Low-level ¹⁶⁶^mHo measurements with AMS for the ECHo-project

The Electron Capture in ¹⁶³Ho experiment (ECHo) aims at measuring the mass of νe by analysing the EC spectrum of the long-lived radionuclide ¹⁶³Ho (T_{1/2=4570 a) with a metallic magnetic calorimeter (MMC). For the determination of a reasonable upper limit for the neutrino mass it is mandatory to keep any contamination with the long-lived radionuclide ^166mHo nine orders of magnitude below the ¹⁶³Ho content. The ion-implantation of ultra-pure ¹⁶³Ho into a MMC for the experiment is carried out by the RISIKO mass separator. The separation from ¹⁶⁶mHo, however, cannot be quantified to such low levels as needed. Here we present our approach to determine the corresponding low isotopic ratio with accelerator mass spectrometry (AMS). This requires the formation of negative ions, we find the highest negative ion yield for the anion HoO₂⁻. For first tests ¹⁶⁵Ho was implanted by RISIKO in various metal foils and we obtained results for the Ho detection efficieny. This allows for extrapolations for the expected measurement limit of the ¹⁶⁶^mHo/¹⁶³Ho ratio.}