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The European Research Council has recently funded HOLMES, a new experiment to directly measure the neutrino mass. HOLMES will perform a calorimetric measurement of the energy released in the decay of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{163}$$\end{document}163Ho. The calorimetric measurement eliminates systematic uncertainties arising from the use of external beta sources, as in experiments with beta spectrometers. This measurement was proposed in 1982 by A. De Rujula and M. Lusignoli, but only recently the detector technological progress allowed to design a sensitive experiment. HOLMES will deploy a large array of low temperature microcalorimeters with implanted \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{163}$$\end{document}163Ho nuclei. The resulting mass sensitivity will be as low as 0.4 eV. HOLMES 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 0.1 eV. We outline here the project with its technical challenges and perspectives.

Assessing the absolute neutrino mass scale is one of the major challenges in particle physics and astrophysics nowadays. A powerful tool to directly estimate the effective electron neutrino mass consists in the calorimetric measurement of the energy released within a nuclear beta decay. The progresses made over the last few decades on low-temperature detector technologies have permitted to design experiments with expected sensitivities on the neutrino mass below 1 eV/c2 with the calorimetric approach. Despite the remarkable performances in both energy (∼ eV at keV) and time resolutions $(\sim\! 1\ \mu\text{s})$ on the single channel, a large number of detectors working in parallel is required to reach a sub-eV sensitivity. Microwave frequency-domain readout provides a powerful technique to read out large arrays of low-temperature detectors, such as transition edge sensors (TESs) or microwave kinetic inductance detectors (MKIDs). In this way, the multiplex factor is only limited by the bandwidth of the available commercial fast digitizers. The microwave multiplexing system will be used to read out the TES array of the HOLMES experiment, which is made of 1000 163Ho-implanted microcalorimeters. HOLMES is a new experiment that aims to measure the electron neutrino mass by means of the electron capture decay of 163Ho with an expected sensitivity of the order of the eV/c2.

The European Research Council has recently funded HOLMES, a project with the aim of performing a calorimetric measurement of the electron neutrino mass measuring the energy released in the electron capture decay of 163Ho. The baseline for HOLMES are microcalorimeters coupled to transition edge sensors read-out with rf-SQUIDs, for microwave multiplexing purposes. A promising alternative solution is based on superconducting microwave resonators that have undergone rapid development in the last decade. These detectors, called Microwave Kinetic Inductance Detectors (MKIDs), are inherently multiplexed in the frequency domain and suitable for even larger-scale pixel arrays, with theoretical high energy resolution and fast response. The aim of our activity is to develop arrays of microresonator detectors for X-ray spectroscopy and suitable for the calorimetric measurement of the energy spectra of 163Ho. Superconductive multilayer films composed by a sequence of pure Titanium and stoichiometric TiN layers show many ideal properties for MKIDs, such as low loss, large sheet resistance, large kinetic inductance, and tunable critical temperature $$T_\mathrm{c}$$ . We developed Ti/TiN multilayer microresonators with $$T_\mathrm{c}$$ within the range from 70 mK to 4.5 K and with good uniformity. In this contribution, we present the design solutions adopted, the fabrication processes, and the characterization results.

HOLMES is a new experiment to directly measure the neutrino mass with a sensitivity as low as 0.4 eV. HOLMES will perform a calorimetric measurement of the energy released in the electron capture decay of 163 Ho. HOLMES will deploy a large array of low temperature microcalorimeters with implanted 163 Ho nuclei. HOLMES baseline detector is an array of 1000 microcalorimeters each with an implanted 163 Ho activity of about 300 Bq, an energy resolution FWHM of about 1 eV at the spectrum end-point ( Q ≈ 2.5 keV), and a time resolution of about 1 μs. Matching these performances requires a careful optimization of all components, from the microcalorimeters to the signal processing algorithms. We outline here the project technical challenges and the present status of the development.

The main goal in the HOLMES experiment is the neutrino mass measurement using an array of 1000 micro-calorimeters with standard metallic absorber. A good isotope for such measurement is the 163 Ho, those isotopes embedded in the metallic absorber will be 10 11 –10 13 . Since 163 Ho is not available in nature, a dedicated process must be set up to produce the amount needed for this neutrino mass experiment. The process with the highest born-up cross-section is the neutron irradiation of Er 2 O 3 enriched in 162 Er: 162 Er(n,γ) 163 Er → 163 Ho+ν e , where the decay is an EC with half-life of about 75 min and the (n,γ) is about 20 barns for thermal neutron. After the neutron irradiation in the oxide powder there are several radioactive isotopes which are potentially disturbing because of the background that they cause below 5 keV. The chemical separation of holmium from the irradiation enriched Er 2 O 3 powder is therefore mandatory and will be performed by means of ion exchange chromatography. On the end of those processes the oxide powder enriched in 162 Er will have the 163 Ho isotope number required. The holmium chemical state influences the end point of the EC spectrum, in order to avoid such effect it is necessary to embed in the absorber only the metallic isotope. Reduction and distillation technique allowed us to obtain a pure metallic holmium, starting from natural oxide holmium. This technique will be applied on the irradiated oxide powder to obtain the metallic 163 Ho, ready to be embedded in the micro-calorimeter absorber.

The determination of the neutrino mass is still an open issue in particle physics. The calorimetric measurement of the energy released in a nuclear beta decay allows measuring the whole energy, except the fraction carried away by the neutrino: due to the energy conservation, a finite neutrino mass mν causes the energy spectrum to be truncated at Q- mν, where Q is the transition energy of the decay. The electron capture of 163Ho (Q∼ 2.5keV) is an ideal decay, due to the high fraction of events close to the endpoint (i.e., the maximum energy of the relaxation energy spectrum). In order to achieve enough statistics, a large number of detectors (∼104) are required. Superconducting microwave microresonators are detectors suitable for large-scale multiplexed frequency-domain readout, with theoretical energy and time resolution on the order of electronvolts and microseconds, respectively. Our aim is to develop arrays of microresonator detectors applicable to the calorimetric measurement of the energy spectrum of 163Ho. Currently, a study aimed at the selection of the best design and material for the detectors is in progress. In order to obtain low- Tc detectors, with Tc ranging between ∼0.5 and 2 K, different Ti/TiN (titanium nitride) multilayer films were produced. The reduced Tc was obtained by superposing thin layers of stoichiometric TiN to pure Ti layers, and the Tc was tuned by varying the ratio between the thickness of the layers. In this contribution, a comparison between the measurements (critical temperature, gap parameter, and X-ray energy spectra) made with stoichiometric and substoichiometric TiN and Ti/TiN multilayer film microresonators is presented.