GERDA: GERmanium Detector ArraySearching for 02 decay

Xiang LiuMax-Planck-Institut für Physik

1. 02 introduction2. GERDA setup & detectors3. Low background & high sensitivity4. R&D 5. Conclusion

SNOW 2006, May 2-6th, 2006

Neutrino mixing & 02 decay

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Open questions:Absolute mass scale?Normal hierarchy or inversed?Nature: Dirac or Majorana? 02 decay might address all.

22 decay: (A,Z) (A,Z+2) +2e+ 2 allowed by SM & exists.02 decay: (A,Z) (A,Z+2) +2e if s Majorana & have mass. L=2Many isotopes can be used to search for 02. Best limit from Heidelberg-Moscow experiment (HdMo) with 76Ge.

Lightest neutrino mass (eV)m

(

eV)

Effective Majorana neutrino mass: m

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GERDAPhase I: Existing detectors Phase II: new detectorsFuture: 1 ton experiment

T12

1 G0 E 0 ,Z M0 2m

2

G0: phase space integralM0: nuclear matrix element<m>= Vei

2 mi (Vei: neutrino mixing matrix)

Search 02 & measure T1/2 Phase IPhase II

GERDA Setup

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02 rare processLarge amount of target massHigh efficiency detecting signalLow background

76Ge 8% in nature,need enrichment (reduce bg)

Detector = source, >95% signal effi. Excellent (FWHM ~2 keV at 1.3 MeV) , high Q value (2.04 MeV)

existing detectors & experiences from HdMo & IGEX.

LNGS Hall A(3800 m.w.e.) GERDA new features:

Ge detectors directly submerged in LN2/LAr (as cooling & shielding)

Segmented Ge detectors (phase-II)

Clean room & Lock

Scintillator veto(top of clean room)

Čerenkov veto(PMTs in water tank) Water tank

Ge detector array

LN2/LAr cryostat

Detectors & supports

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Phase-I: HdMo & IGEX detectors, sum ~15kg, non-segmentedPhase-II:New detectors, sum ~25kg, 18 segments (3z6)

Materials near detectors: as little as possible, as pure as possible.

readout cable

HV cable

suspensionstrings

holder

(Phase II)

Status of Existing Phase-I Detectors

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IGEX detectors testedinside Vacuum storage Can. FWHM 2.2 - 2.3keV at 1332keVHdMo detectors stored at LNGS, all tested.One HdMo detector taken out from Vacuum Can for testing.

HdMo Setup ANG1 Ge crystalHdMo claimSignal: 28.75 6.86 eventsBg level: 0.11 cts/[keV kg year]

If claim true, Phase-I will haveSignal: ~13 eventsBg level: <0.01 cts/[keV kg year] <3 events in 10keV window at Q value.

Status of Phase-II Detectors

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37.5kg Ge, 87.25% 76Ge, purity >99.9% (in GeO2) enriched in Russia, transported to MPI Munich on April 3rd 2006, now stored underground (200m.w.e)Next steps: purification, crystal pulling & detector fabrication.

High sensitivity = Low background

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Phase I

Phase II

Exposure [kg year] bg. Level [keV kg year] Limit on m [ev]

Phase I 30 10-2 0.5

Phase II 100 10-3 0.2

a)b)

a) Assessment of uncertainties in QRPA 02-decay nuclear matrix elementsV.A. Rodin, A. Faessler, F. Simkovic, P. Vogel [Nucl-th 0503063]

b) Search for Neutrinoless Double Beta Decay with Enriched 76Ge in Gran Sasso 1990-2003, H.V. Klapdor-Kleingrothaus, I.V. Krivosheina, A. Dietz and O. Chkvorets, Phys. Lett. B586 (2004) 198-212

Use segments to reject background

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Background : Mostly Compton scattering 2~3 cm mean free path (mostly multi-segment)

02 signal: 2 electrons deposit energy very localized (<1mm)

Source Reduction

factor208Tl (in Ge) 13

60Co (in Ge) 3868Ge (in Ge) 18

210Pb ( on Ge surface) 1

208Tl (in holder) 5

60Co (in holder) 157 (27)

208Tl (in cable) 7

MC simulation of bg.

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MC package (MaGe) for both Gerda & Majorana.Upper limit on contamination levels for all materials calculated.Bg. Index calculated with available materials.

Water tank

Cryo vessel

(Phase II)

R&D: Ge detector properties

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Several test facilities being built with detectors directly in LN2/LAr.

Both n & p-type Ge cycled multi times in LN2 & LAr Similar behavior in LN2/LAr. No deterioration of performance.6-segment p-type detector in LN2.

Co60

One 18-segment n-type detector is under study.

Conclusion

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Open questions in : absolute mass? hierarchy? Majorana or Dirac?02 might answer all these questions.

GERDA searches for 02 decay in 76Ge.Ge detectors are submerged directly in LN2/LAr (cooling & shielding).IGEX & HdMo detectors for Phase-I.Segmented Ge detectors for Phase-II.Goal for bg. rate: 10-3 cts/[keV kg year] (Phase-II), difficult but feasible.With this bg. rate, sensitivity of 0.2eV on m expected.

Several test facilities for GERDA are being built.Both n- & p-type detectors cycled in LN2/LAr without obvious deterioration.

Once the GERDA concept is proven, global experiment with 1t Ge might be pursued (MoU with Majorana exists).