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A bstract Neutrinoless double-beta decay is a key process in particle physics. Its experimental investigation is the only viable method that can establish the Majorana nature of neutrinos, providing at the same time a sensitive inclusive test of lepton number violation. CROSS (Cryogenic Rare-event Observatory with Surface Sensitivity) aims at developing and testing a new bolometric technology to be applied to future large-scale experiments searching for neutrinoless double-beta decay of the promising nuclei 100 Mo and 130 Te. The limiting factor in large-scale bolometric searches for this rare process is the background induced by surface radioactive contamination, as shown by the results of the CUORE experiment. The basic concept of CROSS consists of rejecting this challenging background component by pulse-shape discrimination, assisted by a proper coating of the faces of the crystal containing the isotope of interest and serving as energy absorber of the bolometric detector. In this paper, we demonstrate that ultra-pure superconductive Al films deposited on the crystal surfaces act successfully as pulse-shape modifiers, both with fast and slow phonon sensors. Rejection factors higher than 99.9% of α surface radioactivity have been demonstrated in a series of prototypes based on crystals of Li 2 MoO 4 and TeO 2 . We have also shown that point-like energy depositions can be identified up to a distance of ∼ 1 mm from the coated surface. The present program envisions an intermediate experiment to be installed underground in the Canfranc laboratory (Spain) in a CROSS-dedicated facility. This experiment, comprising ∼ 3 × 10 25 nuclei of 100 Mo, will be a general test of the CROSS technology as well as a worldwide competitive search for neutrinoless double-beta decay, with sensitivity to the effective Majorana mass down to 70 meV in the most favorable conditions.

The future Ricochet experiment aims at searching for new physics in the electroweak sector by providing a high precision measurement of the Coherent Elastic Neutrino-Nucleus Scattering (CENNS) process down to the sub-100 eV nuclear recoil energy range. The experiment will deploy a kg-scale low-energy-threshold detector array combining Ge and Zn target crystals 8.8 m away from the 58 MW research nuclear reactor core of the Institut Laue Langevin (ILL) in Grenoble, France. Currently, the Ricochet Collaboration is characterizing the backgrounds at its future experimental site in order to optimize the experiment’s shielding design. The most threatening background component, which cannot be actively rejected by particle identification, consists of keV-scale neutron-induced nuclear recoils. These initial fast neutrons are generated by the reactor core and surrounding experiments (reactogenics), and by the cosmic rays producing primary neutrons and muon-induced neutrons in the surrounding materials. In this paper, we present the Ricochet neutron background characterization using 3 He proportional counters which exhibit a high sensitivity to thermal, epithermal and fast neutrons. We compare these measurements to the Ricochet Geant4 simulations to validate our reactogenic and cosmogenic neutron background estimations. Eventually, we present our estimated neutron background for the future Ricochet experiment and the resulting CENNS detection significance. Our results show that depending on the effectiveness of the muon veto, we expect a total nuclear recoil background rate between 44 ± 3 and 9 ± 2 events/day/kg in the CENNS region of interest, i.e. between 50 eV and 1 keV. We therefore found that the Ricochet experiment should reach a statistical significance of 4.6 to 13.6  σ for the detection of CENNS after one reactor cycle, when only the limiting neutron background is considered.

Random coincidences of events could be one of the main sources of background in the search for neutrino-less double-beta decay of 100 Mo with macro-bolometers, due to their modest time resolution. Scintillating bolometers as those based on Li 2 MoO 4 crystals and employed in the CROSS and CUPID experiments can eventually exploit the coincident fast signal detected in a light detector to reduce this background. However, the scintillation provides a modest signal-to-noise ratio, making difficult a pile-up pulse-shape recognition and rejection at timescales shorter than a few ms. Neganov–Trofimov–Luke assisted light detectors (NTL-LDs) offer the possibility to effectively increase the signal-to-noise ratio, preserving a fast time-response, and enhance the capability of pile-up rejection via pulse shape analysis. In this article we present: (a) an experimental work performed with a Li 2 MoO 4 scintillating bolometer, studied in the framework of the CROSS experiment, and utilizing a NTL-LD; (b) a simulation method to reproduce, synthetically, randomly coincident two-neutrino double-beta decay events; (c) a new analysis method based on a pulse-shape discrimination algorithm capable of providing high pile-up rejection efficiencies. We finally show how the NTL-LDs offer a balanced solution between performance and complexity to reach background index ∼ 10 - 4 counts/keV/kg/year with 280 g Li 2 MoO 4 ( 100 Mo enriched) bolometers at 3034 keV, the Q β β of the double-beta decay, and target the goal of a next generation experiment like CUPID.

The Crab (Calibrated nuclear Recoils for Accurate Bolometry) project aims to precisely characterize the response of cryogenic detectors to sub-keV nuclear recoils of direct interest for coherent neutrino-nucleus scattering and dark matter search experiments. The Crab method relies on the radiative capture of thermal neutrons in the target detector, resulting in a nuclear recoil with a well-defined energy. We present a new experimental setup installed at the TRIGA Mark-II reactor at Atominstitut (Vienna), providing a low intensity beam of thermal neutrons sent to the target cryogenic detector mounted inside a wet dilution refrigerator Kelvinox 100. After the presentation of all components of the setup we report the analysis of first commissioning data with CaWO 4 detectors of the Nucleus experiment. They show stable operation of the cryostat and detectors on a week-scale. Due to an energy resolution currently limited to 20 eV we use neutron beam induced events at high energy, in the 10 to 100 keV range, to demonstrate the excellent agreement between the data and simulation and the accurate understanding of external background. Thanks to these data we also propose an updated decay scheme of the low-lying excited states of 187 W. Finally, we present the first evidence of neutron-capture induced coincidences between BaF 2 γ -detectors installed around the dewar and the inner cryogenic detector. These promising results pave the way for an extensive physics program with various detector materials, like CaWO 4 , Al 2 O 3 , Ge and Si.

Random coincidences of nuclear events can be one of the main background sources in low-temperature calorimetric experiments looking for neutrinoless double-beta decay, especially in those searches based on scintillating bolometers embedding the promising double-beta candidate 100  Mo, because of the relatively short half-life of the two-neutrino double-beta decay of this nucleus. We show in this work that randomly coinciding events of the two-neutrino double-beta decay of 100  Mo in enriched Li 2 100 MoO 4 detectors can be effectively discriminated by pulse-shape analysis in the light channel if the scintillating bolometer is provided with a Neganov–Luke light detector, which can improve the signal-to-noise ratio by a large factor, assumed here at the level of ∼ 750 on the basis of preliminary experimental results obtained with these devices. The achieved pile-up rejection efficiency results in a very low contribution, of the order of ∼ 6 × 10 - 5  counts/(keV · kg · y), to the background counting rate in the region of interest for a large volume ( ∼ 90  cm 3 ) Li 2 100 MoO 4 detector. This background level is very encouraging in view of a possible use of the Li 2 100 MoO 4 solution for a bolometric tonne-scale next-generation experiment as that proposed in the CUPID project.

Neutrinoless double-beta ( 0 ν β β ) decay is a hypothetical rare nuclear transition ( T 1 / 2 > 10 25 – 10 26 year). Its observation would provide an important insight into the nature of neutrinos (Dirac or Majorana particle) demonstrating that the lepton number is not conserved. This decay can be investigated with bolometers embedding the double-beta decay isotope ( 76 Ge , 82 Se , 100 Mo , 116 Cd , 130 Te ...), which perform as low-temperature calorimeters (few tens of mK) detecting particle interactions via a small temperature rise read out by a dedicated thermometer. Cryogenic Rare-event Observatory with Surface Sensitivity (CROSS) aims at the development of bolometric detectors (based on Li 2 MoO 4 and TeO 2 crystals) capable of discriminating surface α and β interactions by exploiting superconducting properties of Al film deposited on the detector surface. We report in this paper the results of tests on prototypes performed at CSNSM (Orsay, France) that showed the capability of a-few- μ m -thick superconducting Al film deposited on crystal surface to discriminate surface α from bulk events, thus providing the detector with the required pulse shape discrimination capability. The CROSS technology would further improve the background suppression and simplify the detector construction (no auxiliary light detector is needed to reject alpha surface events) with a view to future competitive double-beta decay searches.

For the first time, a cadmium tungstate crystal scintillator enriched in [Formula omitted]Cd has been succesfully tested as a scintillating bolometer. The measurement was performed above ground at a temperature of 18 mK. The crystal mass was 34.5 g and the enrichment level [Formula omitted]82 %. Despite a substantial pile-up effect due to above-ground operation, the detector demonstrated high energy resolution (2-7 keV FWHM in 0.2-2.6 MeV [Formula omitted] energy range and 7.5 keV FWHM at the [Formula omitted]Cd double-beta decay transition energy of 2813 keV), a powerful particle identification capability and a high level of internal radio-purity. These results prove that cadmium tungstate is a promising detector material for a next-generation neutrinoless double-beta decay bolometric experiment, like that proposed in the CUPID project (CUORE Upgrade with Particle IDentification).

Cadmium-116 is one of the favorable candidates for neutrinoless double-beta decay ( 0 ν β β ) searches from both theoretical and experimental points of view, in particular thanks to the high energy of the decay (2813.49 keV), the possibility of the industrial enrichment in 116 Cd and its use in the well-established production of cadmium tungstate crystal scintillators. In this work, we present low-temperature tests of two 0.6 kg 116 CdWO 4 crystals enriched in 116 Cd to 82 % as scintillating bolometers. These detectors were operated underground, with one at the Laboratoire Souterrain de Modane (LSM) in France and the second at the Laboratorio Subterraneo de Canfranc (LSC) in Spain. The two crystals are coupled to bolometric Ge light detectors in order to register the scintillation light. The double readout of heat and scintillation enables reduction in the background in the region of interest by discriminating between different populations of particles. The main goal of these tests is the study of the crystals’ radiopurity and the detectors’ performance. The achieved results are extremely promising, in particular, the detectors demonstrate a high energy resolution (11–16 keV FWHM at 2615 keV) and a high-efficiency discrimination of the alpha background ( ∼ 20 σ ). These results, achieved for the first time with large mass enriched 116 CdWO 4 crystals, demonstrate prospects of the bolometric technology for high-sensitivity searches of 116 Cd 0 ν β β decay.

Random coincidences of nuclear events can be one of the main background sources in low-temperature calorimetric experiments looking for neutrinoless double-beta decay, especially in those searches based on scintillating bolometers embedding the promising double-beta candidate [Formula omitted] Mo, because of the relatively short half-life of the two-neutrino double-beta decay of this nucleus. We show in this work that randomly coinciding events of the two-neutrino double-beta decay of [Formula omitted] Mo in enriched Li [Formula omitted] detectors can be effectively discriminated by pulse-shape analysis in the light channel if the scintillating bolometer is provided with a Neganov-Luke light detector, which can improve the signal-to-noise ratio by a large factor, assumed here at the level of [Formula omitted] on the basis of preliminary experimental results obtained with these devices. The achieved pile-up rejection efficiency results in a very low contribution, of the order of [Formula omitted] counts/(keV [Formula omitted]kg [Formula omitted]y), to the background counting rate in the region of interest for a large volume ( [Formula omitted] cm [Formula omitted]) Li [Formula omitted] detector. This background level is very encouraging in view of a possible use of the Li [Formula omitted] solution for a bolometric tonne-scale next-generation experiment as that proposed in the CUPID project.

Scintillating bolometers of LiF and Al2O3 can monitor the fast neutrons flux in WIMPs searches. With both materials we merge the traditional fast neutron detection methods of induced reactions and scattering. The ROSEBUD collaboration devoted an underground run in the old Canfranc laboratory to study the response of LiF and Al2O3 to fast neutrons from 252Cf. Both bolometers were used simultaneously in a common experimental set-up resembling those of current WIMPs searches, which could give valuable insights into future WIMPs searches with cryogenic detectors as EURECA.