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

For accelerator mass spectrometry (AMS) measurements of 26Al 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 26MgO− 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 26Mg and acceptance of 26Al. 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 26Mg suppression factor of 103 and an acceptance of 26Al of >50% and therefore provides a significant advantage over the degrader foil method (sFoil(26Mg)≈3·102,τFoil(26Al)≈30%). In combination with the 26Mg separation of 104 in the GIC it is now possible to measure samples with a Mg content in the ppm range down to 26Al/27Al ≈10-14 with an overall efficiency that is approximately 5× improved with respect to the previous TANDEM setup.