The Development of ⁹³Zr/⁹³Nb Isobar Separation Technique for Future ⁹³Zr AMS Measurement

⁹³Zr and the stable Zr isotopes (except ⁹⁶Zr) are traditionally thought to be mainly produced by the s-process in Asymptotic Giant Branch stars and secondarily, by the r-process in massive stars. Other nucleosynthesis processes may also produce a significant amount of these isotopes. There are significant disagreements between different stellar models regarding ⁹³Zr and the other stable Zr isotopes production. To better understand the nucleosynthesis of Zr and refine stellar models, precise knowledge of the neutron capture cross sections on Zr isotopes are very important. These cross sections are also important for understanding the large amount of ⁹³Zr produced in nuclear reactors, which makes it an important radionuclide in nuclear waste management.

One promising tool for measuring both the Zr isotope neutron capture cross sections and the radioactive ⁹³Zr concentration in nuclear waste is accelerator mass spectrometry (AMS). The main challenges in the AMS measurement of the ⁹³Zr isotope are the separation of ⁹³Zr from stable Zr isotopes (limited by the facility infrastructure) and the separation of ⁹³Zr from its stable isobar ⁹³Nb (one atomic number difference).

The development of a technique for future ⁹³Zr AMS measurements has been performed at the Nuclear Science Laboratory (NSL) of the University of Notre Dame. The combination of a Gas-Filled Magnet and gas ionization chamber techniques are used, while other experimental methods like the projectile X-ray emission when hitting a target are also explored. A dedicated small ionization chamber was built for the ⁹³Zr experiment. Initial tests have shown that this detector has improved resolution compared with the original detector.

This experiment faces three obstacles: the current injection magnet lacks sufficient mass resolution for this mass region; the available beam energy is not high enough; and the energy resolution of the detector needs to be improved. As a result, the current achieved ⁹³Zr/Zr sensitivity is at ~ 10^-5. A ⁹³Nb reduction chemistry has also been developed. It is tested by the AMS method at the Vienna Environmental Research Accelerator laboratory to be a factor of 1000, which brings the sensitivity level to ~ 10^-8.

To further improve the ⁹³Zr experiment sensitivity, work is continuing on improving the detector performances and the upgrade of the NSL low energy injection system is in progress. After these upgrades, the sensitivity is expected to reach ~ 10^-11.