Transcript Slide 1

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
Outline
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Introduction to the double beta decay
and neutrino mass problem
Goals of GERDA
GERDA design
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Status of selected subprojects
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Phase I detectors
Phase II detectors
Front-end electronics
Background in GERDA
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Cryostat
Water tank and the muon veto
Cleanroom and the lock system
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: me < 2.2 eV  KATRIN
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Cosmology, Large Scale Structure WMAP & SDSS:
cosmological bounds m < 0.8 eV
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Neutrinoless double beta decay evidence/claims??
Majorana  mass: <mee>  0.4 eV
M. Wojcik, Jagellonian University: GERDA status report
Neutrino mass hierarchy
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< mee> (100 – 500) meV – claim of an observation of
0 in 76Ge
suggests quasi-degenerate spectrum of neutrino masses
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< mee> (20 – 55) meV – calculated using atmospheric
neutrino oscillation parameters
suggests inverted neutrino mass hierarchy or the normalhierarchy – 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>
QRPA, RQRPA: V.Rodin, A. Faessler, F. Š., P. Vogel, Nucl. Phys. A 793, 213 (2007)
M. Wojcik,
Jagellonian
University:
GERDA status report
Shell model: E. Caurieratal.
Rev. Mod.
Phys. 77,
427 (2005).
Open questions
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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
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Investigation of neutrino-less double beta decay of
76Ge
Significant reduction of background around Q down
to 10-3 cts/(kgkeVy)
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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:
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O(1 ton) experiment in worldwide collaboration (cooperation
with Majorana)
M. Wojcik, Jagellonian University: GERDA status report
Collaboration
~70 physicists
13 institutions
6 countries
M. Wojcik, Jagellonian University: GERDA status report
Why
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Enrichment of 76Ge possible
Germanium semiconductor diodes
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76Ge?
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
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Phase I:
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Phase II:
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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
Adding new segmented diodes (total: ~40 kg 76Ge)
Demonstration of bkg-level <10-3 count/(kg·keV·y)
Phase III:
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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
phase
II
phase
I
Background reduction
is critical!
KKDC
claim
M. Wojcik, Jagellonian University: GERDA status report
GERDA design
Clean room
Lock
Water tank (650
m3 H2O)
Cryostat
(70 m3 LAr)
M. Wojcik, Jagellonian University: GERDA status report
Cryostat
Copper shield
Vacuum-insulated double
wall stainless steel cryostat
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
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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
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IGEX and HdM diodes were
removed from their cryostats
Dimensions were measured
Construction of dedicated lowmass 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
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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)
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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
Phase II detectors
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37.5 kg enrGe produced
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~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:
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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
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Requirements:
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Candidates:
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Low noise, low radioactivity, low power consumption,
operational at 87 K
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
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External background
- g 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
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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
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T1/2:
271 d
Decay
EC
Radiation X – 10,3 keV
68Ga
68 min
+(90%) EC(10%)
 – 2,9 MeV
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68Zn
stable
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
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60Ni
T1/2: 5.27 y
Decay: Radiation:  (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
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~ 3 m purified water (650 m3)
~ 4 cm SS cryostat + 2 walls
~ 3 cm Copper shield
~ 2 m LAr (70 m3)
Tl concentration
208Tl
< 1 mBq/kg
208Tl < 10 mBq/kg
208Tl ~ 10 µBq/kg
Tl ~ 0
Shielding and cooling with LAr is the best solution
‘reduce all impure material close to detectors as much
as possible’
 external g / n /  background < 0.001 cts/(keV·kg·y)
for LAr will be reached
M. Wojcik, Jagellonian University: GERDA status report
External background reduction
Counts/kg/keV/y
Muon-induced background: Prompt background
No -veto!
no cut: 10-2
Anticoincidence
(phase I): 10-3
goal
Segmentation
(phase II):
3·10-4
Energy (keV)
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
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produced from 76Ge by n-capture.
Reduction by delayed coincidence cut (muon, g-rays,
β-decay).
77,77mGe
Others
Bcg Index for LAr [cts/(kg·keV·y)]
1.1 · 10-4
5 · 10-5
M. Wojcik, Jagellonian University: GERDA status report
Liquid Argon purification
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Same principle as N2 purification
Initial 222Rn conc. in Ar higher than in N2
In gas phase achieved:
222Rn
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in Ar: <1 atom/4m3 (STP)
Even sufficient for GERDA phase III
Purification works also in liquid phase
(efficiency lower  more activated carbon
needed)
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
Summary
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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