Towards AMS measurements of $^{91}$Nb, $^{94}$Nb and $^{93}$Mo produced in fusion environment

Abstract

Long-lived radionuclides, such as $^{91}$Nb, $^{94}$Nb and $^{93}$Mo, 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: $^{91}$Zr, $^{94}$Zr and $^{94}$Mo, and $^{93}$Nb. For $^{91,94}$Nb measurements, NbO$_3^-$ is selected, with interferences from ZrO$_3^-$ 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 $^{91}$Zr/$^{93}$Nb and $^{94}$Zr/$^{93}$Nb levels of $1.2\,\times\,10^{-14}$ and $1.8\,\times\,10^{-14}$, respectively, in targets prepared from commercial Nb$_2$O$_5$. MoO$_3$ is suppressed by a factor of 4360, leading to a $^{94}$Mo/$^{93}$Nb interference of $1.28\,\times\,10^{-10}$ in the same targets. For $^{93}$Mo measurements, MoO$_2^-$ is selected, with interference from $^{93}$NbO$_2^-$ suppressed by 637\,nm photons by a factor of $5.5\,\times\,10^6$. This results in a $^{93}$Nb/$^\text{nat}$Mo level of $1.3\,\times\,10^{-13}$ in targets prepared from commercial MoO$_3$. 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.