Review
The CUORICINO and CUORE double beta decay experiments

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Abstract

After an introduction on the various experimental techniques to be adopted in searches for double beta decay, the new approach based on the use of cryogenic low temperature detectors is described. The present results are reported on the limit for neutrinoless double beta decay of 130Te obtained with the large bolometric detector CUORICINO. This setup consists of 44 cubic crystals of natural TeO2 with 5 cm sides and 18 crystals of 3×3×6 cm3. Four of these latter crystals are made with isotopically enriched materials: two in 128Te and two others in 130Te. With a sensitive mass of 40.7 kg, this array is by far the most massive running cryogenic detector. The array is operated at a temperature of ∼10 mK in a dilution refrigerator under a heavy shield in the Gran Sasso Underground Laboratory at a depth of about 3500 m.w.e. The counting rate in the region of neutrinoless double beta decay is 0.18±0.02 counts keV −1 kg−1 y−1, among the lowest in this type of experiment. No evidence for neutrinoless double beta decay is found. The corresponding lower limit for the lifetime of this process is 2×1024 years at 90% C.L. The resultant upper limit on the effective neutrino mass ranges between 0.2 and 1.0 eV, depending on the theoretically calculated nuclear matrix elements. This constraint is the most restrictive one, except for those obtained with Ge diodes, and is comparable to them. The second part of this report is devoted to the present status of the construction of the larger experiment CUORE (Cryogenic Underground Observatory for Rare Events) formed from 988 bolometers with a cubic TeO2 absorber of size 5×5×5 cm3, with a total mass of ∼750 kg. We present technical details of the CUORE setup as well as of its location and our efforts to reduce radioactive backgrounds.

Introduction

The discovery of neutrino oscillations in solar, atmospheric, and reactor experiments [1], [2], [3], [4], [5], [6], [7], [8] indicating that the mass of at least one neutrino is finite has strongly revived interest in neutrinoless double beta decay (DBD). This process in fact would not only indicate the non-conservation of the lepton number, but also provide a test to determine if neutrinos are majorana particles and, if so, enable us in this hypothesis to determine the absolute value of its mass. It becomes therefore imperative to search for a finite value for the effective electron neutrino mass [9], [10], [11], [12], [13], [14]. In astrophysics, the recent results of the full sky microwave maps by WMAP, together with the 2 dF Galaxy Redshift Survey [15], constrain the sum of the masses of the three neutrinos at 0.7–1.7 eV [16], [17], [18], [19], [20]. A claim for a finite mass of 0.56 eV has also been presented [21]. Direct experiments on single beta decay presently constrain the absolute value of this mass to less than 2.2 eV, while a bound of ∼0.2 eV is expected in the KATRIN experiment [22], [23], [24].

A more restrictive limit for the effective mass of majorana neutrinos can undoubtly come from neutrinoless double beta decay (DBD). In its two negatron channel, DBD consists of the direct emission of two electrons from a nucleus (A, Z) decaying to the corresponding isobar (A,Z+2). This process can be searched for when the single beta transition from (A, Z) to (A,Z+1) is energetically forbidden, or at least strongly suppressed, by a large change of the spin-parity state. The process of two neutrino DBD is accompanied by the emission of two electron antineutrinos and therefore conserves lepton number. It is allowed by the standard model of electroweak interactions, and has been found in ten nuclei [24], [25], [26], [27], [28], [29], [30]. In contrast, conservation of the lepton number is violated in the majoron decay, where one or more massless Goldstone bosons accompany the emission of the two electrons, and in the so called neutrinoless DBD, where only the two electrons are emitted. In this case these two particles would share the total transition energy and a peak appears in the sum energy spectrum of the two electrons. In addition, the available phase space is much larger with respect to the two neutrino case, rendering neutrinoless DBD a very powerful way to search for lepton number non-conservation. The expected value for the effective neutrino mass, mν, or its upper limit, is proportional to the square root of the rate, which makes searches for neutrinoless DBD quite difficult. On the other hand, this rate is proportional to the square of the nuclear matrix element, whose evaluation is, at least at present, quite uncertain. Since the uncertainty in the value of the nuclear matrix element reflects itself directly in that of mν, searches for neutrinoless DBD should be carried out on several candidate nuclei. There is another important reason to do that. Natural and environmental radioactivity predict many peaks in any type of background spectrum. Even if a peak appears in the energy region predicted for neutrinoless DBD, one has to exclude the possibility that it could have been mimicked by one so far unknown radioactivity line. Only the presence of another peak at the different energy expected for DBD in other nuclei would unambiguously prove the existence of this very important process. No evidence has been claimed so far for the neutrinoless channel in any nucleus, with the exception of alleged evidence for neutrinoless DBD of 76Ge reported by a subset of the Heidelberg–Moscow collaboration [31], [32], but confronted by other authors [9], [33], [34], and even by a different subset of the same collaboration [35].The previous claim has however been enhanced by a more detailed analysis [36], [37].

DBD can be searched for, indirectly, in radiochemical [38] or geochemical experiments [39], [40], [41], [42], [43], based on the search for the (A,Z+2) product nuclei. These experiments are very sensitive, but indicate only the presence of the daughter nucleus and cannot therefore discriminate between lepton conserving and non-conserving processes or between decays to the ground or excited states of the daughter nucleus.

Direct experiments are based on two different approaches. In the source ≠detector experiment, thin sheets of a double beta active material are inserted in a suitable detector. In the source =detector or “calorimetric” experiments [44], the detector itself is made of a material containing the double beta active nucleus.

The use of cryogenic detectors to search for DBD was suggested in 1984 [45]. These bolometers, as shown in Fig. 1, are made [46], [47], [48], [49] with diamagnetic and dielectric crystals. As a consequence, at low temperature their heat capacity is proportional to the cube of the ratio between the operating and Debye temperatures. In a cryogenic setup this capacity can become so small that even the tiny energy released by a particle in the form of heat generates a measurable temperature increase of the absorber. Cryogenic detectors offer a wide choice of DBD candidates, the only requirement being that the candidate nucleus is part of a compound which can be grown in the form of a crystal with the necessary thermal and mechanical properties. The isotope 130Te is an excellent candidate to search for DBD due to its high transition energy (2530.30±1.99 keV ) [50] and large isotopic abundance (33.8%) [51] which allows a sensitive experiment to be performed with natural tellurium. In addition, the expected signal at 2530.30 keV happens to be in an energy region between the peak and the Compton edge of the 208Tl  γ-rays at 2615 keV, which generally dominates the γ background in this high energy region. Of the various compounds of this element, TeO2 appears to be the most promising due to good mechanical and thermal properties.

A series of experiments with various arrays of 340 g crystals of natural TeO2 have been performed in the Laboratori Nazionali del Gran Sasso. The results of an experiment carried out with an array of 16 crystals of natural Te and four enriched crystals, two in 128Te and two in 130Te, with a total mass of ∼6.8 kg have been published [52].

Section snippets

Experimental details

The CUORICINO array consists of a tower with 13 planes containing 62 crystals of TeO2 operating in Hall A of the Gran Sasso Underground Laboratory [53] in the same dilution refrigerator previously used in our experiment with 20 crystals [52].

As shown in Fig. 2, the structure is as follows: the upper 10 planes and the lowest plane consist of four natural crystals of 5×5×5 cm3, while the 11th and 12th planes have nine 3×3×6 cm3 crystals each. In the 3×3×6 cm3 planes the central crystal is fully

CUORICINO performance

CUORICINO was cooled down at the beginning of 2003. Unfortunately, during this operation, electrical connections to 12 of the 44 5×5×5 cm3 detectors and to one of the 3×3×6 cm3 crystals were lost [60]. This was mainly due to an annoying problem with the soldering of the read-out wires at low temperature at the various stage of thermalization. New thermal links were therefore designed and tested. The cryostat was then warmed up in fall 2003 and all the thermalizers were changed. All but two

Results of CUORICINO

The results reported here refer to the statistics collected up to July 31, 2005 corresponding to ∼5.87 kg y of 130Te. The calibration spectra for the ensemble of large and small detectors are shown in Fig. 4. We would like to point out that the enriched crystals were introduced in the CUORICINO setup to investigate the two neutrino DBD. Even if the present statistics are still inadequate to achieve that aim, it indicates a considerable improvement in the background in the low energy region with

Future experiments and prospects of CUORE

A list of proposed second-generation experiments to search for neutrinoless DBD is reported in Table 1. The predicted sensitivity to the neutrino mass is obviously strongly dependent on the value assumed for the nuclear matrix elements. From the experimental point of view, the predicted sensitivity depends, even more weakly, on the value assumed for the expected background, which is a difficult and sometime debatable issue indeed.

We are concerned with the experiment CUORE (Cryogenic Underground

Present status and perspectives of CUORE

CUORE was approved on April 2, 2004 by the Scientific Committee of the Gran Sasso Underground Laboratory. Its final location was decided on September of the same year. The almost contemporary approval by the advisory Commissione II of INFN (Italian Institute of Nuclear Physics) and allocation of funding allowed us to start R&D activity in January 2005. A tender for the construction of the dilution refrigerator is underway. In August 2005, the Neutrino Scientific Assessment Group (NuSAG) in the

Conclusions

Even if the evidence claimed by Klapdor-Kleingrothaus et al. is confirmed, we believe that the detection of at least another peak at a different transition energy expected for a different nuclear candidate is needed to definitely prove the existence of this lepton violating process. Thermal detectors are unique for this aim, in that they offer the possibility of measuring different DBD candidates. CUORICINO is the most sensitive experiment on neutrinoless DBD currently running. It has the same

Acknowledgement

Gorla and Torres were CEE fellows in the Network on Cryodetectors, under contract HPRN-CT-2002-00322. This experiment has been partly supported by the Commission of European Communities under contracts FMRX-CT98-03167 and HPRN-CT-2002-00322, by the U.S. Dept. of Energy under Contract number DE-AC03-76 SF000 98, and by the National Science Foundation.

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