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 ¹⁴C, ²⁶Al, ¹²⁹I. Additional isotopes are ⁵⁵Fe (t_1/2 = 2.74 years), ⁶⁸Ge (t_1/2 = 270.9 days) and ²⁰²Pb (t_1/2 = 52.5 kyr), with ⁶⁸Ge and ²⁰²Pb never been used in AMS so far. Their respective stable isobars, ⁵⁵Mn, ⁶⁸Zn and ²⁰²Hg do not form stable negative ions. The exceptional sensitivity of AMS for ⁵⁵Fe, ⁶⁸Ge and ^202gPb 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 × 10^-14 were measured for ⁵⁵Fe/⁵⁶Fe, ⁶⁸Ge/⁷⁰Ge and ²⁰²Pb/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 ⁵⁵Fe measurements with an uncertainty of ±1.6% was produced from a certified reference solution. In case of ⁶⁸Ge dedicated neutron activations produced a sufficiently large number of ⁶⁸Ge atoms that allowed quantifying them through the activity of its decay product ⁶⁸Ga. Finally, for ²⁰²Pb, the short-lived isobar ²⁰²Tl was produced via neutron activation and served as a proxy for ²⁰²Pb AMS measurements.