Fluorides or hydrides? ⁴¹Ca performance at VERA's 3-MV AMS facility

Recent improvements in isobaric suppression for medium-mass isotopes, e.g. ⁴¹Ca, offer new possibilities for tandem accelerators with terminal voltages of 3 MV or lower; i.e. when dealing with particle energies ≤1 MeV/amu. In particular, detection of ⁴¹Ca requires sufficient discrimination of the stable isobar ⁴¹K. We explored the limits of ⁴¹Ca detection at our 3-MV AMS facility by means of different types of particle detectors: The ΔTOF method, which is based on the different flight-time of isobars after passing a thick absorber foil. The second method makes use of a new type of compact ionization chamber: ⁴¹K and ⁴¹Ca are separated in energy due to their different energy loss in the detector entrance foil and the detector gas, which is measured via a segmented anode. At VERA we measured ⁴¹Ca/Ca ratios below 10^-13 for commercial CaF₂ material serving as blank samples. CaH₂ sputter targets, with the extraction of CaH₃^-, yielded background ratios as low as ⁴¹Ca/Ca = 1 × 10^-15. The typical measurement precision at VERA for ⁴¹Ca measurements was between 2% and 5%. These results demonstrate that AMS facilities based on 3-MV tandems have reached the sensitivity level of larger AMS facilities for a wide range of applications, with the advantage of high overall efficiency and sample throughput.