Abstract
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 100Mo and 130Te. 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 Li2MoO4 and TeO2. 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 ∼ 3×1025 nuclei of 100Mo, 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.
Highlights
Concepts of the CROSS technologyIn order to observe 0ν2β decay, experimentalists aim at the detection of the two emitted electrons, which share the total transition energy (the so-called Q-value of the process) [1– 3, 25, 26]
We present an innovative bolometric technology to be tested in a demonstrator named CROSS
In CROSS, we will use two types of phonon sensors: (i) a neutron transmutation doped (NTD) Ge thermistor [29], which is mainly sensitive to thermal phonons and can be considered with a good approximation as a thermometer, and (ii) a NbSi thin film [30, 31], which is faster and exhibits a significant sensitivity to athermal phonons
Summary
In order to observe 0ν2β decay, experimentalists aim at the detection of the two emitted electrons, which share the total transition energy (the so-called Q-value of the process) [1– 3, 25, 26]. Bolometers, the instruments chosen for CROSS, are among the most powerful nuclear detectors for the conduction of sensitive 0ν2β decay searches in the calorimetric approach [27]. They can provide high sensitive mass (via large detector arrays), high detection efficiency, high energy resolution and extremely low background thanks to potentially high material radiopurity and methods to reject parasitic events [28]. A Q-value as high as possible is desirable since this places the signal in a lower background region and increases strongly the decay probability. In CROSS, we will use two types of phonon sensors: (i) a neutron transmutation doped (NTD) Ge thermistor [29] (following the scheme adopted in Cuoricino and CUORE [6]), which is mainly sensitive to thermal phonons and can be considered with a good approximation as a thermometer, and (ii) a NbSi thin film [30, 31], which is faster and exhibits a significant sensitivity to athermal phonons.
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