Critical Temperature tuning of Ti/TiN multilayer films suitable for low temperature detectors

We present our current progress on the design and test of Ti/TiN Multilayer for use in Kinetic Inductance Detectors (KIDs). Sensors based on sub-stoichiometric TiN film are commonly used in several applications. However, it is difficult to control the targeted critical temperature $T_C$, to maintain precise control of the nitrogen incorporation process and to obtain a production uniformity. To avoid these problems we investigated multilayer Ti/TiN films that show a high uniformity coupled with high quality factor, kinetic inductance and inertness of TiN. These features are ideal to realize superconductive microresonator detectors for astronomical instruments application but also for the field of neutrino physics. Using pure Ti and stoichiometric TiN, we developed and tested different multilayer configuration, in term of number of Ti/TiN layers and in term of different interlayer thicknesses. The target was to reach a critical temperature $T_C$ around $(1\div 1.5)$ K in order to have a low energy gap and slower recombination time (i.e. low generation-recombination noise). The results prove that the superconductive transition can be tuned in the $(0.5\div 4.6)$ K temperature range properly choosing the Ti thickness in the $(0\div 15)$ nm range, and the TiN thickness in the $(5\div 100)$ nm rang

Development of microwave superconducting microresonators for neutrino mass measurement in the HOLMES framework

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 (TESs) 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 $T_c$. We developed Ti/TiN multilayer microresonators with $T_c$ 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

Superconducting Microresonator Detectors for Neutrino Physics in Milano

Superconducting microwave microresonators are low temperature detectors compatible with large-scale multiplexed frequency domain readout. Our aim is to adapt and further advance the technology of microresonator detectors to develop new devices applied to the problem of measuring the neutrino mass. More specically, we aim to develop detector arrays which can be applied to the calorimetric measurement of the energy spectra of 163Ho EC decay (Q 2-3 keV) for a direct measurement of the neutrino mass. In order to achieve this goal, a study aimed to the selection of the best design and material for the detectors is required. A recent advance in microwave microresonator technology was the discovery that some metal nitrides, such as TiN, possess properties consistent with very high detector sensitivity. In this contribution, our progress on the design and test of Ti/TiN multilayer lms is presented. We report measurements made on stoichiometric TiN, sub-stoichiometric TiN and multilayer Ti/TiN lms including the critical temperature, the gap parameter and the quasi-particle recombination time extrapolated from keV X-ray pulses

Preparation of Papers for Special Issues of IEEE Development of Microresonator Detectors for 163Ho Endpoint Measurement in Milano

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 ~ 2.5 keV) 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

Superconducting Detectors for Neutrino Mass Measurement

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 (∼1 μs) 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

Superconducting Detectors for Neutrino Mass Measurement

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/c^2 with the calorimetric approach. Despite the remarkable performances in both energy (~ eV at keV) and time resolutions (∼1 μs) 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 ^(163)Ho-implanted microcalorimeters. HOLMES is a new experiment that aims to measure the electron neutrino mass by means of the electron capture decay of ^(163)Ho with an expected sensitivity of the order of the eV/c^2

Superconducting Detectors for Neutrino Mass Measurement

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/c^2 with the calorimetric approach. Despite the remarkable performances in both energy (~ eV at keV) and time resolutions (∼1 μs) 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 ^(163)Ho-implanted microcalorimeters. HOLMES is a new experiment that aims to measure the electron neutrino mass by means of the electron capture decay of ^(163)Ho with an expected sensitivity of the order of the eV/c^2