Relative Formation Probabilities for Fluoride and Oxyfluoride Anions of U, Np, Pu and Am in Accelerator Mass Spectrometry Measurements at VERA
Andreas Wiederin1,2, Robin Golser1, Michael Kern1, Karin Hain1, Martin Martschini1, Aya Sakaguchi3, Peter Steier1, Akihiko Yokoyama4
1University of Vienna, Faculty of Physics, Isotope Physics, Vienna, Austria, 2Vienna Doctoral School of Physics, University of Vienna, Vienna, Austria, 3University of Tsukuba, Faculty of Pure and Applied Sciences, Tsukuba, Japan, 4Kanazawa University, Institute of Science and Engineering, Kanazawa, Japan
Fluoride molecular anions have emerged as a potential alternative to oxides for the extraction of actinides for AMS [1, 2]. Research at the Vienna Environmental Research Accelerator (VERA) in this respect has focused on mixing samples prepared as oxides in Fe₂O₃ with PbF₂ for in situ fluoridization during the sputter process. In this context, the relative formation probabilities for a range of (oxy-)fluoride molecular anions of the actinides uranium, neptunium, plutonium, and americium have been systematically investigated. This data helps to identify isobaric contaminations for those elements.
A first application is monitoring the production of the unwanted byproduct ²³⁶U in the irradiation of Th foils with ⁷Li ions, which is considered for the production of ²³⁶Np, a potential isotopic spike for ²³⁷Np. Exploring these relative formation probabilities is an important step towards the separation of U and Np isobars via selective photodetachment or reactive gases in the Ion Laser InterAction Mass Spectrometry (ILIAMS) ion-cooler [3,4].
Test samples were prepared by drying a well-known amount of ²³⁶U, ²³⁷Np, ²⁴²Pu and ²⁴³Am spike solution with 300 µg Fe (as nitric solution), ignition, and mixing with PbF₂ in a mass ratio of 1:9. The required instrumental setups, spanning a mass range from 306 u to 338 u, were scaled from the closest pilot beam (²³⁸UF₅⁻ and ²³⁸UF₂O₂⁻) by adjusting only the electrostatic components of VERA to allow for fast switching. The formation probability for each molecular anion AnFₙOₘ⁻ (An standing for the respective actinide) was determined relative to AnF₅⁻. The remaining material from an irradiation experiment previously measured at VERA in oxide form was mixed with PbF₂ and the distributions of the molecular anions for the trace masses 237 (co-produced ²³⁷Np) and 236 were compared to the spiked samples.
The distribution of the molecular anions was found to be element-specific for U, Np, Pu and Am. With progressive consumption of fluorine, the formation ratios shifted towards lower fluorides and oxyfluorides. Lower mixing ratios (1:4.5) with PbF₂ had a similar effect, while increased ratios (1:18) did not affect the distribution significantly. In sufficiently short (<2.5h) measurements, the molecular anion formation ratios for mass 236 obtained for samples irradiated at the RIKEN Nishina Center are well compatible with Np. This result indicates that ²³⁶Np was successfully produced in the irradiation and chemically separated at the University of Tsukuba. The investigation has also revealed that Uranium can be suppressed by one order of magnitude compared to Np by extracting them from the AMS target as UF₄⁻ and NpF₄⁻. Due to the large difference in formation probabilities, these two molecules are of particular interest for the development of the in-flight separation of ²³⁶U and ²³⁶Np with ILIAMS.
This work was funded by the Austrian Science Fund (FWF): [I 4803-N].
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 M. Martschini, this meeting.
Andreas Wiederin has gained experience with AMS measurements and the chemical sample preparation of actinides during work on his master’s thesis “Analysis of Isotopic Uranium and Plutonium Ratios in the Ocean by Accelerator Mass Spectrometry”. He is now employed as a full-time PhD-student at the University of Vienna in the Isotope Physics Group via the Austrian Science Fund (FWF) project [I 4803-N], which has the aim of developing an isotopic spike for 237Np. His current work is focused on the separation of atomic isobars in the actinide range, primarily 236Np and 236U via selective photodetachment or reactive gases.