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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 - PowerPoint PPT Presentation
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M. Wojcik, Jagellonian University: GERDA status report
Search for neutrino-less double beta decay of 76Ge in the GERDA experiment
M. WojcikInstytute of Physics, Jagiellonian University
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
Double beta decay
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: me < 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 76Gesuggests 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 parameterswould suggest normal neutrino mass hierarchy
M. Wojcik, Jagellonian University: GERDA status report
Neutrino mass hierarchy
quasi-degenerate (QD) mass spectrum
mmin>> (m212)1/2 as well as mmin>>(m32
2)1/2
M. Wojcik, Jagellonian University: GERDA status report
From T1/2 to the <mee>
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).
M. Wojcik, Jagellonian University: GERDA status report
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
Goals of GERDA
Investigation of neutrino-less double beta decay of 76Ge
Significant reduction of background around Q down to 10-3 cts/(kgkeVy) 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
Collaboration
~70 physicists13 institutions 6 countries
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
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
Sensitivity
Assumed energy resolution:
E = 4 keV
Background reduction is critical!
phase II
phase I
KKDC claim
M. Wojcik, Jagellonian University: GERDA status report
GERDA design
Clean room Lock
Cryostat (70 m3 LAr)
Water tank (650 m3 H2O)
M. Wojcik, Jagellonian University: GERDA status report
Cryostat
Vacuum-insulated double wall stainless steel cryostat
Copper shield
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 - 222Rn emanation measurement
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
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
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 -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
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: testsSuppression 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
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 +(90%) EC(10%) Radiation X – 10,3 keV – 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 PhD4.7 60Co/(kg·d) Avinione et al.,
60Co 60Ni
T1/2: 5.27 y
Decay: -
Radiation: (Emax = 2824 keV), (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 reductionPhoton – 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
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
Graded shielding
M. Wojcik, Jagellonian University: GERDA status report
External background reduction
Segmentation(phase II): 3·10-4
Energy (keV)
Counts
/kg
/keV
/y
goal
no cut: 10-2
No -veto!
Anticoincidence(phase I): 10-
3
75% effective muon-veto is sufficient to achieve 10-4 counts/kg/keV/y
Muon-induced background: Prompt background
M. Wojcik, Jagellonian University: GERDA status report
External background reductionMuon-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
LArGe (~1 m3 LAr) in GDL
MC example: Background suppression for contaminations located in detector support
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