AMS of Zr-93: Passive absorber versus gas-filled magnet

Two different isobar separation techniques were tested for the detection of the long-lived fission product Zr-93 (T-1/2 = 1.64 x 10⁶ a) using Accelerator Mass Spectrometry (AMS), i.e. a passive absorber and a gas-filled magnet, respectively. Both techniques were used in combination with a Time-of-Flight path for the identification of the stable neighboring isotopes Zr-92 and Zr-94. The passive absorber was represented by a stack of silicon nitride foils for high flexibility regarding the thickness for optimal isobar separation. Ion beams with a large variety of energies, between 80 and 180 MeV, were provided for this experiment by the tandem accelerator at the Maier-Leibnitz Laboratory in Garching, Germany. With these beams, the stopping powers of Zr-93 and Nb-93 as a function of energy were determined experimentally and compared to the results obtained with the simulation program SRIM. Considerable discrepancies regarding the energy dependence of the two stopping power curves relative to each other were found. The lowest detection limit for Zr-93 achieved with the passive absorber setup was Zr-93/Zr = 1x10^-10. In comparison, by optimizing the gas-filled magnet set-up, Nb-93 was suppressed by around six orders of magnitude and a detection limit of Zr-93/Zr = 5x10^-11 was obtained. To our knowledge, these results represent the lowest detection limit achieved for Zr-93 until now. (authors)