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

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 >500 for Er and of >105 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) %.