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

M. Wojcik, Jagellonian University: GERDA status report 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

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

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

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

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

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

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).

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

M. Wojcik, Jagellonian University: GERDA status report 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

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

M. Wojcik, Jagellonian University: GERDA status report 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

M. Wojcik, Jagellonian University: GERDA status report 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

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

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

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

M. Wojcik, Jagellonian University: GERDA status report 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 M. Wojcik, Jagellonian University: GERDA status report

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

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

M. Wojcik, Jagellonian University: GERDA status report 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

M. Wojcik, Jagellonian University: GERDA status report 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

M. Wojcik, Jagellonian University: GERDA status report 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

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

Front-end electronics 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. M. Wojcik, Jagellonian University: GERDA status report

M. Wojcik, Jagellonian University: GERDA status report Background in GERDA External background -  from U, Th decay chain, especially 2.615 MeV from 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

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  68Zn T1/2: 271 d 68 min stable Decay EC b+(90%) EC(10%) Radiation X – 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

Internal background reduction Cosmogenic 60Co production in natural Ge at sea level: 6.5 60Co/(kg·d) Baudis PhD 4.7 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

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

External background reduction Graded shielding Shielding layer Tl concentration ~ 3 m purified water (650 m3) 208Tl < 1 mBq/kg ~ 4 cm SS cryostat + 2 walls 208Tl < 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

External background reduction Muon-induced background: Prompt background Energy (keV) Counts/kg/keV/y 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

External background reduction Muon-induced background: Delayed background 77Ge produced from 76Ge by n-capture. Reduction by delayed coincidence cut (muon, -rays, β-decay). Bcg Index for LAr [cts/(kg·keV·y)] 77,77mGe 1.1 · 10-4 Others 5 · 10-5 M. Wojcik, Jagellonian University: GERDA status report

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

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

M. Wojcik, Jagellonian University: GERDA status report 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