Transcript Document

GERDA: GERmanium Detector Array
Searching for 02 decay
Xiang Liu
Max-Planck-Institut für Physik
1. 02 introduction
2. GERDA setup
& detectors
3. Low background
& high sensitivity
4. R&D
5. Conclusion
SNOW 2006, May 2-6th, 2006
Neutrino mixing & 02 decay
22 decay: (A,Z)  (A,Z+2) +2e+ 2
allowed by SM & exists.
02 decay: (A,Z)  (A,Z+2) +2e
if s Majorana & have mass. L=2
Many isotopes can be used
to search for 02.
Best limit from Heidelberg-Moscow
experiment (HdMo) with 76Ge.
Open questions:
Absolute mass scale?
Normal hierarchy or inversed?
Nature: Dirac or Majorana?
 02 decay might address all.
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Effective Majorana neutrino mass: m
1
T1  G
0
2
0
E 0 ,ZM
2
m
m (eV)
Search 02 & measure T1/2
Phase I
Phase II
2
Lightest neutrino mass (eV)
G0: phase space integral
M0: nuclear matrix element
<m>=  Vei2 mi
(Vei: neutrino mixing matrix)
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GERDA
Phase I: Existing detectors
Phase II: new detectors
Future: 1 ton experiment
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GERDA Setup
76Ge
02 rare process
Large amount of target mass  8% in nature,need enrichment (reduce bg)
High efficiency detecting signal  Detector = source, >95% signal effi.
Low background
 Excellent  (FWHM ~2 keV at 1.3 MeV) ,
high Q value (2.04 MeV)
 existing detectors & experiences
from HdMo & IGEX.
LNGS Hall A
GERDA new features:
(3800 m.w.e.)
Ge detectors directly submerged
Clean room
in LN2/LAr (as cooling & shielding)
& Lock
Segmented Ge detectors (phase-II)
Scintillator veto
(top of clean room)
Čerenkov veto
(PMTs in water tank)
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Ge detector array
LN2/LAr cryostat
Water tank
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Detectors & supports
(Phase II)
readout
cable
holder
HV cable
suspension
strings
Materials near detectors:
as little as possible,
as pure as possible.
Phase-I: HdMo & IGEX detectors, sum ~15kg, non-segmented
Phase-II:New detectors, sum ~25kg, 18 segments (3z6)
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Status of Existing Phase-I Detectors
IGEX detectors testedinside Vacuum storage Can.
FWHM 2.2 - 2.3keV at 1332keV
HdMo detectors stored at LNGS, all tested.
One HdMo detector taken out from Vacuum Can for testing.
HdMo
Setup
HdMo
claim
Signal: 28.75  6.86 events
Bg level: 0.11 cts/[keV kg year]
ANG1 Ge crystal
If claim true, Phase-I will have
Signal: ~13 events
Bg level: <0.01 cts/[keV kg year]
<3 events in 10keV window
at Q value.
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Status of Phase-II Detectors
37.5kg Ge, 87.25% 76Ge, purity >99.9% (in GeO2) enriched in Russia,
transported to MPI Munich on April 3rd 2006,
now stored underground (200m.w.e)
Next steps: purification, crystal pulling & detector fabrication.
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High sensitivity = Low background
Exposure [kg year]
bg. Level [keV kg year]
Limit on m [ev]
Phase I
30
10-2
0.5
Phase II
100
10-3
0.2
a)
b)
Phase II
Phase I
a) Assessment of uncertainties in QRPA 02-decay nuclear matrix elements
V.A. Rodin, A. Faessler, F. Simkovic, P. Vogel [Nucl-th 0503063]
b) Search for Neutrinoless Double Beta Decay with Enriched 76Ge in Gran Sasso 1990-2003,
H.V. Klapdor-Kleingrothaus, I.V. Krivosheina, A. Dietz and O. Chkvorets, Phys. Lett. B586 (2004) 198-212
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Use segments to reject background
02 signal:
2 electrons deposit energy very localized (<1mm)
Background :
Mostly Compton scattering 2~3 cm mean free path
(mostly multi-segment)
Source
Reduction
factor
208Tl
(in Ge)
13
60Co
(in Ge)
38
68Ge
(in Ge)
18
( on Ge surface)
1
210Pb
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208Tl
(in holder)
5
60Co
(in holder)
157 (27)
208Tl
(in cable)
7
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MC simulation of bg.
Water tank
MC package (MaGe) for both Gerda & Majorana.
Upper limit on contamination levels for all
materials calculated.
Bg. Index calculated with available materials.
Cryo vessel
(Phase II)
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R&D: Ge detector properties
Several test facilities being built with detectors directly in LN2/LAr.
Both n & p-type Ge cycled
multi times in LN2 & LAr
Similar behavior in LN2/LAr.
No deterioration of performance.
6-segment p-type detector in LN2.
Co60
One 18-segment n-type detector is under study.
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Conclusion
Open questions in  : absolute mass? hierarchy? Majorana or Dirac?
02 might answer all these questions.
GERDA searches for 02 decay in 76Ge.
Ge detectors are submerged directly in LN2/LAr (cooling & shielding).
IGEX & HdMo detectors for Phase-I.
Segmented Ge detectors for Phase-II.
Goal for bg. rate: 10-3 cts/[keV kg year] (Phase-II), difficult but feasible.
With this bg. rate, sensitivity of 0.2eV on m expected.
Several test facilities for GERDA are being built.
Both n- & p-type detectors cycled in LN2/LAr without obvious deterioration.
Once the GERDA concept is proven, global experiment with 1t Ge might be
pursued (MoU with Majorana exists).
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