93Zr developments at the Heavy Ion Accelerator Facility at ANU

Abstract The long-lived radionuclide 93 Zr t 1/2  = (1.61 ± 0.05) Ma plays an important role in nuclear astrophysics and nuclear technology. In stellar environments, it is mainly produced by neutron capture on the stable nuclide 92 Zr. On Earth high amounts of radioactive 93 Zr are produced in nuclear power plants directly from 235 U fission, but also by neutron capture on 92 Zr, as Zr-alloys are commonly used as cladding for nuclear fuel rods. Despite its importance, the neutron capture cross section of 92 Zr at thermal and stellar energies (keV) is not well known. Neutron irradiation of 92 Zr and subsequent determination of produced 93 Zr via AMS is a promising approach to resolve this issue. The main challenge in AMS measurements of 93 Zr is the interference from the stable isobar 93 Nb. The high particle energies available with the 14UD tandem accelerator at the Australian National University are ideal to tackle this challenge. Different sample materials, molecular ion species and sample holder materials were tested for their 93 Nb background. Commercial ZrO 2 powder irradiated with thermal neutrons from the reactor at the Atominstitut in Vienna (ATI) was used as reference material for AMS measurements. In contrast to literature reports and γ-activity measurements of 95 Nb, which suggest that chemical Nb reduction works, elevated 93 Nb contents were measured in chemically pre-treated samples. The reasons are under investigation. At the ANU we developed AMS for ∼210 MeV 93 Zr ions using an 8 anode ionisation chamber. We achieved background levels of 93 A/Zr ∼ 10 −12 with acceptance of 2–8% of the 93 Zr ions at the high-energy side. This is more than an order of magnitude better than previously reported. The 93 Nb isobar was suppressed by a factor between 13,000 and 90,000 in the detector. This performance allows measurements of the thermal and stellar neutron-capture cross section of 92 Zr for samples irradiated at the ATI and the Soreq Applied Research Accelerator Facility, respectively, using AMS.