The 0ν2β-decay CROSS experiment: preliminary results and prospects

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 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.

Cosmogenic Activation in Double Beta Decay Experiments

“…The DBD of 130 Te has been largely studied using bolometers [115][116][117]. Thanks to a high natural isotopic abundance of 34%, enrichment is not necessary which makes it feasible to accumulate large amounts of tellurium oxide.…”

“…The decay conserves the lepton number and is detectable for nuclei having an even number of neutrons and protons and the beta transition energetically forbidden or strongly suppressed by the angular momentum change. The DBD with emission of neutrinos, specifically reviewed in [15], has been observed for several nuclei ( 48 Ca, 76 Ge, 82 Se, 96 Zr, 100 Mo, 116 Cd, 128 Te, 130 Te, 136 Xe, 150 Nd and 238 U) with measured half-lives varying approximately from 10 19 to 10 24 y (see recommended values at [16]).…”

“…Section 2 sets up the basics of cosmogenic activation, describing the relevant processes, different approaches to estimate production cross sections and main components of the cosmic ray flux. Germanium, tellurium oxide, and xenon are used in some of the largest DBD detectors investigating the neutrinoless DBD of 76 Ge, 130 Te, and 136 Xe; the studies for cosmogenic activation in these materials are reviewed in Sections 3-5, respectively, while Section 6 presents studies performed for materials used in experiments focused on other DBD emitters like 82 Se, 100 Mo, or 150 Nd. For each material, the status of the corresponding DBD experiments will be briefly summarized, the principal cosmogenic products will be presented, the activation works (based on measurements or different calculations) from nucleons and muons, if available, will be described and the effect of cosmogenics in the background levels will be discussed.…”

High-Resolution Digitization System for the CROSS Experiment

“…CROSS (Cryogenic Rare-event Observatory with Surface Sensitivity) [1] is a bolometric experiment devoted to the search of neutrinoless double-β decay that will be installed in Canfranc Underground Laboratory (LSC, Spain).…”

“…The discrimination of surface events in CROSS is possible thanks to ultra-pure superconductive Aluminum thin foils, deposited on the surface of the crystals, which act as energy absorbers for α surface events, providing a different dynamic in the conversion and propagation of phonons, hence a different signal shape. This technique has been already demonstrated both with Tellurium dioxide (TeO 2 ) [3] and Lithium molybdate (LiMoO 4 ) [1] crystals. The final choice of the 0ν2β candidate material is still under study.…”