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

Abstract

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 σHo-163 to Ho-164m= (23±3) b and σHo-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) %.

Highly efficient isotope separation and ion implantation of 163Ho for the ECHo project

“…The combination of resonance ionization and high-transmission magnetic sector field mass separation will increase the purity of the implanted 163 Ho source to at least 99.9992(4) %. Using starting materials with a 166m Ho: 163 Ho ratio of 2.4(3) × 10 −5 and 3.2(6) × 10 −4 [4], this will lead to a 166m Ho contamination in the MMCs below 3(1) × 10 −10 and 4(2) × 10 −9 , respectively. In Table 2 the abundance of all relevant isotopes are calculated for every separation step.…”

“…A geometric efficiency of 20(2) % has been extracted for the typical arrangement of the ECHo detector array. With the current sample amount of 1.2(2) × 10 18 atoms of 163 Ho after the chemical separation [4], this would deliver 1.7(4) × 10 17 atoms in the detectors by applying ionization, separation and geometric implantation efficiency, which corresponds to an activity of about 800(200) kBq. This is in the range requested for the presently running project phase of ECHo 100k [1].…”

“…The ECHo 163 Ho samples are in the same chemical form as the 165 Ho AAS solution [4]. In Section 5 the isotope compositions of the initial samples are described.…”

“…For this purpose the synthetic radioisotope 163 Ho with a half-life of 4570(50) a [3] is produced by intense neutron irradiation of an enriched 162 Er target in the ILL high flux nuclear reactor. The Ho is then chemically separated from the target material and any other interfering elements [4]. The obtained samples mainly consists of 163 Ho, 165 Ho (stable) and 166m Ho (radioactive).…”

“…Calculated composition change of the two analyzed ECHo 163 Ho samples through resonance ionization (RI) and mass separation (MS). The initial composition is taken from[4]. × 10 −11 RI + MS ECHo chip with 10 Bq in each of the 64 absorbers takes about 1 h.The abundance of the interfering 166m…”