High-sensitivity isobar-free AMS measurements and reference materials for 55Fe, 68Ge and 202gPb

Isobaric interference represents one of the major limitations in mass spectrometry. For a few cases in AMS with tandem accelerators, isobaric interference is completely excluded like the well-known major isotopes C-14, Al-26, I-129. Additional isotopes are Fe-55 (t(1/2) = 2.74 years), Ge-68 (t(1/2) = 270.9 days) and Pb-202 (t(1/2) = 52.5 kyr), with Ge-68 and Pb-202 never been used in AMS so far. Their respective stable isobars, Mn-55, Zn-68 and Hg-202 do not form stable negative ions. The exceptional sensitivity of AMS for Fe-55, Ge-68 and Pb-202g offers important insights into such different fields like nuclear astrophysics, fundamental nuclear physics and technological applications. VERA, a dedicated AMS facility is well suited for developing procedures for new and non-standard isotopes. AMS measurements at the VERA facility established low backgrounds for these radionuclides in natural samples. Limits for isotope ratios of <10(-15), <10(-16) and <= 2 x 10(14) were measured for Fe-55/Fe-56, Ge-68/Ge-70 and Pb-202/Pb, respectively. In order to generate accurate isotope ratios of sample materials, AMS relies on the parallel measurement of reference materials with well-known ratios. A new and highly accurate reference material for Fe-55 measurements with an uncertainty of +/- 1.6% was produced from a certified reference solution. In case of Ge-68 dedicated neutron activations produced a sufficiently large number of Ge-68 atoms that allowed quantifying them through the activity of its decay product Ga-68. Finally, for Pb-202, the short-lived isobar (TI)-T-202 was produced via neutron activation and served as a proxy for Pb-202 AMS measurements. (C) 2012 Elsevier B.V. All rights reserved.