Double beta decay search with TeO 2 bolometers

1. The Future of Neutrino Mass Measurements: Terrestrial, Astrophysical, and Cosmological Measurements in the Next Decade Seattle, February 8-11, 2010 Double beta decay search with TeO2bolometers Andrea Giuliani on behalf of the CUORE collaboration University of Insubria (Como) and INFN Milano-Bicocca

2. 2n Double Beta Decay allowed by the Standard Model already observed –t  1019 – 1021 y  Two decay modes are usually discussed: neutrinoless Double Beta Decay (0n-DBD) never observed(except a discussed claim) t > 1025 y  (A,Z)  (A,Z+2) + 2e- (A,Z)  (A,Z+2) + 2e- + 2ne Process would imply new physics beyond the Standard Model violation of total lepton number conservation mn 0 n  n Observation of 0n-DBD Decay modes for Double Beta Decay Double Beta Decay is a very rare, second-order weak nuclear transition which is possible for a few tens of even-even nuclides

3. S. Pascoli, S. T. Petcov and T. Schwetz, hep-ph/0505226 The size of the challenge  100 - 1000 counts / y ton 76Ge result  1 - 10 counts / y ton 50 meV 20 meV  0.1 - 1 counts / y ton

4. two neutrino DBD continuum with maximum at 1/3 Q sum electron energy / Q Electron sum energy spectra in DBD The shape of the two electron sum energy spectrum enables to distinguish among the two different decay modes For a detector with few keV energy resolution, the target for the background index is of the order of or better than neutrinoless DBD peak enlarged only by the detector energy resolution 10-3 counts / keV kg y Q  2-3 MeV for the most promising candidates In order to explore the inverted hierarchy region, the background in the region of interest needs to be as low as  few counts / y ton

5. Properties of 130Te as a DBD emitter 130Tefeatures as a DBD candidate: large phase space, low background (clean window between full energy and Compton edge of 208Tl photons) excellent feature for reasonable-cost expansion of Double Beta Decay experiments • high natural isotopic abundance (I.A. = 33.87 %) • high transition energy (Q = 2530 keV) • encouraging theoretical calculations for 0n-DBD lifetime • already observed with geo-chemical techniques ( t1/2incl = ( 0.7 - 2.7 )  1021 y) • 2n DBD decay observed by a precursor bolometric experiment (MiDBD) and by NEMO3 at the level t1/2 = ( 5 - 7 )  1020 y mbb 50 meV t  2x1026 y

6. Te dominates in mass the compound Excellent mechanical and thermal properties Energy absorber TeO2 crystal C  2 nJ/K  1 MeV / 0.1 mK Thermometer NTD Ge-thermistor R  100 MW dR/dT  100 kW/mK Heat sink T  10 mK Thermal coupling G  4 nW / K = 4 pW / mK • Temperature signal: DT = E/C 0.1 mK for E = 1 MeV • Bias: I  0.1 nA  Joule power  1 pW Temperature rise  0.25 mK • Voltage signal: DV = I  dR/dT DTDV = 1 mV for E = 1 MeV • Signal recovery time: t = C/G  0.5 s • Noise over signal bandwidth (a few Hz): Vrms = 0.2 mV In real life signal about a factor 2 - 3 smaller Energy resolution (FWHM):  1 keV The bolometric technique for 130Te: detector concepts

7. Thermometer (doped Ge chip) • Energy absorber • single TeO2 crystal • 790 g • 5 x 5 x 5 cm A physical realization of bolometers for DBD Cuoricino basic module

8. R&D final tests for CUORE (hall C) Cuoricino and CUORE Location Cuoricinowas and CUORE will be installed in Laboratori Nazionali del Gran Sasso L'Aquila – ITALY the mountain provides a 3500 m.w.e. shield against cosmic rays CUORE (hall A) Cuoricino (hall A)

9. The CUORICINO set-up CUORICINO = tower of 13 modules, 11 modules x 4 detector (790 g) each 2 modules x 9 detector (340 g) each M =  41 kg   5 x 1025130Te nuclides Coldest point Cold finger Tower Lead shield Same cryostat and similar structure as previous pilot experiment

10. CUORICINO physics results • use new more accurate Q-value: 2527.5 keV Phys. Rev. Lett. 102, 212502 (2009) , Phys. Rev. C 80, 025501 (2009) • updated statistics through Mar 2008 (shut down in July 2008) 60Co sum peak 2505 keV  3 FWHM from DBD Q-value MT = 18.14 kg 130Te  y Background sum spectrum of all the detectors in the DBD region 130Te - 0nbb BKG rate: 0.180.01 counts / keV kg y Nucl. Phys. A 766, 107 (2006) [Erratum-ibid. A 793, 213 (2007)] mbb< 206 – 720 meV T1/20v (y) > 2.94  1024 y (90% C.L.)

11. CUORE = closely packed array of 988 detectors 19 towers - 13 modules/tower - 4 detectors/module M = 741 kg   1027130Te nuclides Compact structure, ideal for active shielding From CUORICINO to CUORE(Cryogenic Underground Observatory for Rare Events) Each tower is a CUORICINO-like detector Custom dilution refrigerator

12. The CUORE schedule 2008 2009 2010 2011 2012 CUORE-0 PREPARATION CUORE-0 RUNNING HUT CRYOSTAT CRYSTALS OTHER DETECTOR ELEMENTS (THERMISTORS, HEATERS, HOLDERS…) CLEAN ROOM ASSEMBLY AND INTEGRATION DATA TAKING

13. CUORE sensitivity Surface background problem partially / fully solved Background in 0nbb region 10-3 c/keV/kg/y Background in 0nbb region 10-2 c/keV/kg/y Detector resolution  5 keV Live time 5 years T1/20n (130Te) > 2.1 x 1026 y mββ < 44 - 73 meV T1/20n (130Te) > 6 x 1026 y mββ < 25 - 43 meV More recent and reliable NME calculations IBM2 44 meV QRPA Jyuvaeskulae 49 meV QRPA Tuebingen et al. 53 meV ISM 73 meV IBM2 25 meV QRPA Jyuvaeskulae 29 meV QRPA Tuebingen et al. 31 meV ISM 43 meV

14. Simulation of the external background Cuore external background - arXiv:0912.0452v2 – accepted by Astroparticle Physics

15. The CUORE background components Background in DBD region ( 10-3 counts/keV kg y ) Component External gammas < 0.39 Apparatus gammas < 1 Crystal bulk < 0.1 < 1 Close-to-det. material bulk < 3 Crystal surface Close-to-det. material surface  20 – 40 External neutrons 0.270 ± 0.022 (8.56 ± 6.06)×10−3 Muons 17.3 ± 0.3 0.104 ± 0.022 Anticoincidence Total

16. The Cuoricino background and the alpha surface radioactivity 208Tl 214Bi 60Co p.u. ~ 0.11 c / keV kg y Gamma region

17. Strategies for the control of the surface background from inert materials (A) Action on the source  surface cleaning – plastic wrapping Mechanical action / Electropolishing Chemical etching / Plasma cleaning Strategy adopted by the CUORE collaboration to reject surface background  “Legnaro” cleaning method [CUORE baseline]  “Gran Sasso” cleaning method  Polyethelene wrapping (B) Action on the detectors  events identification Possible R&D subjects aiming at fully exploiting the potential of TeO2bolometers in a second phase Composite surface sensitive bolometers Thin-film equipped surface sensitive bolometers  Cherenkov light for beta identification

18. (A) Surface-radioactivity source control: state of the art Three-tower run: direct comparison of the three methods for source control Preliminary Results: The three methods are similar, but Legnaro and wrapping are better: baseline option is confirmed. Wrapping Raw background in 3-4 MeV region:  0.06-0.07 counts/keV kg y GranSasso Safe and conservative extrapolation for CUORE background in bb region: 0.04 counts/keV kg y Legnaro

19. Action on the detectors Some personal considerations on plausible options to reject the surface background

20. Two TeO2 auxiliary bolometers are read in parallel (B) Event identification: composite surface sensitive bolometers • Protect each crystal surface with a thin auxiliary bolometerand read out simultaneous signal • from the main and the auxiliary bolometer a scatter plot separates surface and bulk events Appl. Phys. Lett. 86,134106(2005) Nucl. Instr. Meth. A559, 355 (2006) Pulse amplitude TeO2 alphas betas+gammas Pulse amplitude • The thermistor on the thin absorber is removed: the slab works as a signal shape modifier Underground tests performed on CUORE size elementary modules Dramatic simplification 210Pb contamination monochromatic alphas Encouraging but not conclusive results X Pulse decay time TeO2 It is worthwhile to study systematically this approach bulk events Pulse amplitude

21. (B) Event identification: thin-film equipped surface sensitive bolometers • Deposit on each crystal surface a NbSi thin film which works as temperature sensor, but is • sensitive to athermal phonons for surface events  PSD separates surface and bulk events 60 keV gamma It works in small TeO2 prototype, but unpractical 5.4 MeV alpha Pulse shape parameter TeO2 surface • Six independently • read out films are • required • Specific heat of NbSi • is high, difficult to • work at 10 mK NbSi film bulk Pulse amplitude J. Low Temp. Phys. 151, 871 (2008) • Deposit a passive film on each surface and read out TeO2 temperature with ordinary sensor Dramatic simplification The goal is to get a shape-changed signal when part of the energy is released in or near the deposited film  film as a pulse shape modifier • Rely on purely thermal effect  play with film material (heat • capacity) and film-crystal thermal conductance  Same approach as • slab addition, but cleaner and more reproducible TeO2 • Exploit quasi-particle life-time in a superconductive film (e.g. Al) Pulse shape At the center of a two-year program of a Marie Curie fellowship [ARBRES]

22. (B) Event identification: Cherenkov light for electron tagging Optical properties of TeO2 crystals Threshold for Cherenkov emission: 50 keV for an electron 400 MeV for an alpha particle • Transparent from 350 nm to infrared • n=2.4 Cherenkov effect is potentially able to discriminate betas from alphas! Eur. Phys. J. C 65, 359 (2010) How many photons in the 2 - 3.5 eV interval?  125 photons for a 0nbb decay event It looks within the reach of the bolometric light detectors developed for Dark Matter (CRESST) Caution:scintillation light for TeO2 was claimed several years ago Nucl. Instr. Meth. A 520 159 (2004) The results for a gamma calibration seems compatible with Cherenkov light emission BUT a small light yield from alpha particle (a few photons/5MeV) suggests that also scintillation is present. Anyway, the beta/gamma light yield ratio is of the order of  25 Attempts to increase low temperature scintillation in TeO2 by Nb and Mn doping failed

23. CUORE sensitivity with improved TeO2bolometers TeO2 (isotope: 130Te) Successful R&D (enriched crystals and alpha tagging) BKG in DBD region = 1x10-3 counts/keV kg y Enrichment cost: 15 €/g at 99% Assumptions for CUORE upgrade: mββ S(mββ) [meV] Sensitivity to mββ (QRPA-Tuebingen et al.) as a function of the enrichment cost and detector mass 35 30 Maximum occupation of CUORE cryostat 25 20 15 Detector total mass [kg] 200 400 600 800 1000 2 4 6 8 10 12 Enrichment cost [M€]

24. Conclusions • TeO2-based bolometers represent a well established technique, very • competitive for neutrinoless double beta decay search • Cuoricino, stopped in 2008, is one of the most sensitive double beta decay • search ever run • Cuoricino demonstrates the feasibility of a large scale bolometric detector • (CUORE) with high energy resolution and competitive sensitivity (approaching • the inverted hierarchy region) • CUORE, a next generation detector, is under construction and will start to take • data in 2012/2013 • The CUORE sensitivity can be extended to cover fully the inverted hierarchy • region with enrichment and rejection of surface radioactivity → the TeO2 • approach is highly competitive in general and in the specific field of • bolometric searches