1. 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

2. 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

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

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

5. 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

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. 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

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

12. 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

13. 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

14. 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

15. 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

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

17. 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

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. 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

21. 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

22. M. Wojcik, Jagellonian University: GERDA status report
Phase II detectors
37.5 kg enrGe produced
~87% 76Ge enrichment in form of GeO2
Chemical purity: % (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. 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

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

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  Ga  68Zn T1/2: d 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 = 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 PhD Co/(kg·d) Avinione et al.,
60Co  Ni
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 = 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 layer Tl concentration
~ 3 m purified water (650 m3) Tl < 1 mBq/kg
~ 4 cm SS cryostat + 2 walls Tl < 10 mBq/kg
~ 3 cm Copper shield Tl ~ 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 < cts/(keV·kg·y) for LAr will be reached
M. Wojcik, Jagellonian University: GERDA status report

30. 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

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).
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

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. 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