Improved ⁴¹Ca AMS measurements at DREAMS

Sensitivity of ⁴¹Ca measurements at the 6 MV AMS system at HZDR, DREAMS, using calcium fluoride (CaF₂) targets, is mainly limited by 2 factors: the total efficiency of the measurements; and the fraction of ions of its isobar ⁴¹K which mimic the signal of ⁴¹Ca in the gas ionization chamber detector.
The addition of lead fluoride (PbF₂) to the target mixture has been proven to boost the production of different (MFₙ)- ions. At DREAMS, changing the previously used mixture of CaF₂+Ag (1:4 w/w) by CaF₂+Ag+PbF₂ (1:4:4 w/w), ionization efficiency is increased from ∼0.15% to ∼0.45%.
The ⁴¹K suppression by the detector can also be improved, even without changes in the instrumentation itself. With an optimized analysis of the 4-dimensional signals from the gas ionization chamber detector, the suppression factor can be increased, at least, a factor 2: from 2 × 10⁴ to 4 × 10⁴.
The reported changes improve the total efficiency of ⁴¹Ca detection as well as the suppression of the ⁴¹K isobar and lead to a ⁴¹Ca/⁴⁰Ca sensitivity of 2-3 × 10⁻¹⁵ with an overall efficiency of ∼0.03%.