Accelerator mass spectrometry of 26Al at 6 MV using AlO- ions and a gas-filled magnet

For accelerator mass spectrometry (AMS) measurements of Al-26 the use of AlO- ions holds considerable promise given their ionization yield which is about an order of magnitude higher compared to the commonly used Al- ions. However, the strong isobaric interference introduced by (MgO-)-Mg-26 ions must be reduced before the identification in a gas ionization chamber (GIC). At the ETH Zurich 6 MV EN TANDEM accelerator the foil degrader method as well as the gas-filled magnet (GFM) method were systematically investigated with respect to suppression of Mg-26 and acceptance of Al-26. Additionally a new, large-acceptance GIC was designed to capture as large a fraction of the broad beams as possible. The GFM method achieves a Mg-26 suppression factor of 10(3) and an acceptance of (26)AI of >50% and therefore provides a significant advantage over the degrader foil method (s(Foll)(Mg-26) approximate to 3.10(2), tau(Foil)(Al-26) approximate to 30%). In combination with the Mg-26 separation of 10(4) in the GIC it is now possible to measure samples with a Mg content in the ppm range down to Al-26/Al-27 approximate to 10(-14) with an overall efficiency that is approximately 5x improved with respect to the previous TANDEM setup. (C) 2017 Elsevier B.V. All rights reserved.