Towards AMS measurements of 91Nb, 94Nb and 93Mo produced in fusion environment

Long-lived radionuclides, such as ⁹¹Nb, ⁹⁴Nb and ⁹³Mo, are expected to be produced in nuclear fusion reactors by reactions of high-energy neutrons with the structural material. Accurate predictions of waste categorization require experimental validation of simulation codes like FISPACT-II. This work explores the use of Ion-Laser InterAction Mass Spectrometry (ILIAMS) at the Vienna Environmental Research Accelerator (VERA) to measure these three radionuclides by accelerator mass spectrometry (AMS). The ILIAMS setup employs laser photodetachment to suppress their respective stable isobars: ⁹¹Zr, ⁹⁴Zr and ⁹⁴Mo, and ⁹³Nb.

For ^91,94Nb measurements, NbO₃⁻ is selected, with interferences from ZrO₃⁻ suppressed by collisions with the He buffer gas in the ion cooler. The suppression can be enhanced by overlapping a 355 nm laser with the ion beam. The lower limit for the suppression factor is 37000. In that way, we reach ⁹¹Zr/⁹³Nb and ⁹⁴Zr/⁹³Nb levels of 1.2 × 10⁻¹⁴ and 1.8 × 10⁻¹⁴, respectively, in targets prepared from commercial Nb₂O₅. MoO₃⁻ is suppressed by a factor of 4360, leading to a ⁹⁴Mo/⁹³Nb interference of 1.28 × 10⁻¹⁰ in the same targets.

For ⁹³Mo measurements, MoO₂⁻ is selected, with interference from NbO₂⁻ suppressed by 637 nm photons by a factor of 5.5 × 10⁶. This results in a ⁹³Nb/^natMo level of 1.3 × 10⁻¹³ in targets prepared from commercial MoO₃.

Suppression factors as high as this are not achieved by isobar suppression techniques based on differences in energy loss, not even by AMS facilities with terminal voltages above 8.5 MV.