Scintillating bolometric technique for the neutrino-less double beta decay search: The LUCIFER/CUPID-0 experiment

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Abstract

CUPID is a proposed future tonne-scale bolometric neutrino-less double beta decay (0νββ) experiment to probe the Majorana nature of neutrinos and discover lepton number violation in the so-called inverted hierarchy region of the neutrino mass. In order to improve the sensitivity with respect to the current bolometric experiments, the source mass must be increased and the backgrounds in the region of interest must be dramatically reduced. The background suppression can be achieved discriminating β/γ against α events by means of the different light yield produced in the interactions within a scintillating bolometer. The increase in the number of 0νββ emitters demands for crystals grown with enriched material. LUCIFER/CUPID-0, the first demonstrator of CUPID, aims at running the first array of enriched scintillating Zn82Se bolometers (total mass of about 7 kg of 82Se) with a background level as low as 10−3 counts/(keV kg y) in the energy region of interest. We present the results of the first measurement performed on three Zn82Se enriched scintillating bolometers operated deep underground in the Hall C of the Laboratori Nazionali del Gran Sasso.

Introduction

The most sensitive bolometric experiment searching for neutrino-less double beta decay (0 νββ) will be CUORE (Cryogenic Underground Observatory for Rare Events) [1] located in the Laboratori Nazionali del Gran Sasso (LNGS). With an array of 988TeO2 bolometers (5×5×5 cm)3 corresponding to 206 kg of 130Te, and a background level of 0.01 counts/(keV kg y) CUORE will be able to reach a sensitivity on the effective Majorana mass of neutrino (mββ) of about (0.05÷0.13) eV, which represent the beginning of the inverted hierarchy region of the neutrino mass. The main background in the energy region of interest (about 2.5 MeV) will come from α particles produced by surface contaminations in 238U and 232Th (and their daughters) on the crystals and on the copper structure that supports the array. In addition, since the Q-value of the 130Te 0 νββ decay [2] lies between the 208Tl γ line at 2615 keV and its Compton edge, also multiCompton events contribute to the background budget of the experiment.

Next generation 0 νββ experiments [3] aim to reach a sensitivity on mββ at a level of 0.01 eV, in order to completely explore the inverted hierarchy region.

To reach this ambitious goal the source mass must be increased and the background in the region of interest dramatically reduced. To increase the number of 0 νββ emitters crystals grown with enriched material are needed. The background suppression can be achieved rejecting the α interactions by performing a particle identification and simultaneously choosing isotopes with a Q-value above the 2615 keV line (208Tl).

The CUORE Upgrade with Particle IDentification (CUPID) collaboration aims to run the first demonstrator of a future inverted hierarchy explorer, operating 30 crystals of Zn82Se as scintillating bolometer. This detector, named LUCIFER/CUPID-0, will study the 0 νββ of 82Se which is considered one of the most interesting candidates because of its high Q-value, 2998 keV [4]. Se was enriched up to 95.4% in order to overcome the rather low isotopic abundance of 82Se (8.73%). The 82Se was embedded in 440 g ZnSe crystals that can be operated as scintillating bolometers [5]. A scintillating bolometer is a crystal that, beside working as calorimeter at cryogenic temperatures, also emits scintillation light; the light yield (LY) produced in the particle interaction is strongly dependent on the particle itself: for a fixed energy released in the bolometer the LY of a β/γ particle is very different from an α one allowing an active background rejection. To detect the emitted light, an additional detector must be faced to the main bolometer: this light detector is also a bolometer, consisting of a thin germanium wafer as absorber and a thermal sensor of the same type as the one used for the main bolometer [6].

With a total mass of about 7 kg of 82Se and an expected background level of about 10−3 counts/(keV kg y) the CUPID-0 detector will be able to reach a sensitivity of the same order of magnitude of CUORE, despite it is 56 times smaller in terms of detector mass and 30 times smaller in terms of 0 νββ emitters [7].

In the following sections we describe the CUPID-0 detector, and the results of the test performed on the first three Zn82Se bolometers.

Section snippets

CUPID-0 detector

The CUPID-0 detector will be composed by five towers, each hosting six Zn82Se, and being surrounded by 3 M Vikuti reflective foils to increase the light collection efficiency. Each crystal will be monitored by two germanium light detectors [6] (Ge-LD). The 30 Zn82Se crystals are cylinders 5.5 cm in height and 4.4 cm in diameter and the 36 Ge-LD are germanium disks with 4.4 cm in diameter and 170  μm thickness; a SiO2 layer is deposited on one face of the germanium disk to increase the light

Experimental set-up of the first Zn82Se test

The first three enriched Zn82Se bolometers, each one equipped with two Ge-LD are assembled using the same detector design shown in Fig. 1a and anchored to the mixing chamber of the CUORE/LUCIFER R&D dilution refrigerator located in the Hall C of LNGS [11]. The read-out of the thermistors is performed using the Cuoricino electronics [12]. The voltage signals, amplified and filtered by means of an anti-aliasing 6-pole active Bessel filter (120 dB/decade), were acquired by a NI PXI-6284 18-bits ADC

Results

The energy resolution computed on the γ peaks produced by the 232Th calibration source is reported in Fig. 2-Left for the Zn82Se-01.

Fitting with a line this energy trend we are able to extrapolate an energy resolution at the 0 νββ energy that results about 30 keV FWHM. The remaining two Zn82Se crystals showed consistent results.

The 55Fe spectrum acquired by one of the four Ge-LDs is shown in Fig. 2-Right: the two peaks at 5.9 and 6.4 keV are fitted using two Gaussian functions. We obtained σ=70±8

Conclusion and future perspectives

We tested for the first time three enriched Zn82Se crystals working as scintillating bolometers. The performances in terms of energy resolution and particle identification are excellent. This test represents the first step in the realization of the final detector CUPID-0, that will be able to demonstrate the potential of this technique for a future tonne-scale experiment with the potential to explore completely the inverted hierarchy region of neutrino masses.

Acknowledgments

This work was supported by the LUCIFER experiment, funded by ERC under the European Union Seventh Framework Programme (FP7/2007-2013)/ERC Grant agreement no. 247115, funded within the ASPERA 2nd Common Call for R&D Activities and INFN.

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