Measurement Of ～(93)Zr With Accelerator Mass Spectrometry

⁹³Zr with a very long half-life (1.5×10⁶ a) is a pure beta-particle-emitting radionuclide, which is produced by nuclear fission and neutron activation of the stable isotope ⁹²Zr. Zr is a constituent of the structural components of nuclear reactor vessels. Due to the existence of the stable isobar ⁹³Nb, it is difficult to detect ⁹³Zr with conventional mass spectrometry technique. AMS is one of the most promising methods to detect trace amounts of ⁹³Zr.A method for the measurement of ⁹³Zr is being developed by the Accelerator Mass Spectrometry (AMS) Laboratory group at China Institute of Atomic Energy (CIAE). AMS provides ultra sensitivity for this nuclide with high rejection of interferences.Efforts in sample preparation have been focused on standard samples and blank sample. The ⁹³Zr/⁹²Zr ratio in the original sample was determined by measurement of X-rays associated with the isomeric transition of Niobium-93m. The ⁹³Zr was sequentially diluted with natural Zr to obtain a series of standard samples with ⁹³Zr/⁹²Zr ratios ranging from 10^-7 to 10^-12. A Silica gel chromatography method for radiochemical separation of Zr has been developed to reduce content of stable isobar ⁹³Nb, the main interference for the AMS measurement of ⁹³Zr. Monitored using ⁹⁵Zr tracer, Zirconium recovery is more than 80%, and the decontamination factor for Nb is more than 100.To lower the background level of Nb, target needles made of organic glass and supporting plates made of polyvinyl fluoride were used during pressing pellet into standard aluminum AMS sample holder.A challenge in AMS measurements of ⁹³Zr is high background from the isobaric interference ⁹³Nb. AMS provides a significant advantage in this regard in that the high energy of the resulting ions allows the use of nuclear technique for identification and rejection of interferences. For the energies attainable with the CIAE accelerator mass spectrometer, a reduction of ⁹³Nb interference by up to one order of magnitude is obtained. The expected sensitivity for blank sample is in the order of 10^-10 for ⁹³Zr/⁹²Zr ratio, depending on the chemical recovery of zirconium and the level of residual niobium.AMS measurements are performed using the HI-13 tandem accelerator at CIAE. The accelerating voltage is 8.2 MV and carbon foil stripping is used. The high energy spectrometer is set to select ⁹³Zr¹⁰⁺ions at 89.1 MeV. Ion counting is performed using a gas ionization detector filled to 95 Torr with P10 (90% argon, 10% methane). Differential energy loss is measured in 4 anodes. Two dimensional spectra are used for particle identification. ⁹³Zr and ⁹³Nb are partially resolved at the energies available with the HI-13 tandem.Under current conditions, the sensitivity for the atomic ratio of ⁹³Zr/⁹²Zr is 10^-10.