Double beta decay search Double beta decay search with TeOwith TeO22 bolometers bolometers

Andrea Giulianion 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 DecadeSeattle, February 8-11, 2010

Decay modes for Double Beta DecayDecay modes for Double Beta Decay

(A,Z) (A,Z+2) + 2e- neutrinoless Double Beta Decay (0-DBD)never observed (except a discussed claim)

> 1025 y

(A,Z) (A,Z+2) + 2e- + 2e

2Double Beta Decay allowed by the Standard Model

already observed – 1019 – 1021 y

Two decay modes are usually discussed:

Double Beta Decay is a very rare, second-order weak nuclear transitionwhich 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/0505226

The size of the challengeThe size of the challenge

100 - 1000 counts / y ton

1 - 10 counts / y ton

0.1 - 1 counts / y ton

76Ge result

50 meV

20 meV

Electron sum energy spectra in DBDElectron sum energy spectra in DBDThe 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 candidatessum electron energy / Q

two neutrino DBDcontinuum with maximum at 1/3 Q

neutrinoless DBDpeak 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

Properties of Properties of 130130Te as a DBD emitterTe as a DBD emitter

high natural isotopic abundance (I.A. = 33.87 %)

high transition energy ( Q = 2530 keV )

encouraging theoretical calculations for 0-DBD lifetime

already observed with geo-chemical techniques ( 1/2 incl = ( 0.7 - 2.7 ) 1021 y)

2 DBD decay observed by a precursor bolometric experiment (MiDBD) and by NEMO3 at the level 1/2 = ( 5 - 7 ) 1020 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 208Tl photons)

m 50 meV 2x1026 y

130Te features as a DBD candidate:

The bolometric technique for The bolometric technique for 130130Te: detector conceptsTe: 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): Vrms = 0.2 V Signal recovery time: = C/G 0.5 s

In real life signal about a factor 2 - 3 smaller

Energy resolution (FWHM): 1 keV

Heat sinkT 10 mK

Thermal couplingG 4 nW / K = 4 pW / mK

ThermometerNTD Ge-thermistor

R 100 MdR/dT 100 k/mK

Energy absorberTeO2 crystal

C 2 nJ/K 1 MeV / 0.1 mK

Te dominates in mass the compoundExcellent mechanical and thermal properties

A physical realization of bolometers for DBDA physical realization of bolometers for DBD

Energy absorbersingle TeO2 crystal 790 g 5 x 5 x 5 cm

Thermometer(doped Ge chip)

Cuoricino basic moduleCuoricino basic module

Cuoricino and CUORE LocationCuoricino and CUORE Location

Cuoricino was and CUORE will be installed in

Laboratori Nazionali del Gran SassoL'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) eachM = 41 kg 5 x 1025 130Te nuclides

The CUORICINO set-upThe CUORICINO set-up

Cold finger

Tower

Lead shield

Same cryostatand similar

structureas previous

pilot experiment

Coldest point

CUORICINO physics resultsCUORICINO physics results

MT = 18.14 kg 130Te y

T1/20v (y) > 2.94 1024 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: 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 peak2505 keV

3 FWHM from DBD Q-value

130Te - 0

Nucl. Phys. A 766, 107 (2006) [Erratum-ibid. A 793, 213 (2007)]

BKG rate: 0.180.01 counts / keV kg y

CUORE = closely packed array of 988 detectors 19 towers - 13 modules/tower - 4 detectors/moduleM = 741 kg 1027 130Te nuclides

Compact structure, ideal for active shielding

From CUORICINO to CUOREFrom CUORICINO to CUORE((CCryogenic ryogenic UUnderground nderground OObservatory for bservatory for RRare are EEventsvents))

Each tower is a CUORICINO-like detectorCustom dilution refrigerator

2008 2009 2010 2011 2012

HUT

CRYSTALS

OTHER DETECTOR ELEMENTS (THERMISTORS, HEATERS, HOLDERS…)

CRYOSTAT

CLEAN ROOM

CUORE-0 PREPARATION

ASSEMBLY AND INTEGRATION

DATA TAKING

The CUORE scheduleThe CUORE schedule

CUORE-0 RUNNING

Background in 0 region10-2 c/keV/kg/y

T1/20 (130Te) > 2.1 x 1026 ymββ < 44 - 73 meV

Background in 0 region10-3 c/keV/kg/y

T1/20 (130Te) > 6 x 1026 y

mββ < 25 - 43 meV

Surface background problem partially / fully

solved

IBM2 44 meVQRPA Jyuvaeskulae 49 meVQRPA Tuebingen et al. 53 meVISM 73 meV

IBM2 25 meVQRPA Jyuvaeskulae 29 meVQRPA Tuebingen et al. 31 meVISM 43 meV

Detector resolution 5 keVLive time 5 years

More recent and reliable NME calculations

CUORE sensitivityCUORE sensitivity

Simulation of the external backgroundSimulation of the external background

Cuore external background - arXiv:0912.0452v2 – accepted by Astroparticle Physics

The CUORE background componentsThe CUORE background components

Component Background in DBD region( 10-3 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 – 40

0.270 ± 0.022 (8.56 ± 6.06)×10−3

17.3 ± 0.3 0.104 ± 0.022

Close-to-det. material bulk

Total Anticoincidence

The Cuoricino background and the alpha surface radioactivityThe Cuoricino background and the alpha surface radioactivity

214Bi

60Co p.u.

208Tl~ 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 Strategies for the control of the surface background from inert materials 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 TeO2 bolometers in a second phase

(A) Surface-radioactivity source control: state of the art(A) Surface-radioactivity source control: state of the artThree-tower run: direct comparison of the three methods for source control

WrappingWrapping

Gran Gran SassoSasso

LegnaroLegnaro

Prelim

inary

Prelim

inary

Results:The three methods are similar, but Legnaro and wrapping are better: baseline option is confirmed.

Raw background in 3-4 MeV region: 0.06-0.07 counts/keV kg y

Safe and conservative extrapolation for CUORE background in region:

0.04 counts/keV kg y

Action on the detectorsAction on the detectors

Some personal considerations on plausible options

to reject the surface background

(B) Event identification: composite surface sensitive bolometers(B) Event identification: composite surface sensitive bolometersProtect 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 TeO2 auxiliary bolometers are read in parallel

Pulse amplitude

Pulse amplitudebetas+gammas

alphas

X

The thermistor on the thin absorber is removed: the slab works as a signal shape modifier

Pulse decay time

Pulse amplitude

210Pb 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)

TeO2

TeO2

(B) Event identification: thin-film equipped surface sensitive bolometers(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 TeO2 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 TeO2 temperature with ordinary sensorDramatic

simplificationThe 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]

TeO2

TeO2

(B) Event identification: Cherenkov light for electron tagging(B) Event identification: Cherenkov light for electron tagging

Optical properties of TeO2 crystals Transparent from 350 nm to infrared n=2.4

Threshold for Cherenkov emission:50 keV for an electron400 MeV for an alpha particle

Cherenkov effect is potentially able to discriminate betas from alphas!

How many photons in the 2 - 3.5 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 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

Eur. Phys. J. C 65, 359 (2010)

CUORE sensitivity with improved TeOCUORE sensitivity with improved TeO22 bolometers bolometers

Assumptions for CUORE upgrade:

TeO2 (isotope: 130Te)Successful R&D (enriched crystals and alpha tagging) BKG in DBD region = 1x10-3 counts/keV kg yEnrichment cost: 15 €/g at 99%

mββ

Sensitivity to mββ (QRPA-Tuebingen et al.) as a function of the enrichment cost and detector mass35

30

25

20

15200 400 600 800 10002 4 6 8 10 12

(mββ)[meV]

Enrichment cost [M€]Detector total mass [kg]

Maximum occupation of

CUORE cryostat

ConclusionsConclusions

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