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Search for neutrino-less double beta decay of 76 Ge in the GERDA experiment

Search for neutrino-less double beta decay of 76 Ge in the GERDA experiment. M. Wojcik Instytut e of Physics , Jagiello nian University. Outline. Introduction to the double beta decay and neutrino mass problem Goals of GERDA GERDA design

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Search for neutrino-less double beta decay of 76 Ge in the GERDA experiment

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  1. Search for neutrino-less double beta decay of 76Ge in the GERDA experiment M. Wojcik Instytute ofPhysics, Jagiellonian University M. Wojcik, Jagellonian University: GERDA status report

  2. Outline • Introduction to the double beta decay and neutrino mass problem • Goals of GERDA • GERDA design • Cryostat • Water tank and the muon veto • Cleanroom and the lock system • Status of selected subprojects • Phase I detectors • Phase II detectors • Front-end electronics • Background in GERDA • Internal • External • Argon purity • Liquid Argon scintillation • Summary M. Wojcik, Jagellonian University: GERDA status report

  3. Double beta decay M. Wojcik, Jagellonian University: GERDA status report

  4. Double beta decay M. Wojcik, Jagellonian University: GERDA status report

  5. Absolute neutrino mass scale • 3H beta-decay, electron energy measurementMainz/Troisk Experiment: me < 2.2 eV  KATRIN • Cosmology, Large Scale StructureWMAP & SDSS: cosmological bounds m < 0.8 eV • Neutrinoless double beta decayevidence/claims?? Majorana  mass: <mee>  0.4 eV M. Wojcik, Jagellonian University: GERDA status report

  6. Neutrino mass hierarchy • < mee> (100 – 500)meV – claim of an observation of 0 in 76Ge suggests quasi-degenerate spectrum of neutrino masses • < mee> (20 – 55)meV – calculated using atmospheric neutrino oscillation parameters suggests inverted neutrino mass hierarchy or the normal-hierarchy – very near QD region • < mee> (2 – 5)meV – calculated using solar neutrino oscillation parameters would suggest normal neutrino mass hierarchy M. Wojcik, Jagellonian University: GERDA status report

  7. Neutrino mass hierarchy quasi-degenerate (QD) mass spectrum mmin>> (m212)1/2 as well as mmin>>(m322)1/2 M. Wojcik, Jagellonian University: GERDA status report

  8. From T1/2 to the <mee> M. Wojcik, Jagellonian University: GERDA status report QRPA, RQRPA: V.Rodin, A. Faessler, F. Š., P. Vogel, Nucl. Phys. A 793, 213 (2007) Shell model: E. Caurieratal. Rev. Mod. Phys. 77, 427 (2005).

  9. Open questions • What is the nature of neutrino? Dirac or Majorana? • Which mass hierarchy is realized in nature? • What is the absolute mass-scale for neutrinos? A neutrinoless double beta decay experiment, like GERDA has the potential to answer all three questions M. Wojcik, Jagellonian University: GERDA status report

  10. Goals of GERDA • Investigation of neutrino-less double beta decay of 76Ge • Significant reduction of background around Qbb down to 10-3 cts/(kgkeVy) • Use of bare diodes in cryogenic liquid (LAr) of very high radiopurity • Use of segmented detectors • Passive/active background suppresion • If KKDC-evidence not confirmed: • O(1 ton) experiment in worldwide collaboration (cooperation with Majorana) M. Wojcik, Jagellonian University: GERDA status report

  11. Collaboration ~70 physicists 13 institutions 6 countries M. Wojcik, Jagellonian University: GERDA status report

  12. Why 76Ge? • Enrichment of 76Ge possible • Germanium semiconductor diodes • source = detector • excellent energy resolution • ultrapure material (monocrystal) • Long experience in low-level Germanium spectrometry M. Wojcik, Jagellonian University: GERDA status report

  13. Phases of GERDA • Phase I: • Use of existing 76Ge-diodes from Heidelberg-Moscow and IGEX-experiments • 17.9 kg enriched diodes  ~15 kg 76Ge • Background-free probe of KKDC evidence • Phase II: • Adding new segmented diodes (total: ~40 kg 76Ge) • Demonstration of bkg-level <10-3 count/(kg·keV·y) • Phase III: • If KKDC-evidence not confirmed: O(1 ton) experiment in worldwide collaboration M. Wojcik, Jagellonian University: GERDA status report

  14. phase II phase I KKDC claim Sensitivity Assumed energy resolution: E = 4 keV Background reduction is critical! M. Wojcik, Jagellonian University: GERDA status report

  15. GERDA design Lock Clean room Water tank (650 m3 H2O) Cryostat (70 m3 LAr) M. Wojcik, Jagellonian University: GERDA status report

  16. Cryostat Copper shield Vacuum-insulated double wall stainless steel cryostat M. Wojcik, Jagellonian University: GERDA status report

  17. Cryostat • The cryostat has been constructed • Pressure test for inner and outer vessel • have been performed • Evaporation test at SIMIC sucessfully done • First 222Rn emanation test done (~ 30 mBq) • Vessel is ready for transportation to GS • Next steps at GS: • - 222Rn emanation measurement • - Evaporation test • - Copper mounting • - 222Rn emanation measurement M. Wojcik, Jagellonian University: GERDA status report

  18. Water tank and muon veto • Passive shield • Filled with ultra-pure water • 66 PMTs: Cherenkov detector • Plastic scintillator on top • Bottom plate has been installed M. Wojcik, Jagellonian University: GERDA status report

  19. Cleanroom and the lock system M. Wojcik, Jagellonian University: GERDA status report

  20. Phase I diodes • IGEX and HdM diodes were removed from their cryostats • Dimensions were measured • Construction of dedicated low-mass holder for each diode • Reprocessing of all diodes at manufacturer (so far two has been processed) • Storage underground during reprocessing (HADES) • 17.9 kg enriched and 15 kg non-enriched crystals (GENIUS-TF) are available M. Wojcik, Jagellonian University: GERDA status report

  21. Phase I prototypes • Establishing handling procedures • Optimization of thermal cyclings (> 40 cycles performed) • Design and tests of the low-mass detector holder • Long-term stability tests • Study of leakage current (LC) • Detector handling procedure • Irradiation with g-sources and LEDs • Problems with increasing LC seems to be under control • The same performance in LAr and LN2 observed M. Wojcik, Jagellonian University: GERDA status report

  22. Phase II detectors • 37.5 kg enrGe produced • ~87% 76Ge enrichment in form of GeO2 • Chemical purity: 99.95 % (not yet sufficient) • Underground storage • Investigation of different options for crystal pulling – IKZ Berlin most probable • 18-fold n-type diodes preferred: • Segmentation easier • Thin outside dead layer  little loss of active mass M. Wojcik, Jagellonian University: GERDA status report

  23. Phase II detectors: tests Suppression of events from external 60Co and 228Th source (10 cm distance) M. Wojcik, Jagellonian University: GERDA status report

  24. Requirements: Low noise, low radioactivity, low power consumption, operational at 87 K Candidates: F-CSA104 (fully integrated, under tests) PZ-0/PZ-1 (not yet ready for tests) IPA4 (under tests) CSA-77 (partly cold, under tests) Tests in different configurations, with different cables etc. Front-end electronics M. Wojcik, Jagellonian University: GERDA status report

  25. Background in GERDA • External background • -  from U, Th decay chain, especially 2.615 MeVfrom 208Tl in concrete, rock, steel... • - neutrons from (,n) reaction and fission in concrete, rock and from  induced reactions • external background will be reduced by passive and active shield • Internal background • - cosmogenic isotopes produced in spallation reactions at the surface, 68Ge and 60Co with half lifetimes ~year(s) • - surface and bulk Ge contamination • internal background will be reduced by anticoincidence between segments and puls shape discrimination M. Wojcik, Jagellonian University: GERDA status report

  26. Internal background reduction Cosmogenic 68Ge product. in 76Ge at surface: ~1 68Ge/(kg·d) (Avinione et al., Nucl. Phys B (Proc. Suppl) 28A (1992) 280) 68Ge 68Ga 68ZnT1/2: 271 d 68 min stable DecayECb+(90%) EC(10%) RadiationX – 10,3 keV b – 2,9 MeV After 6 months exposure at surface and 6 months storage underground 58 decays/(kg·y) in 1st year Bck. index = 0.012 cts/(keV·kg·y) = 12 x goal! M. Wojcik, Jagellonian University: GERDA status report

  27. Internal background reduction • Cosmogenic 60Co production in natural Ge at sea level: • 6.5 60Co/(kg·d) Baudis PhD4.7 60Co/(kg·d) Avinione et al., • 60Co 60Ni • T1/2: 5.27 y • Decay:b- • Radiation: b(Emax = 2824 keV), g(1172 keV, 1332 keV) • After 30 days of exposure at sea level 15 decays/(kg·y)Bck. index = 0.0025 cts/(keV·kg·y) = 2.5 x goal! • As short as possible exposure to cosmic rays!!! M. Wojcik, Jagellonian University: GERDA status report

  28. Internal background reduction • Photon – Electron discrimination • Signal: local energy deposition – single site event • Gamma background: compton scattering – multi site event Anti-coincidence between segments suppr. factor ~10 Puls shape analysis suppr. factor ~2 M. Wojcik, Jagellonian University: GERDA status report

  29. External background reduction Graded shielding • Shielding layerTl concentration • ~ 3 m purified water (650 m3) 208Tl < 1 mBq/kg • ~ 4 cm SS cryostat + 2walls208Tl < 10 mBq/kg • ~ 3 cm Copper shield 208Tl ~ 10 µBq/kg • ~ 2 m LAr (70 m3) Tl ~ 0 • Shielding and cooling with LAr is the best solution • ‘reduce all impure material close to detectors as much as possible’ • external /n/ background < 0.001 cts/(keV·kg·y) for LAr will be reached M. Wojcik, Jagellonian University: GERDA status report

  30. Counts/kg/keV/y Energy (keV) External background reduction Muon-induced background: Prompt background No m-veto! no cut: 10-2 Anticoincidence (phase I): 10-3 Segmentation (phase II): 3·10-4 goal 75% effective muon-veto is sufficient to achieve 10-4 counts/kg/keV/y M. Wojcik, Jagellonian University: GERDA status report

  31. External background reduction Muon-induced background: Delayed background • 77Ge produced from 76Ge by n-capture. • Reduction by delayed coincidence cut (muon, -rays, β-decay). M. Wojcik, Jagellonian University: GERDA status report

  32. Liquid Argon purification • Same principle as N2 purification • Initial 222Rn conc. in Ar higher than in N2 • In gas phase achieved: • Even sufficient for GERDA phase III • Purification works also in liquid phase (efficiency lower  more activated carbon needed) 222Rn in Ar: <1 atom/4m3 (STP) M. Wojcik, Jagellonian University: GERDA status report

  33. Liquid Argon scintillation MC example: Background suppression for contaminations located in detector support LArGe (~1 m3 LAr) in GDL M. Wojcik, Jagellonian University: GERDA status report

  34. Summary • Main goal: background reduction by a factor of 102 – 103 comparing to previous Ge experiments • The cryostat has been contructed, transportation to Gran Sasso very soon • Construction of the water tank has started (bottom plate is ready) • Reprocessing of existing diodes is ongoing at Canberra • Handling of bare diodes under control (LC problem) • 76Ge for Phase II detectors is available, crystal pulling is under discussion • Various background reduction techniques are under investigations • Start of data taking: 2009 M. Wojcik, Jagellonian University: GERDA status report

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