Accelerator mass spectrometry measurements of ⁹³Zr for astrophysical and nuclear technology applications

Zirconium-93 is a long-lived radionuclide with a half-life of (1.61 ± 0.05) × 10⁶ yr. Production cross sections for ⁹³Zr by neutron capture on stable ⁹²Zr in the keV and thermal energy ranges are important input parameters for astrophysical network calculations and nuclear industry, respectively. Despite their importance, existing experimental data suffer from large uncertainties. Here, the combination of neutron activation and accelerator mass spectrometry (AMS) is presented as an alternative method to online time-of-flight measurements for the determination of these cross sections. The main challenges for AMS of ⁹³Zr are the interference from the stable isobar ⁹³Nb and the production of suitable reference material. At the Heavy Ion Accelerator Facility (HIAF) the first challenge is tackled with the available high particle energies and by investigating different Zr compounds and extracted molecular beams. Using ZrF₅⁻ beams extracted from ZrF₄ sample material, it is shown that the Nb background can be reduced by up to two orders of magnitude relative to the extraction of ZrO⁻ beams from ZrO₂. Using the 13⁺ charge state and particle energies of ∼190 MeV, ⁹³Zr/Zr background levels in the 10⁻¹² range are regularly achieved at HIAF. The ZrF₅⁻ output from ZrF₄ samples may be highly variable but can be stabilised using an intimate mixture of ZrF₄ and PbF₂ at a ratio of 1:10 that has been dried down from a concentrated hydrofluoric acid solution. An option to produce a well-characterised ⁹³Zr reference material by utilising the well-known ²³⁵U-fission yield of ⁹³Zr is discussed. Once this remaining challenge is resolved, accurate measurements of the astrophysically and technologically relevant neutron-capture cross sections of ⁹²Zr using AMS will be possible at HIAF.