Preliminary results on double beta decay (DBD) of  130 Te, obtained in the first run of the CUORICINO experiment are presented. The set-up consists of an array of 62 crystals of TeO 2  operating as bolometers in a deep underground dilution unit at a temperature of about 10 mK. Due to a total mass of about 41 kg, CUORICINO represents by far the most massive running cryogenic mass to search for rare events. The achieved lower limit on the neutrinoless DBD is 5.5⋅10 23  years, that corresponds to a limit on the Majorana effective mass between 0.37 and 1.9 eV. Performances of the detectors together with the sensitivity estimation are discussed

A. de Waard

A. Fascilla

A. Giuliani

A. Morales

A. Nucciotti

A. R. Smith

C. Arnaboldi

C. Brofferio

C. Bucci

C. Rosenfeld

Carlos Pobes

D. R. Artusa

E. Olivieri

Edoardo Pasca

Eric B. Norman

Ettore Fiorini

Eugene E. Haller

Ezio Previtali

F. Capozzi

F. T. Avignone

G. Pessina

G. Ventura

Giorgio Frossati

H. A. Farach

I. C. Bandac

I. G. Irastorza

Jeffrey W. Beeman

L. Carbone

L. Torres

Lara Risegari

M. Balata

M. Pavan

M. Pedretti

M. Pyle

Marco Barucci

Monica Sisti

Oliviero Cremonesi

P. Gorla

R. J. Creswick

R. J. McDonald

S. Cebrián

S. Pirro

Simone Capelli

V. Palmieri

English

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First results of the CUORICINO experimentS. Pirro∗a†, C. Arnaboldia, D.R. Artusab , F.T. Avignone IIIb, M. Balatac, I. Bandacb, M. Baruccid ,J. Beemane, C. Brofferioa, C. Buccic, S. Capellia, F. Capozzia, L. Carbonea, S. Cebrianf ,O. Cremonesia, R. J. Creswickb , A. de Waardg, H. A. Farachb, A. Fascillah, E. Fiorinia, G. Frossatig,A. Giulianih, P. Gorlaf ,c, E. E. Halleri,e, I. G. Irastorzaf , R. J. McDonalde, A. Moralesf , E. B. Normane,A. Nucciottia, E. Olivierid, V. Palmierij, E. Pascad, M. Pavana, M. Pedrettii , G. Pessinaa, E. Previtalia,C. Pobesf , M. Pylec, L. Risegarid, C. Rosenfeldb, M. Sistia, A. R. Smithe, L. Torresf , and G. VenturadaDipartimento di Fisica dell’Università di Milano-Bicocca e Sezione di Milano dell’INFN, MilanoI-20126, ItalybDepartment of Physics and Astronomy, University of South Carolina, Columbia, South Carolina, USA29208cLaboratori Nazionali del Gran Sasso, Assergi (L’Aquila), I-67010 ItalydDipartimento di Fisica dell’Università di Firenze e Sezione di Firenze dell’INFN, Firenze I-50125, ItalyeLawrence Berkeley National Laboratory, Berkeley, California, 94720, USAfLaboratorio de Fisica Nuclear y Altas Energias, Universidàd de Zaragoza, 50009 Zaragoza, SpaingKamerling Onnes Laboratory, Leiden University, 2300 RAQ, Leiden, The NetherlandshDipartimento di Scienze Chimiche, Fisiche e Matematiche dell’Università dell’Insubria e Sezione diMilano dell’INFN, Como I-22100, ItalyiDepartment of Materials Science and Mineral Engineering, University of California, Berkeley, CA94720, USAjLaboratori Nazionali di Legnaro, Via Romea 4, I-35020 Legnaro (Padova), ItalyPreliminary results on double beta decay (DBD) of 130Te, obtained in the first run of the CUORICINOexperiment are presented. The set-up consists of an array of 62 crystals of TeO2 operating as bolometers in adeep underground dilution unit at a temperature of about 10 mK. Due to a total mass of about 41 kg, CUORICINOrepresents by far the most massive running cryogenic mass to search for rare events. The achieved lower limit onthe neutrinoless DBD is 5.5·1023 years, that corresponds to a limit on the Majorana effective mass between 0.37and 1.9 eV. Performances of the detectors together with the sensitivity estimation are discussed.∗Thanks are due to the Laboratori Nazionali delGran Sasso for generous hospitality and to M. Perego,G.Galotta, S. Parmeggiano, R. Gaigher, A. Rotilio, B.Romualdi, and especially to P. Nuvolone and S. Nisi forcontinuous and constructive help in various stages of thisexperiment. We also gratefully acknowledge Dr. AngeloAlessandrello for his generous and fundamental help in thefirst 15 years of the bolometric adventure. This experimenthas been partially supported by the Commission of Euro-pean Communities under contract FMRX–CT98–0167.†Correspond to Stefano.Pirro@mib.infn.it1. INTRODUCTIONThe evidence of a neutrino rest mass representsone of the most exciting discoveries in the field ofparticle physics.Measurement of this tiny mass [1], however,represents a big challenge from an experimen-tal point of view. Oscillation experiments pro-vide only differences between squared neutrinoNuclear Physics B (Proc. Suppl.) 138 (2005) 210–2130920-5632/$ – see front matter © 2004 Elsevier B.V. All rights reserved.www.elsevierphysics.comdoi:10.1016/j.nuclphysbps.2004.11.050masses. Double beta decay is a rare radioactivetransition from an even nucleus (A, Z) to its iso-bar (A, Z +2), with the simultaneous emission oftwo electrons. The discovery of the neutrinolessDBD (0ν-DBD), however, will provide not onlythe ultimate answer about the nature (Dirac orMajorana) of the neutrino, but will also allow asensitivity on the mass, down to few meV [2]. 0ν-DBD searches actually measure the effective neu-trino mass |mee|=∑kmνk |Uek|2eiαk . Therefore0ν-DBD does not measure the electron neutrinomass directly, but it can set the absolute scale forthe neutrino mass eigenstates νk, using the neu-trino mixing matrix Uek and some hypotheses onthe Majorana CP phases eiαk .In the experimental approach, the detector con-tains, or is made of, the isotope candidate toDBD, and the signature of the decay will be theappearance of a monochromatic line in the back-ground spectrum, corresponding to the transitionenergy of the decay, shared by the two electrons.Using thermal detectors, a search on a wide choiceof 0ν-DBD isotopes is possible.The study of 130Te is one of the most promis-ing approaches for new-generation DBD experi-ments. In fact, considering the reasonably hightransition energy (2.528 MeV, in the Comptonedge of the highest γ line due to natural radioac-tivity ) and the nuclear matrix elements, 130TeDBD rate is predicted to be about four/five timesfaster than for 76Ge [3], and it is one of the fastestamong the most interesting candidates. In largemass DBD experiments, the availability of theisotope nuclide is a critical point. The naturalisotopic abundance of 130Te is 34 %, much higherthan those of the other interesting DBD candi-dates. Therefore, a significant 130Te experimentcan be performed, even without isotopic enrich-ment, which is often economically prohibitive inlarge quantities.2. EXPERIMENTAL SET-UPThe array of CUORICINO consists in a towerwith 13 planes containing 62 TeO2 crystals oper-ating in the Gran Sasso Underground Laboratory,in the same dilution refrigerator used in our pre-vious experiment [4]. As shown in Fig. 1, thepassing−troughcold fingerSpringMixing Chamber3x3x6 cm planesRoman Lead( 8 mK)Figure 1. CUORICINO set-up.structure is as follows: the upper ten planes andthe lowest one are made of 4 crystals of 5×5×5cm3, while the 11th and 12th ones are made ofnine 3×3×6 cm3 crystals. In these two planes, thecentral crystal is fully surrounded by the nearbyones. It will be useful to investigate the effect ofsurface contamination of the crystals and of thecopper frame using coincidence and anticoinci-dence analysis. Four of this small crystals are iso-topically enriched (two 75% of 130Te and two 82%of 128Te) in order to search for the 2νDBD decaymode with the method of background subtrac-tion. The temperature sensors are neutron trans-mutation doped Ge thermistors of 3×3×1 mm3,specifically prepared in order to present similarthermal characteristics, and thermally coupled toeach crystal with nine ∼0.6 mm diameter epoxyglue spots. A resistor of ∼50 kΩ, realized with aS. Pirro et al. / Nuclear Physics B (Proc. Suppl.) 138 (2005) 210–213 211heavily doped meander on a 1 mm3 silicon chip,is attached to each absorber and acts as a heaterto stabilize the gain of bolometer [5]. The toweris mechanically decoupled from the cryostat in or-der to avoid vibration from the cryogenic facility,which will induce noise on the detectors [6]. Thetower is, therefore, connected, through a 25 cmcopper bar passing through the mixing chamber,to a steel spring fixed at the 50 mK plate (seeFig.1). The longitudinal oscillation frequency ofthe system is ∼ 1.8 Hz. The thermal link is en-sured by four 40×10×0.05 mm3 copper foils con-nected from the MC to the tower.500 1000 1500 2000 2500Energy  [keV]103050Counts/(keV kg year)2700Counts51015202500 260024000ββ0νFigure 2. Background sum spectrum correspond-ing to a statistic of 3.1 kg×year.3. PHYSICS RESULTSCUORICINO was cooled down at the begin-ning of 2003; unfortunately during the coolingprocedure some of the contacts of the read outwires inside the cryostat were lost, so that only32 of the large crystal and 16 of the small onescould be read. The performances of the remain-ing detectors are quite good: the average FWHMenergy resolution measured on the 208Tl γ line at2615 keV (near the expected 0ν-DBD region) is∼7 keV for the large crystal and ∼9 keV for thesmall ones.The results reported here refer to the first run,totalling effective statistics corresponding to ∼2.9kg y−1 for the large crystals and ∼0.26 kg y−1 forthe small ones. The background counting rates inthe region of 0ν-DBD are, respectively, 0.20±.03and 0.20±.1 c kg−1 y−1 for the large and smallcrystals. The sum of all the background spectra,reported in Fig. 2, shows the γ lines due to 60Co,40K, and those due to the 238U and 232Th chains.Also visible are the activation lines of 57Co, 58Co,60Co and 54Mn; as can be seen from the inset, nopeaks appear in the region of interest. A max-imum likelihood procedure used to establish themaximum number of 0ν-DBD events, compatiblewith the measured background, allows setting anupper limit of 5.5·1023 years for neutrinoless DBDof 130Te at 90% CL. The corresponding limit ofthe effective Majorana mass ranges between 0.37and 1.9 eV, due to the uncertainties on the nu-clear matrix elements. The sources of backgroundin the region of interest are mainly due to de-graded α’s from the copper surfaces facing thedetector (50±20 %), 232Th γ’s from the cryostat(30±10 %) and α’s from the surfaces of crystals(20±10 %).4. CONCLUSIONSThe first run of CUORICINO, lasting onlythree months, is now temporarily stopped. We re-placed all the wires in order to have the full masssensitivity. The next run is foreseen to start datataking in January 2004. With the first run, wealready partially covered the corresponding spanof the experiments on neutrinoless DBD of 76Ge.The expected three years of sensitivity for thefull mass of CUORICINO is ∼1·1025 years, cor-responding to a sensitivity on the Majorana massbetween 0.1 and 0.45 eV.REFERENCES1. F. Feruglio, A. Strumia, F. Vissani Nucl.Phys. B 637 (2002) 345 (Addendum: B 659(2003) 359)2. S.R. Elliott and P. Vogel Annu. Rev. Nucl.Part Sci. 52 (2002) 115-151, hep-ph/0202264.S. Pirro et al. / Nuclear Physics B (Proc. Suppl.) 138 (2005) 210–2132123. J. Suhonen and O. Civitarese, Phys. Rep. 300(1998) 123 and ref. therein.4. C. Arnaboldi et al., Phys. Lett. B 557 (2003)167.5. A. Alessandrello et al., Nucl. Instr. and Meth.A 412 (1998) 454.6. S. Pirro et al., Nucl. Instrum. and Meth. A444 (2000) 331.S. Pirro et al. / Nuclear Physics B (Proc. Suppl.) 138 (2005) 210–213 213

First results of the CUORICINO experiment

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First results of the CUORICINO experiment

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