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 ¹⁶³Ho. They rely on the availability of large, radiochemically pure samples of ¹⁶³Ho. Here, we describe the production, separation, characterization, and sample production within the Electron Capture in Holmium-163 (ECHo) project. ¹⁶³Ho has been produced by thermal neutron activation of enriched, prepurified ¹⁶²Er 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 ¹⁶²Er target-material and from the main byproducts ¹⁷⁰Tm and ¹⁷¹Tm, which are co-produced in GBq amounts. Decontamination factors of >500 for Er and of >10⁵ for Tm and yields of 3.6·10¹⁶ and 1.2·10¹⁸ atoms of ¹⁶³Ho 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 ¹⁶³Ho 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 ¹⁶⁴Ho. Specific samples were produced for further purification by mass separation to isolate ¹⁶³Ho 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) %.