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.
In preparation for the CROSS experiment to search fordouble-beta (2β) decay of 100Mo, we upgraded alow-background facility at the Canfranc underground laboratory(Spain) by installing a magnetic ...damping-based detector suspensioninside a pulse-tube cryostat. We tested the suspension inlow-temperature (15 mK) measurements with two scintillatingbolometers made of large-volume crystals of 116CdWO4(reference of the set-up) and Li2MoO4 (R&D of the CLYMENEproject in view of the CUPID 2β experiment), each coupled to athin Ge bolometric light detector. Despite the evidence of aresidual pulse-tube-induced noise, picked up by cabling, we achievedhigh performance with all tested devices. In particular, the energyresolution for 2615 keV γ-ray measured with both116CdWO4 and Li2MoO4 bolometers is ∼6 keV FWHM,among the best-reported results for thermal detectors based on suchcompounds representing a great interest to searches for 2βdecay in 116Cd (Q-value is 2813 keV) and 100Mo(3034 keV) respectively. Highly efficient particle identification(α vs. β/γ) is achieved also with thescintillating bolometer made of a low light yield crystal(Li2MoO4), thanks to the low noise (10 eV RMS) exhibited bythe light detector exploiting the Neganov-Trofimov-Luke effect forsignal amplification. We also found a rather high level ofradiopurity in the Li2MoO4 crystal; only traces of 210Poand 226Ra were detected (∼0.1 mBq/kg each), while the228Th activity is expected to be at least an order of magnitudelower, as well as a 40K activity is below 6 mBq/kg.
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.
CUORE (Cryogenic Underground Observatory for Rare Events) is a 1-ton experiment to search for Neutrinoless Double Beta Decay of 130Te using 988 TeO2 bolometers. It aims at reaching the sensitivity on ...the effective neutrino mass of the order of few tens of meV. CUORICINO, a single CUORE tower running since 2003 in the Gran Sasso Underground Laboratory (Italy), plays an important role as a standing alone experiment for developing the future CUORE setup. The latest limit obtained on the Majorana mass is presented. The effort in improving the detector sensitivity and background reduction is also reported.
Abstract
The CROSS experiment is proposing to use a new technology of surface sensitive bolometers for low-background neutrinoless double beta decay searches. Efficient rejection of surface
α
and
β
...events will allow to reach background in the region of interest below than 10
−4
cnts/keV/kg/yr. The isotopes of interest, which are
130
Te and
100
Mo, are investigated with TeO
2
and Li
2
MoO
4
bolometers. The surface sensitivity is achieved thanks to the evaporation of thin metallic film on the crystal surface that modifies the pulse shape of near-surface events. An investigation of various pulse shape parameters was performed. The analysis shows that one of the best parameters for discrimination is the integrated area of the raw signal both for TeO
2
and Li
2
MoO
4
with Pd-Al (10 nm - 100 nm) bi-layer.
Used for both proton decay searches and neutrino physics, large water Cherenkov (WC) detectors have been very successful tools in particle physics. They are notable for their large masses and charged ...particle detection capabilities. While current WC detectors reconstruct charged particle tracks over a wide energy range, they cannot efficiently detect neutrons. Gadolinium (Gd) has the largest thermal neutron capture cross section of all stable nuclei and produces an 8 MeV gamma cascade that can be detected with high efficiency. Because of the many new physics opportunities that neutron tagging with a Gd salt dissolved in water would open up, a large-scale R&D program called EGADS was established to demonstrate this technique’s feasibility. EGADS features all the components of a WC detector, chiefly a 200-ton stainless steel water tank furnished with 240 photo-detectors, DAQ, and a water system that removes all impurities from water while keeping Gd in solution. In this paper we discuss the milestones towards demonstrating the feasibility of this novel technique, and the features of EGADS in detail.
An array of twelve 0.28 kg lithium molybdate (LMO) low-temperature bolometers equipped with 16 bolometric Ge light detectors, aiming at optimization of detector structure for CROSS and CUPID ...double-beta decay experiments, was constructed and tested in a low-background pulse-tube-based cryostat at the Canfranc underground laboratory in Spain. Performance of the scintillating bolometers was studied depending on the size of phonon NTD-Ge sensors glued to both LMO and Ge absorbers, shape of the Ge light detectors (circular vs. square, from two suppliers), in different light collection conditions (with and without reflector, with aluminum coated LMO crystal surface). The scintillating bolometer array was operated over 8 months in the low-background conditions that allowed to probe a very low, μBq/kg, level of the LMO crystals radioactive contamination by 228Th and 226Ra.
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