December 4, 2007Béla Majorovits,MPI für Physik, München Excellence Cluster Universe – Science...

December 4, 2007Béla Majorovits,

MPI für Physik, München

Excellence Cluster Universe – Science Week 2007

The Germanium Detector Array (GERDA) for the search of neutrinoless double Beta

Decay

• Neutrinos and the Neutrinoless Double Beta-Decay

• Present Experimental Status

• Principles of the GERDA Experiment

• Status and Prospects of GERDA




Neutrinos in the Universe

Neutrinos are nearly as abundant in our universe as photons!

Even a small rest mass has large cosmological influence!

Most prominent Neutrino sources:•Relic from Big Bang: 100 per cm3•Neutrinos from the sun at earth: 6 1010 cm2s-1•Neutrinos from 1GW nuclear reactors: 6 1011 cm2s;1 in 100m

•Supernova Neutrinos; Up to 1059 emitted within 10s




Neutrino-oscillation experiments have taught us: Neutrinos must have a non vanishing rest mass!

Neutrino Mass Hierarchy

Normal hierarchy Δm32 > 0 eV

Inverted hierarchy Δm32 < 0 eV

But we do not know the sign of Δm32

Mass hierarchy is still unknown

We only have information on the squared mass difference between the eigenstates

Absolute mass scale still unknown

Are Neutrinos their own Antiparticles, ie Majorana particles?

Nature of the Neutrinos still unknow




Signature: Sharp peak at Q-value of the decay

Neutrinoless Double Beta-Decay

• Double Beta-Decay is an allowed 2nd order weak process

• It’s half life is of the order 1010 times the age of the universe

• If Neutrinos are massive and their own anti-particles, ie Majorana particles, the process could occur without the emission of Neutrinos

1/ = G(Q,Z) |Mnucl|2 <mee>2




Δm32 > 0 eV

Δm32 < 0 eV degen

era

cy

F.Feruglio et al., Nucl. Phys. B 637(2002)

90% CL

In order to discriminate between normal and inverted hierarchy, we need an experiment with sensitivity down to 10 meV !

Lightest neutrino mass [eV]

<m

ee>

[eV

]

K.K. Claim

Neutrinoless Double Beta-Decay and the Mass Hierarchy




Present Experimental Status

Experiment

Underground Laboratory

Isotope

T1/2 [1021 y] <mee> [eV]

Elegant VI Oto (Japan) 48Ca > 95 < 7.2 - 44.7

Heidelberg-Moscow

Gran Sasso (Italy)

76Ge >19000 evidence:

11900

< 0.35 - 1.2 0.44

IGEX Canfranc (Italy) 76Ge > 16000 < 0.3 - 1.5

NEMO-III Frejus (France) 82Se > 140 < 1.7 - 4.9

NEMO-III 100Mo > 460 < 0.7 - 2.8

CdWO4 scintillator

Solotvina (Ukrain)

116Cd > 170 < 1.5 - 1.7

Cuoricino Gran Sasso 130Te > 1800 < 0.4 - 1.9

DAMA Gran Sasso 136Xe > 1200 < 2.9




11.5 kg of enriched Ge detectors

71.7 kg yrs of data

Upper limit:

T1/2≥1.9 * 1025 years (90% C.L.)

4.2 σ claim: T1/2= 1.19 * 1025 years

<mee> = 440 meV (KK matrix el.)

Evidence for Neutrinoless Double Beta-Decay

HPGe-detectors, enriched in 76Ge in conventional low background copper cryostat.

Data taking: 1990 - 2003

The Heidelberg-Moscow Experimet:




The principle idea of the GERDA experiment:

Use the cryo-liquid as cooling medium and shield simultaneously:

GERmanium Detector Array: GERDA

Increase sensitivity in order to confirm or refute the claim

--> Reduce bkg-index by at least two orders of magnitude to 10-3 Cts/(kg keV year)

--> Increase target mass

G. Heusser, Ann. Rev. Nucl.

Part. Sci. 45(1995)543

--> Radioactive background can be drastically reduced

- LN and LAr can be produced with very high purity

- Materiel of conventional cryostat is removed from

detector surrounding




Germanium detectors

Water / Myon-Veto (Č)

Clean room / lock

Steel-tank + Cu liningLiquid argon


•Place array of naked HPGe-detectors enriched in 76Ge in the center of a stainless steel cryostat filled with LAr.

•Inner copper lining as radiation shield against gammas from cryostat.

•Surround the whole setup with water tank to shield against external gammas, neutrons and muons (water Cerenkov).

10 m





Phase I Phase II

Target mass of enriched material [kg]

18 kg from Hd-Mo and IGEX detectors

Additional 20 kg

Envisioned background[Counts/(kg keV y)]

0.01(limited by cosmogenic 60Co)

0.001(improvement by segmentation and further material selections)

Exposure 15 kg y 100 kg y

Confirm or refute claim

Push sensitivity, prove low background capability of technique

Discov. potential: T1/25 •1025 yrs,

Limit setting: to 1.5 • 1026 yrs.

For Rodin et al. matrix element, mass sensitivity about 200 meV




Heidelberg-Moscow detectors

IGEX detectors

Phase I Detectors:

• Acquired HdMo and IGEX detectors• Constructed detector holder out of low level

materials• Dismounting of detectors from cryostats without

problem• Prototype (nat.) detector works well in LAr• Detectors being refurbrished by Canberra




Germanium detectors can be segmented --> Background identification through identification of

multiply Compton-scattered photons by coincidences

Signal: Background:

PhaseII detectors: Segmentation

Phase II detectors will be 18 fold segmented:

3-fold in height, 6-fold in φ




Phase II: Detector development18-fold segmented prototype detector works fine. New contacting scheme verified in conventional surrounding:

Good energy resolution for core and 18 all segments:

3 keV @ 1.3 MeV --> 0.2%

I. Abt. et al, nucl-ex/0701004,Accepted for publication in NIM A

Novel low mass contacting scheme with Cu on Kapton.

Material balance:31g Cu, 7g Teflon, 2.5 g Kapton cable




Phase II: Results with PrototypeCompton Background recognition works as expected:

Photon Peak is reduced in single segment spectrum, whereas Double Escape Peak remains

Suppression factors (SF) as expected from MCAll events

Single segment events

Double Escape Peak (mostlySSE)

Photon Peak (mostly MSE)

All events

Single segment events

I. Abt. Et al, nucl-ex/0701005,Submitted to NIM A




Gran Sasso Underground Laboratory

1400 m rock overburden : ~3500 mwe to shield against cosmic radiation

Laboratori Nationale di Fisica Nucleare




Status: Cryostat Production




Inverted hierarchy

Normal hierarchy

Degen

erat

e

Lightest neutrino (m1,m3) in eV

mee in

eV

1. We will confirm or rule out the Klapdor-Kleingrothaus et al. claim (Phase I)

2. If not confirmed and background reduction to the level 10-3/(kg yr keV) demonstrated (Phase II), go for

3. Phase III (ca. 1 ton, 20 meV level)K.K. Claim

GERDA III

GERDA I,II

Different M.E.

GERDA sensitivity

December 4, 2007Béla Majorovits,MPI für Physik, München Excellence Cluster Universe – Science...

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