Double beta decay search with TeO 2 bolometers Andrea Giuliani on behalf of the CUORE collaboration University of Insubria (Como) and INFN Milano-Bicocca The Future of Neutrino Mass Measurements: Terrestrial, Astrophysical, and Cosmological Measurements in the Next Decade Seattle, February 8-11, 2010

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

S. Pascoli, S. T. Petcov and T. Schwetz, hep-ph/ The size of the challenge  counts / y ton  counts / y ton  counts / y ton 76 Ge result 50 meV 20 meV

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

Properties of 130 Te as a DBD emitter  high natural isotopic abundance (I.A. = %)  high transition energy ( Q = 2530 keV )  encouraging theoretical calculations for 0 -DBD lifetime  already observed with geo-chemical techniques (  1/2 incl = ( )  y)  2 DBD decay observed by a precursor bolometric experiment (MiDBD) and by NEMO3 at the level  1/2 = ( )  y excellent feature for reasonable-cost expansion of Double Beta Decay experiments large phase space, low background (clean window between full energy and Compton edge of 208 Tl photons) m   50 meV  2x10 26 y 130 Te features as a DBD candidate:

The bolometric technique for 130 Te: detector concepts  Temperature signal:  T = 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:  V = I  dR/dT   T   V = 1 mV for E = 1 MeV  Noise over signal bandwidth (a few Hz): V rms = 0.2  V  Signal recovery time:  = C/G  0.5 s In real life signal about a factor smaller Energy resolution (FWHM):  1 keV Heat sink T  10 mK Thermal coupling G  4 nW / K = 4 pW / mK Thermometer NTD Ge-thermistor R  100 M  dR/dT  100 k  /mK Energy absorber TeO 2 crystal C  2 nJ/K  1 MeV / 0.1 mK Te dominates in mass the compound Excellent mechanical and thermal properties

A physical realization of bolometers for DBD Energy absorber single TeO 2 crystal  790 g  5 x 5 x 5 cm Thermometer (doped Ge chip) Cuoricino basic module

Cuoricino and CUORE Location Cuoricino was 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 R&D final tests for CUORE (hall C) CUORE (hall A) Cuoricino (hall A)

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 Te nuclides The CUORICINO set-up Cold finger Tower Lead shield Same cryostat and similar structure as previous pilot experiment Coldest point

CUORICINO physics results MT = kg 130 Te  y T 1/2 0v (y) > 2.94  y (90% C.L.) m  < 206 – 720 meV Background sum spectrum of all the detectors in the DBD region  use new more accurate Q-value: keV Phys. Rev. Lett. 102, (2009), Phys. Rev. C 80, (2009)  updated statistics through Mar 2008 (shut down in July 2008) 60 Co sum peak 2505 keV  3 FWHM from DBD Q-value 130 Te - 0  Nucl. Phys. A 766, 107 (2006) [Erratum-ibid. A 793, 213 (2007)] BKG rate: 0.18  0.01 counts / keV kg y

CUORE = closely packed array of 988 detectors 19 towers - 13 modules/tower - 4 detectors/module M = 741 kg   Te 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

HUT CRYSTALS OTHER DETECTOR ELEMENTS (THERMISTORS, HEATERS, HOLDERS…) CRYOSTAT CLEAN ROOM CUORE-0 PREPARATION ASSEMBLY AND INTEGRATION DATA TAKING The CUORE schedule CUORE-0 RUNNING

Background in 0  region c/keV/kg/y T 1/2 0 ( 130 Te) > 2.1 x y m ββ < meV Background in 0  region c/keV/kg/y T 1/2 0 ( 130 Te) > 6 x y m ββ < meV Surface background problem partially / fully solved IBM244 meV QRPA Jyuvaeskulae49 meV QRPA Tuebingen et al.53 meV ISM73 meV IBM225 meV QRPA Jyuvaeskulae29 meV QRPA Tuebingen et al.31 meV ISM43 meV Detector resolution  5 keV Live time 5 years More recent and reliable NME calculations CUORE sensitivity

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

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

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

(A) Action on the source  surface cleaning – plastic wrapping (B) Action on the detectors  events identification Mechanical action / Electropolishing Chemical etching / Plasma cleaning  “Legnaro” cleaning method [CUORE baseline]  “Gran Sasso” cleaning method  Composite surface sensitive bolometers Strategies for the control of the surface background from inert materials  Polyethelene wrapping  Thin-film equipped surface sensitive bolometers  Cherenkov light for beta identification Strategy adopted by the CUORE collaboration to reject surface background Possible R&D subjects aiming at fully exploiting the potential of TeO 2 bolometers in a second phase

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

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

(B) Event identification: composite surface sensitive bolometers  Protect each crystal surface with a thin auxiliary bolometer and read out simultaneous signal from the main and the auxiliary bolometer  a scatter plot separates surface and bulk events Two TeO 2 auxiliary bolometers are read in parallel Pulse amplitude betas+gammas alphas X  The thermistor on the thin absorber is removed: the slab works as a signal shape modifier Pulse decay time Pulse amplitude 210 Pb contamination monochromatic alphas bulk events Dramatic simplification Underground tests performed on CUORE size elementary modules Encouraging but not conclusive results It is worthwhile to study systematically this approach Appl. Phys. Lett. 86,134106(2005) Nucl. Instr. Meth. A559, 355 (2006) TeO 2

(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 Pulse shape parameter Pulse amplitude It works in small TeO 2 prototype, but unpractical  Six independently read out films are required  Specific heat of NbSi is high, difficult to work at 10 mK bulk surface 5.4 MeV alpha 60 keV gamma NbSi film J. Low Temp. Phys. 151, 871 (2008)  Deposit a passive film on each surface and read out TeO 2 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  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] TeO 2

(B) Event identification: Cherenkov light for electron tagging Optical properties of TeO 2 crystals  Transparent from 350 nm to infrared  n=2.4 Threshold for Cherenkov emission: 50 keV for an electron 400 MeV for an alpha particle Cherenkov effect is potentially able to discriminate betas from alphas! How many photons in the eV interval?  125 photons for a  decay event It looks within the reach of the bolometric light detectors developed for Dark Matter (CRESST) Caution: scintillation light for TeO 2 was claimed several years ago Nucl. Instr. Meth. A (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 TeO 2 by Nb and Mn doping failed Eur. Phys. J. C 65, 359 (2010)

CUORE sensitivity with improved TeO 2 bolometers Assumptions for CUORE upgrade: TeO 2 (isotope: 130 Te) Successful R&D (enriched crystals and alpha tagging) BKG in DBD region = 1x10 -3 counts/keV kg y Enrichment cost: 15 €/g at 99% m ββ Sensitivity to m ββ (QRPA-Tuebingen et al.) as a function of the enrichment cost and detector mass  (m ββ ) [meV] Enrichment cost [M€] Detector total mass [kg] Maximum occupation of CUORE cryostat

Conclusions  TeO 2 -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 TeO 2 approach is highly competitive in general and in the specific field of bolometric searches