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Several experiments on the study of the electron neutrino mass are based on high-statistics measurements of the energy spectrum following electron capture of the radionuclide  163 Ho. They rely on the availability of large, radiochemically pure samples of  163 Ho. Here, we describe the production, separation, characterization, and sample production within the Electron Capture in Holmium-163 (ECHo) project.  163 Ho has been produced by thermal neutron activation of enriched, prepurified  162 Er targets in the high flux reactor of the Institut Laue-Langevin, Grenoble, France, in irradiations lasting up to 54 days. Irradiated targets were chemically processed by means of extraction chromatography, which allowed separating the formed Ho from the  162 Er target-material and from the main byproducts  170 Tm and  171 Tm, which are co-produced in GBq amounts. Decontamination factors of &gt;500 for Er and of &gt;10 5  for Tm and yields of 3.6·10 16  and 1.2·10 18  atoms of  163 Ho were obtained, corresponding to a recovery yield of 95 % of Ho in the chemical separation. The Ho-fraction was characterized by means of γ-ray spectrometry, Inductively-Coupled-Plasma Mass Spectrometry (ICP-MS), Resonance Ionization Mass Spectrometry (RIMS) and Neutron Activation Analysis (NAA). In this process, the thermal neutron capture cross section of  163 Ho was measured to σ Ho-163 to Ho-164m = (23±3) b and σ Ho-163 to Ho-164g = (156±9) b for the formation of the two isomers of  164 Ho. Specific samples were produced for further purification by mass separation to isolate  163 Ho from the Ho-isotope mixture, as needed for obtaining the energy spectrum within ECHo. The partial efficiency for this second separation step is (32±5) %.

Production, isolation and characterization of radiochemically pure 163 Ho samples for the ECHo-project