The HOLMES experiment aims at directly measuring the electron neutrino mass using the electron capture (EC) decay of 163Ho. HOLMES performs a calorimetric measurement of the energy released in the decay of 163Ho. This allows to measure all the atomic de-excitation energy, except the fraction carried away by the neutrino. With a transition energy of about only 2.8 keV, 163Ho is a promising isotope. The direct measurement exploits only energy and momentum conservation and it is therefore completely model-independent. At the same time, the calorimetric measurement eliminates systematic uncertainties arising from the use of external beta sources, as in experiments with beta spectrometers, and minimizes the effect of the atomic de-excitation process uncertainties.
The baseline of the HOLMES experiment is to use TES microcalorimeters with about 300 Bq of 163Ho fully embedded in their absorbers. HOLMES will deploy an array of about 1000 low temperature microcalorimeters with implanted 163Ho nuclei. HOLMES may reach a statistical sensitivity of about 1.5 eV and it will be an important step forward in the direct neutrino mass measurement with a calorimetric approach as an alternative to spectrometry. It will also establish the potential of this approach to extend the sensitivity down to 0.1eV.
The HOLMES experiment is located in the Laboratories of INFN Genova and INFN Milano Bicocca / University of Milano-Bicocca.
The 163Ho isotope necessary for HOLMES is produced by neutron irradiation of Er2O3 enriched in 162Er at the ILL (Grenoble, France) high neutron flux nuclear reactor. The total inventory required for HOLMES is about 150 MBq of 163Ho, which could account for the total need for the experiment. The holmium produced after irradiation is chemically separated at PSI in a hot-cell by means of a specially developed efficient process.
To perform a calorimetric measurement of the EC spectrum of 163Ho, the isotopes must be embedded in the absorber of the low temperature microcalorimeters. The system for embedding the isotope is made of an ion implanter and a holmium evaporation chamber that will produce the metallic target for the ion implanter source.
The ion implanter features a Penning sputter ion source and a magnetic mass selection sector, to achieve an optimal mass separation for 163Ho. This allows to separate 163Ho from other trace contaminants not removed by chemical methods at PSI, such as the radioactive isotope 166mHo.
The implanter is integrated with a compact sputtering system to deposit the final gold layer which fully encapsulate the 163Ho source.
The metallic cathode for the ion source will be made out of metallic holmium pellets containing 163Ho which are produced in the holmium evaporation chamber by thermo-reduction and distillation at about 1600°C of the Ho2O3 extracted at PSI from the irradiated Er2O3.
The detectors used for the HOLMES experiment are TES microcalorimeters with gold absorbers. The devices are provided by NIST (Boulder, Co, USA) with a 1μm gold layer.