the combined effects of Ru-suppression in the sample preparation method, ion source and laser photodetachment.
As there is no stable Tc isotope, a normalization to an isotopic Tc spike material, i.e. ⁹⁷Tc (t½ = 4.2·10⁶ a), or a stable reference isotope of another element needs to be established. Unfortunately, due to the high background from ⁹⁷Mo, a normalization to ⁹⁷Tc has so far not been possible. Instead, we are normalizing to the ⁹³Nb current, as NbF₅⁻ is not detached by the 532 nm laser. The chemical recovery is monitored using a ⁹⁵ᵐTc spike (t½ = 61 d, provided by the University of Cologne) and gamma-spectrometry. This way, we have recently shown that varying fractions of Tc are lost during calcination at temperatures around 400°C. By reducing the temperature to 300°C, higher Tc yields could be more reproducibly obtained. In the near future, we plan tests with heating under argon atmosphere for reducing the oxygen content of the AMS target. This is expected to increase and stabilize the production yield for both, NbF₅⁻ and TcF₅⁻, and thereby improve the normalization.
This work was funded by the Austrian Science Fund (FWF). [AP 3161421]
I am a senior post-doc at VERA (University of Vienna) where I’m leading two externally funded projects on the detection of Tc-99 at environmental levels using ILIAMS and on the production of an isotopic spike for Np-237 analysis. I received my PhD from the Technical University of Munich in 2016 for the investigation of Pu releases from the Fukushima accident.