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

1. 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

2. Neutrinos in the Universe • 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 Neutrinos are nearly as abundant in our universe as photons!  Even a small rest mass has large cosmological influence!

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

4. 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

5. 90% CL F.Feruglio et al., Nucl. Phys. B 637(2002) In order to discriminate between normal and inverted hierarchy, we need an experiment with sensitivity down to 10 meV! degeneracy Δm32 < 0 eV K.K. Claim Δm32 > 0 eV Neutrinoless Double Beta-Decay and the Mass Hierarchy <mee> [eV] Lightest neutrino mass [eV]

6. Present Experimental Status

7. 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: 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.)

8. 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 The principle idea of the GERDA experiment: Use the cryo-liquid as cooling medium and shield simultaneously: --> 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 G. Heusser, Ann. Rev. Nucl. Part. Sci. 45(1995)543

9. Clean room / lock Germanium detectors Steel-tank + Cu lining Liquid argon Water / Myon-Veto (Č) GERmanium Detector Array: GERDA • 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

10. GERmanium Detector Array: GERDA

11. IGEX detectors Heidelberg-Moscow 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

12. PhaseII detectors: Segmentation Germanium detectors can be segmented --> Background identification through identification of multiply Compton-scattered photons by coincidences Signal: Background: Phase II detectors will be 18 fold segmented: 3-fold in height, 6-fold in φ

13. Phase II: Detector development Novel low mass contacting scheme with Cu on Kapton. Material balance: 31g Cu, 7g Teflon, 2.5 g Kapton cable 18-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

14. All events Single segment events All events Single segment events Double Escape Peak (mostlySSE) Photon Peak (mostly MSE) Phase II: Results with Prototype Compton Background recognition works as expected: Photon Peak is reduced in single segment spectrum, whereas Double Escape Peak remains Suppression factors (SF) as expected from MC I. Abt. Et al, nucl-ex/0701005, Submitted to NIM A

15. Gran Sasso Underground Laboratory Laboratori Nationale di Fisica Nucleare 1400 m rock overburden : ~3500 mwe to shield against cosmic radiation

16. Status: Cryostat Production

17. Degenerate meein eV K.K. Claim GERDA I,II Inverted hierarchy GERDA III Normal hierarchy Lightest neutrino (m1,m3) in eV GERDA sensitivity We will confirm or rule out the Klapdor-Kleingrothaus et al. claim (Phase I) If not confirmed and background reduction to the level 10-3/(kg yr keV) demonstrated (Phase II), go for Phase III (ca. 1 ton, 20 meV level) Different M.E.