Present and future of double-beta decay searches with bolometric detectors

Thanks to the excellent energy resolution, high efficiency and versatility, bolometric detectors are primed for the search of neutrinoless double-beta decay (0νDBD). The most advanced bolometric experiment, CUORE, is studying the 0νDBD of 130Te using a 741 kg array of TeO2 crystals. CUORE points to a 90% CL sensitivity on the half-life of 0νDBD of 9.5 × 1025 yr in 5 yr, corresponding to an upper limit on the neutrino Majorana mass of 50–130 meV. This sensitivity will allow to touch, but not to explore, the region corresponding to the inverted hierarchy mass scenario. In this document I present the status of CUORE and the possible upgrades of the bolometric technology in view of a next generation experiment

LUCIFER: A Scintillating Bolometer Array for the Search of Neutrinoless Double Beta Decay

One of the main limitations in the study of 0vDBD is the presence of a radioactive background in the energy region of interest. This limit can be overcome by the technological approach of the LUCIFER project, which is based the double read-out of the heat and scintillation light produced by ZnSe scintillating bolometers. This experiment aims at a background lower than 10−3counts/keV/kg/y in the energy region of the 0νDBD of 82Se. Such a low background level will provide a sensitivity on the effective neutrino mass of the order of 100 meV. In the following, the results of the recent R&D activity are discussed, the single module for the LUCIFER detector is described, and the process for the production of 82Se-enriched ZnSe crystals is presented

Measurements and optimization of the light yield of a TeO2_2 crystal

Bolometers have proven to be good instruments to search for rare processes because of their excellent energy resolution and their extremely low intrinsic background. In this kind of detectors, the capability of discriminating alpha particles from electrons represents an important aspect for the background reduction. One possibility for obtaining such a discrimination is provided by the detection of the Cherenkov light which, at the low energies of the natural radioactivity, is only emitted by electrons. This paper describes the method developed to evaluate the amount of light produced by a crystal of TeO$_2$ when hit by a 511 keV photon. The experimental measurements and the results of a detailed simulation of the crystal and the readout system are shown and compared. A light yield of about 52 Cherenkov photons per deposited MeV was measured. The effect of wrapping the crystal with a PTFE layer, with the aim of maximizing the light collection, is also presented

Phonon-Mediated KIDs as Light Detectors for Rare-Event Search: The CALDER Project

Background suppression plays a crucial role in experiments searching for rare events, like neutrino-less double beta decay (0 $$\nu$$ DBD) and dark matter. Large mass bolometers that are among the most competitive devices in this field would largely benefit from the development of ultrasensitive light detectors, as the combined readout of the bolometric and light signals enables the particle identification. The CALDER collaboration is developing cryogenic light detectors that will match the requirements of next generation experiments: noise lower than 20 eV RMS, large active area (several cm $$^{2}$$ ), wide temperature range of operation, and ease in fabricating and operating a thousand of detectors. For this purpose, we are exploiting the excellent energy resolution and the natural multiplexed read-out provided by kinetic inductance detectors (KIDs). These devices can be operated in a phonon-mediated approach, in which KIDs are coupled to a large insulating substrate in order to increase the active surface from a few mm $$^{2}$$ to 25 cm $$^{2}$$ . Our current best prototype, based on aluminum LEKIDs, reached a baseline sensitivity of 80 eV with an overall efficiency of about 20 %

Prediction of the service life of brick/stone masonry damaged by salt crystallisation: application of a stochastic model

An attempt has been made within a EC Contract, to establish the maximum salt content in brick and stone masonry, below which the surface protection treatments do not fail. Crystallisation tests were carried out on treated and untreated brick and limestone masonry specimens. A large number of tests were previously carried out on the single units used for the masonry specimens. Salt solutions with two low concentrations of sodium sulphate were inserted in masonry wallettes treated with a water based water repellent or with a consolidant. On the basis of the recorded experimental data, a suitable damage parameter describing the material deterioration process has been chosen. The parameter assumed is the loss of surface material. The deterioration process could be interpreted as a stochastic process L(t,l), function of time t and damage l. In this way, for differen t damage levels l it is possible to build the fragility curve. By using this approach the magnitude of the expected damage over time and the occurrence time of it can be predicted. The results will allow for the investigation on the durability of materials with respect to the prediction treatments and on the decay process of single and composite materials

New application of superconductors: high sensitivity cryogenic light detectors

In this paper we describe the current status of the CALDER project, which is developing ultra-sensitive light detectors based on superconductors for cryogenic applications. When we apply an AC current to a superconductor, the Cooper pairs oscillate and acquire kinetic inductance, that can be measured by inserting the superconductor in a LC circuit with high merit factor. Interactions in the superconductor can break the Cooper pairs, causing sizable variations in the kinetic inductance and, thus, in the response of the LC circuit. The continuous monitoring of the amplitude and frequency modulation allows to reconstruct the incident energy with excellent sensitivity. This concept is at the basis of Kinetic Inductance Detectors (KIDs), that are characterized by natural aptitude to multiplexed read-out (several sensors can be tuned to different resonant frequencies and coupled to the same line), resolution of few eV, stable behavior over a wide temperature range, and ease in fabrication. We present the results obtained by the CALDER collaboration with 2x2 cm2 substrates sampled by 1 or 4 Aluminum KIDs. We show that the performances of the first prototypes are already competitive with those of other commonly used light detectors, and we discuss the strategies for a further improvement

Characterization of the KID-Based Light Detectors of CALDER

The aim of the Cryogenic wide-Area Light Detectors with Excellent Resolution (CALDER) project is the development of light detectors with active area of $5\times5$ cm$^2$ and noise energy resolution smaller than 20 eV RMS, implementing phonon-mediated kinetic inductance detectors. The detectors are developed to improve the background suppression in large-mass bolometric experiments such as CUORE, via the double read-out of the light and the heat released by particles interacting in the bolometers. In this work, we present the characterization of the first light detectors developed by CALDER. We describe the analysis tools to evaluate the resonator parameters (resonant frequency and quality factors) taking into account simultaneously all the resonance distortions introduced by the read-out chain (as the feed-line impedance and its mismatch) and by the power stored in the resonator itself. We detail the method for the selection of the optimal point for the detector operation (maximizing the signal-to-noise ratio). Finally, we present the response of the detector to optical pulses in the energy range of 0-30 keV

TeO2_2 bolometers with Cherenkov signal tagging: towards next-generation neutrinoless double beta decay experiments

CUORE, an array of 988 TeO$_2$ bolometers, is about to be one of the most sensitive experiments searching for neutrinoless double-beta decay. Its sensitivity could be further improved by removing the background from $\alpha$ radioactivity. A few years ago it has been pointed out that the signal from $\beta$s can be tagged by detecting the emitted Cherenkov light, which is not produced by $\alpha$s. In this paper we confirm this possibility. For the first time we measured the Cherenkov light emitted by a CUORE crystal, and found it to be 100 eV at the $Q$-value of the decay. To completely reject the $\alpha$ background, we compute that one needs light detectors with baseline noise below 20 eV RMS, a value which is 3-4 times smaller than the average noise of the bolometric light detectors we are using. We point out that an improved light detector technology must be developed to obtain TeO$_2$ bolometric experiments able to probe the inverted hierarchy of neutrino masses.Comment: 5 pages, 4 figures. Added referee correction