Neutrinoless Double Beta Decay Experimental...

Andrea Giuliani

University of Insubria (Como) and INFN Milano-Bicocca

Italy

NeutrinolessNeutrinoless Double Beta DecayDouble Beta DecayExperimental techniquesExperimental techniques

Varenna, June 2008

The CUORE background modelThe CUORE background model

The sources of the background

1. Radioactive contamination in the detector materials (bulk and surface)2. Radioactive contamination in the set-up, shielding included3. Neutrons from rock radioactivity4. Muon-induced neutrons

Monte Carlo simulation of the CUORE background based on:

1. CUORE baseline structure and geometry2. Gamma and alpha counting with HPGe and Si-barrier detectors 3. Cuoricino experience ⇒ Cuoricino background model4. Specific measurements with dedicated detectors in test refrigerator in LNGS

Results…..

The CUORE background componentsThe CUORE background components

Component Background in DBD region( 10-3 counts/keV kg y )

Environmental gamma

Apparatus gamma

Crystal bulk

Crystal surfaces

Inert det. material bulk

Inert det. material surface

Neutrons

Muons

< 1

< 1

< 0.1

< 3

< 1

~ 20 – 40

~ 0.01

~ 0.01

The CUORE background componentsThe CUORE background components

Component Background in DBD region( 10-3 counts/keV kg y )

Environmental gamma

Apparatus gamma

Crystal bulk

Crystal surfaces

Inert det. material bulk

Inert det. material surface

Neutrons

Muons

< 1

< 1

< 0.1

< 3

< 1

~ 20 – 40

~ 0.01

~ 0.01

The only limiting factor

The The CuoricinoCuoricino background background and the surface radioactivity modeland the surface radioactivity model

214Bi

60Co p.u.

208Tl~ 0.2 c / keV kg y

Gamma region

Reconstruction of the Reconstruction of the CuoricinoCuoricino spectrum spectrum in the region 2.5 in the region 2.5 –– 6.5 MeV 6.5 MeV

0νDBD

Reconstruction of the Reconstruction of the CuoricinoCuoricino spectrum spectrum in the region 2.5 in the region 2.5 –– 6.5 MeV 6.5 MeV

0νDBD

Additional component required

here

The additional component: The additional component: inert material surface contamination inert material surface contamination

In order to explain the 2.0 - 4 MeV region BKG, one has to introduce 238U or 210Pb surface contamination of the copper structure facing the detectors

surface contamination level: ~ 100 pg/g vs bulk c.l. : < 1 (0.1) pg/g for Cu (TeO2)

contamination depth: ~ 10 μm in agreement with direct measurement on Cu

(A) Passive methods ⇒ surface cleaning

(B) Active methods (“reserve weapons”) ⇒ events ID

Mechanical actionChemical etching / electrolitical processesPassivation

“Legnaro” method

“Gran Sasso” method

surface sensitive bolometers (Como)

scintillating bolometers ableto separate α from elettrons / γ (LNGS / Roma)

Strategies for the control of the surface backgroundStrategies for the control of the surface backgroundfrom inert materials from inert materials

It is difficult and quite demanding in terms of time and money to verifythe effectiveness of a method to reduce the surface radioactiviy

Assuming CUORICINO background

A ≈ 1.7 counts / cm2 ysup3 – 4 MeV

runs in hall C(A) Direct counting

contamination in U – Th of the order of ng/g

ICPMS analysis

(B) Concentration measurements

Two methods:

The diagnostic problemThe diagnostic problem

Not appliable at all for 210Pb or 226Ra

Laser ablation in conjunction to HR-ICPMS was proved to be a powerful method for surface activity diagnosticPlasma cleaning is under tests.

LegnaroLegnaro method: plasma cleaning and ICPMS diagnosismethod: plasma cleaning and ICPMS diagnosis

T ⇒ Mechanical barrel polishing by TumblingE ⇒ Removal of ~100 micron by ElectropolishingC ⇒ Removal of ~ tenths of microns by Chemical EtchingM ⇒ Removal of few microns by Magnetron Sputtering in UHVI ⇒ Removal on nanometric scale by Ion Beam Cleaning in UHV

Receipt for cleaning: several ingredients

CUORICINO: T + C

CUORE: T + E + C + M + (I on the site - LNGS)

Next generation 76Ge double beta decay experiment at Gran SassoSignificant reduction of background around Qββ to ≤10-3

cts/(kg⋅keV⋅y)Contamination in previous experiments mainly in cryostat / diode holder→ Bare diodes in cryogenic liquid (LAr)Cryogenic liquids have very high radiopurity

From HM / IGEX to GERDAFrom HM / IGEX to GERDA((TheThe GERGERmaniummanium DDetectoretector AArrayrray))

GERDA phases and sensitivityGERDA phases and sensitivity

Phase I: operate refurbished HM & IGEX enriched detectors (~20 kg)

Phase II: additional ~20 kg 76Ge diodes (segmented detectors)

Under commissioning Background: 0.01 counts/ keV kg yScrutinize 76Ge claim with the same nuclide (exclude 99% c.l. or confirm 5σ)Half life sensitivity: 3 x 1025 yStart data taking: 2009

Phase III (depending on physics results of Phase I/II)

⟨M⟩ < 20 - 50 meV

⟨M⟩ < 90 - 290 meVBackground: 0.001 counts / keV kg ySensitivity after 100 kg y (~3 years): 2 x 1026 y

⇒ ~ 1 ton experiment in world wide collaboration with MAJORANA

phase II

phase I KKDC claim

ΔEFWHM=4 keV

0 50 100 150 2000

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

no background

y)⋅ keV⋅ counts /(kg−410



Claim

⋅Exposure [kg years]

90%

pro

b. u

pp

er li

mit

< m

ee>

[eV

]

Assuming <M 0ν> = 3.92 [ Erratum: Nucl.Phys.A 766 (2006) 107 ]Inverted hierarchy range

using <M0ν>=3.92

V.A. Rodin at al., Nucl. Phys. A 366 (2006) 107-131.

Erratum:Nucl. Phys. A 793 (2007) 213-215.

phase I

phase II

KKDC claim

In 2006 3 IGEX diodes and 5 HM diodes were removed from their cryostats

Dimensions were measured

Construction of dedicated low-mass holder for each diode

The GERDA detectorsThe GERDA detectors

Phase I prototype Phase I prototype testingtesting

Low mass detector holder developed and testedDefinition of detector handling protocolOptimization of thermal cyclings>40 warming and cooling

cycles carried out

Same performance in LN2/LAr

Issue of the radiationIssue of the radiation--induced leakage currentinduced leakage current

Four irradiations without applying HV: the leakage current is recovered

p+ (B)∼0.3 μm

Passivationlayer

p-Ge

n+ (Li) ∼0.7 mm (not on scale)

Problem much more serious if irradiation of the bottom part

Origin of the problem: the passivation layer

Collection of +/- charges changes conductivity of passivation layer

+ (-) charges collected on the inner (outer) part of the passivation layer

γ radiation-induced decrease of LC: UV light from LAr scintillation

++++++++++ -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --+4 +4 kVkV0 0 kVkV

Tentative Tentative explainationexplaination

Calibration 1 week → negligible increase of LC during live-time of GERDA (<10 pA)(Note: ΔLC ∼ 1 nA → 1.6 keV deterioration)

SolutionSolution

GERDA designGERDA design

Cleanroom Lock

Water tank (650 m3 H2O)

Cryostat (70 m3 LAr)

GERDA designGERDA design

Germanium-detector array

Vacuum-insulated double wall stainless steel cryostat

Additional inner copper shield

Liquid argon

test three 18-fold segmented detectors immersed directly in LN (full GERDA string)

test 18-segment p-type detector

detector response to IR & UV light

R&D on phase II detectorsR&D on phase II detectors

EXO 200 sensitivityEXO 200 sensitivity

Assumptions on detector performance and background:200 kg of Xe enriched to 80% in 136σ(E)/E = 1.4% obtained in EXO R&DLow but finite radioactive background: 20 events/year in the ±2σ interval centered

around the 2.481 MeV endpointNegligible background from 2ν DBD (T1/2>1·1022yr R.Bernabei et al. measurement)

⟨M⟩ < 270 - 380 meVBackground: 40 counts in 2 ySensitivity (2 years, 90% c.l.): 6.4 x 1025 y

QRPA NSM

If 76Ge claim is correct :Worst case (QRPA, upper limit) 15 events (40 events bkg) ⇒2σBest case (NSM, lower limit) 162 events (40 events bkg) ⇒ 11σ

DBD option in SNO+ is sometimes referred to as SNO++

it is possible to add ββ isotopes to liquid scintillator, for example– dissolve Xe gas– organometallic chemistry (Nd, Se, Te)– dispersion of nanoparticles (Nd2O3, TeO2)

SNO+ collaboration researched these options and decided that the best isotope and technique is to make a Nd-loaded liquid scintillator

SNO++: a very powerful newcomerSNO++: a very powerful newcomer

a liquid scintillator detector has poor energy resolution; but enormousquantities of isotope (high statistics) and low backgrounds help compensate

large, homogeneous liquid detector leads to well-defined background model

possibly source in–source out capability

using the technique that was developed originally for LENS and now also used for Gd-loaded scintillator

SNO+ collaboration managed to load Nd into pseudocumene and in linear alkylbenzene (>1% concentration)

with 1% Nd loading (natural Nd) a very good neutrinoless double beta decay sensitivity is predicted, but…

SNO++: conceptsSNO++: concepts

a liquid scintillator detector has poor energy resolution; but enormousquantities of isotope (high statistics) and low backgrounds help compensate

large, homogeneous liquid detector leads to well-defined background model

possibly source in–source out capability

using the technique that was developed originally for LENS and now also used for Gd-loaded scintillator

SNO+ collaboration managed to load Nd into pseudocumene and in linear alkylbenzene (>1% concentration)

with 1% Nd loading (natural Nd) a very good neutrinoless double beta decay sensitivity is predicted, but…

at 1% loading (natural Nd), there is too much light absorption by Nd⇒ 47 ± 6 pe/MeV(from Monte Carlo)

at 0.1% loading (isotopically enriched to 56%) ⇒ 400 ± 21 pe/MeV(from Monte Carlo)

good enough to do the experiment

SNO++: conceptsSNO++: concepts

1 yr, 500 kg isotope, mν = 150 meV

SNO++: sensitivitiesSNO++: sensitivities

enriched natural

Open questionsLarge scale enrichment: laser isotope separation possible in France using a dismissed facility for U ⇒ the facility must be re-operated and tuned to 150Nd150Nd nuclear matrix elements

CUORICINO Background

γ-region α-region130Te76Ge 100Mo116Cd

Environmental “underground” Background:

238U and 232Th trace contaminations

82Se

Scintillating Scintillating bolometersbolometers

α

β/γ


180W α

β/γ

Heater

α


Light Detector

3x3x3 CdWO4

3x3x6 CdWO4


Calibration results on CdWO4

0.2% FWHM @ 2615 keV

232Th + 56Co Calibration

Heat

Calibration results on CdWO4

2.9 % FWHM @ 2615 keV

232Th + 56Co Calibration

Scitillation

2.9% FWHM is the best result ever achieved with CdWO4 as scintillator

CdWO4- some considerations

CdWO4- some considerations II


E-rotated

θ

Erotaded=Heat cosθ + Light sinθ

FWHM @ 2615 improves by ~ 40 % !!!!!


E-rotated

θ

Erotaded=Heat cosθ + Light sinθ

Outline of the lecturesOutline of the lectures

Neutrino mass and Double Beta Decay

Experimental challenge and strategies

Present situation

Overview of the future projects

Some very promising experimental approaches

Prospects and conclusions

(super) CUORE,GERDA III,maybe SNO++...

Prediction of the Moore’s law for the sensitivityPrediction of the Moore’s law for the sensitivity

Future scenarios and branching points in terms of discoveryFuture scenarios and branching points in terms of discovery

100 - 500 meV

15 - 50 meV

2 - 5 meV

sensitivity to ⟨M⟩

degenerate hierarchy100-200 kg isotope - 5 year scaleCUORE – GERDA I / II - EXO-200 - SUPERNEMO

inverted hierarchy - atmospheric ΔM2 region1000 kg isotope - 10 year scalestraightforward in some cases (enriched CUORE)GERDA phase III – EXO final - ….

direct hierarchy - solar ΔM2 regionbig leap in sensitivity - new approaches required100 tons of isotopes is the typical scalediscovery if neutrino is a Majorana particleunpredictable time scale

experimental situation


100 - 500 meV

15 - 50 meV

2 - 5 meV






if this range holds (or if 76Ge claim is right):- SUPERNEMO may investigate the mechanism (82Se or 150Nd)- GERDA phase I / II will see it in 76Ge- EXO-200 will see it in 136Xe- SNO++, if done, could see it in 150Nd- CUORE will see it in 130Te and may do multi-isotope searches simultaneously(130Te - 116Cd - 100Mo)large scale enrichment requiredreduction of uncertainties in NME

precision measurement era for 0ν-DBD!


100 - 500 meV

15 - 50 meV

2 - 5 meV





experimental situationif this range holds:- SUPERNEMO could marginally see it in 82Se or 150Nd- SNO++, if done, could see it in 150Nd- GERDA phase III could see it in 76Ge- CUORE could see it in a couple of isotopes in sequence(after 130Te, 116Cd ?)

discovery in 3 or 4 isotopes necessary (and possible...)to confirm the observation and to improve ⟨Mββ⟩ estimate


100 - 500 meV

15 - 50 meV

2 - 5 meV






if this range holds:- new strategies have to be developed- it is worthwhile to start to elaborate them now- next generation experiments are precious for the selection of the future approaches

- a large investment in enrichment is mandatory

What about a ~ 5 What about a ~ 5 meVmeV experiment?experiment?

50 meV 5 x 1026 y

5 meV 5 x 1028 y

⟨Mββ⟩ T0ν“average” NMEfor an “average” nucleus

if we require 10 counts in 5 years with 0 background

experiment with 1029 nuclei and 0 background

for example 30 ton of TeO2 enriched100 ton of TeO2 natural

selection of the most promising techniquedesign of the experiment

only “one bit” information ⇒ direct hierarchy - Majorana nature of neutrino

Σ [e

V]

⟨Mββ

⟩[eV

]

⟨Mβ⟩

[eV

]Exciting times for neutrino masses:

degeneracy will be deeply probed

discovery potential in case ofinverted hierarchy

(HM,CUORICINO, NEMO)

COSMOLOGY

SINGLE

β

DOUBLE

β

Conclusions Conclusions

PLANCK +larger surveys

KATRIN, MARE CUORE, GERDA, etc.

Neutrinoless Double Beta Decay Experimental...

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