status of the gerda experiment · status of the gerda experiment ... a 366 (2006) 107-131 erratum:...
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September 14, 2007 H. Simgen, TAUP 2007 / Sendai 1
Status of the GERDA experiment
Hardy SimgenMax-Planck-Institut für Kernphysik
Heidelberg
on behalf of theGERDA collaboration
September 14, 2007 H. Simgen, TAUP 2007 / Sendai 2
Outline� Introduction and motivation� Goals and design of GERDA� Main hardware components of GERDA:
� Cryostat and water tank� Cleanroom and lock system� GERDA detector laboratory (GDL)
� Status of subprojects:� Detector preparation for phase I� Development of phase II detectors� Further running R&D programs
� Schedule and summary
September 14, 2007 H. Simgen, TAUP 2007 / Sendai 3
~70 physicists13 institutions
6 countries
The GERDA collaboration
September 14, 2007 H. Simgen, TAUP 2007 / Sendai 4
GERDA – A quick overview
� Next generation 76Ge double beta decay experiment at Gran Sasso
� Significant reduction of background around Qββ to ≤10-3 cts/(kg⋅keV⋅y)
� Contamination in previous experiments mainly in cryostat / diode holder
→ Bare diodes in cryogenic liquid (LAr)� Cryogenic liquids have very high radiopurity
September 14, 2007 H. Simgen, TAUP 2007 / Sendai 5
Why Germanium?
� Enrichment of 76Ge possible (natural abundance: 7.4%)
� Germanium semiconductor diodes� source = detector� excellent energy resolution� ultrapure material (monocrystal)
� long experience in low-level Germanium spectrometry
September 14, 2007 H. Simgen, TAUP 2007 / Sendai 6
Previous 76Ge 0νββνββνββνββ experimentsIGEX experiment:
C. Aalseth et al., Phys. Rev. D 65, 092007.T1/2 > 1.6 1025 y (90% C.L.)
Heidelberg-Moscow experiment:H.V.Klapdor-Kleingrothaus et al., Phys.Lett. B586 (2004) 198.T1/2 = (0.7 - 4.2) 1025 y (3 range)
2039 keV
Scrutinize claim with same & different isotopes!
September 14, 2007 H. Simgen, TAUP 2007 / Sendai 7
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 <1 count/(kg·keV·y)
� If KKDC-evidence not confirmed:� Goal: O(1 ton) experiment in worldwide
collaboration (cooperation with Majorana)
September 14, 2007 H. Simgen, TAUP 2007 / Sendai 8
GERDA sensitivity
assumed energy resolution:
∆E = 4 keV
Background reduction!!!
phase II
phase I KKDC claim
September 14, 2007 H. Simgen, TAUP 2007 / Sendai 9
0 50 100 150 2000
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
no background
y)⋅ keV⋅ counts /(kg−410
y)⋅ keV⋅ counts /(kg−310
y)⋅ keV⋅ counts /(kg−210
Claim
⋅Exposure [kg years]
90%
pro
b. u
pper
lim
it <
mee
> [
eV]
Assuming <M 0ν> = 3.92 [ Erratum: Nucl.Phys.A 766 (2006) 107 ]Inverted hierarchy range
GERDA sensitivity
using <M0ν>=3.92
V.A. Rodin at al., Nucl. Phys. A 366 (2006) 107-131
Erratum:Nucl. Phys. A 793 (2007) 213-215
phase I
phase II
KKDC claim
September 14, 2007 H. Simgen, TAUP 2007 / Sendai 10
GERDA design
Cleanroom Lock
Water tank (650 m3 H2O)
Cryostat (70 m3 LAr)
September 14, 2007 H. Simgen, TAUP 2007 / Sendai 11
GERDA design
Germanium-detectors
Vacuum-insulated double wall stainless steel cryostat
Additional inner copper shield
Liquid argon
September 14, 2007 H. Simgen, TAUP 2007 / Sendai 12
Gas purification for BOREXINO
N2 production rate: 100 m3/h
222Rn: <1 atom/4m3 (STP)
activated carbon
columns
September 14, 2007 H. Simgen, TAUP 2007 / Sendai 13
Argon purification from 222Rn� 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)
222Rn in Ar: <1 atom/4m3 (STP)
G. Zuzel: “Low-level techniques applied in the experiments looking for rare events”, Wed. 12.09, Solar neutrinos & low background techniques
September 14, 2007 H. Simgen, TAUP 2007 / Sendai 14
Stainless steel (SS) cryostat
� Ordered in Dec. 2006� 4 vessel heads produced� Welding certification in progress� Delivery: Beginning of 2008
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� SS contains U/Th/K-contaminations (and 60Co)
� Most dangerous: 208Tl (214Bi)� LAr (higher density than LN2)�
208Tl requirements of stainless steel (SS 1.4571) for� Vessel heads: <10 mBq/kg� Cylindrical part: <5 mBq/kg
Radioactivity of the SS cryostat
<5 mBq/kg
208Tl requirements
<10 mBq/kg
September 14, 2007 H. Simgen, TAUP 2007 / Sendai 16
Screening results of stainless steel samples (SS 1.4571)
cylin
dric
al
part
vess
el
head
s
September 14, 2007 H. Simgen, TAUP 2007 / Sendai 17
Water tank and muon veto
� Passive shield (reduces amount of LAr)
� Filled with ultrapure water� Equipped with 66 PMTs:
Cherenkov detector� Plastic scintillator on top� Construction has started
(bottom plate installation)
September 14, 2007 H. Simgen, TAUP 2007 / Sendai 18
Cleanroom on top of water vessel
September 14, 2007 H. Simgen, TAUP 2007 / Sendai 19
The lock system
September 14, 2007 H. Simgen, TAUP 2007 / Sendai 20
GERDA site at Gran Sasso
September 14, 2007 H. Simgen, TAUP 2007 / Sendai 21
Construction in hall A started
Water tank bottom plate (August 2007)
September 14, 2007 H. Simgen, TAUP 2007 / Sendai 22
GERDA detector lab (GDL) at Gran Sasso
September 14, 2007 H. Simgen, TAUP 2007 / Sendai 23
Enriched diodes for phase I
� In 2006 3 IGEX diodes and 5 HdMdiodes were removed from their cryostats
� Dimensions were measured
� Construction of dedicated low-mass holder for each diode
September 14, 2007 H. Simgen, TAUP 2007 / Sendai 24
Reprocessing of enriched and non-enriched diodes for phase I
� Different design of Hd-Moscow and IGEX diodes
� Reprocessing of all diodes at manufacturer
� Underground storage in between� 17.9 kg enriched and 15 kg non-
enriched crystals under processing
September 14, 2007 H. Simgen, TAUP 2007 / Sendai 25
Phase I prototype testing
� Low mass detector holder developed and tested
� Definition of detector handling protocol
� Optimization of thermal cyclings� >40 warming and cooling
cycles carried out � Passivation layer only
refurbished twiceSame performance in LN2/LAr
September 14, 2007 H. Simgen, TAUP 2007 / Sendai 26
Phase I prototype testing
� Study of leakage current (LC) with respect to� Detector handling procedure
� Irradiation with γ-sources
� Prototype detector continuously operated in LAr under varying irradiation conditions since Feb 07
� Present LC similar to initial value (few tens of pA)
September 14, 2007 H. Simgen, TAUP 2007 / Sendai 27
GERDA phase II
� September 2005: 37.5 kg enrGe produced� ~87% 76Ge enrichment� in form of GeO2
� Chemical purity: 99.95 % (not yet sufficient)
� Underground storage until further processing steps are defined
� Investigation of different options for crystal pulling
September 14, 2007 H. Simgen, TAUP 2007 / Sendai 28
Development of true-axial segmented detectors
� ββ-decay is single-site event, γ-background mostly multi-site event ⇒ Discrimination by segmentation
� Available detectors for testing:� 6-fold φ–segmented p-type crystal� two 18-fold (6φ, 3z) segmented
detectors (n- and p-type)
� 18-fold n-type preferred:� Segmentation easier� Thin outside dead layer
⇒ little loss of active mass
September 14, 2007 H. Simgen, TAUP 2007 / Sendai 29
Results obtained with 18-fold segmented n-type detector
E [keV]0 500 1000 1500 2000 2500 3000
dN/d
E
1
10
210
310
410
510All events
Single segment events
E [keV]1560 1580 1600 1620 1640 1660
dN/d
E
0
500
1000
1500
2000
2500All events
Single segment events
� Suppression of events from external 60Co and 228Th source (10 cm distance).
60Co 228Th
September 14, 2007 H. Simgen, TAUP 2007 / Sendai 30
Background reduction by LAr-scintillation
Energy [keV]0 500 1000 1500 2000 2500
-510
-410
-310
-210
-110
rate [hz] Ge signal
Ge signal with LAr veto
bkgd data
bkgd data with LAr veto
232Th-source
September 14, 2007 H. Simgen, TAUP 2007 / Sendai 31
Liquid argon scintillation –Work in progress
� Increase of photo-electron yield:� by fluor coating (1100 pe/MeV achieved)� by Xe doping
� Characterization of α, β, γ and neutron interactions by pulse shape analysis
� Preparation for LArGe in GDL @ Gran Sasso:� Study of LAr scintillation in ultralow-background
environment� operational beginning of 2008
September 14, 2007 H. Simgen, TAUP 2007 / Sendai 32
LArGe in GDL @ Gran Sasso
1
10
10 2
10 3
10 4
10 5
10 6
10 7
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
208Tl
MC example: Background suppression for contaminations located in detector support
3·10²
Factor 300 reduction in ROI
September 14, 2007 H. Simgen, TAUP 2007 / Sendai 33
LArGe @ Gran Sasso
September 14, 2007 H. Simgen, TAUP 2007 / Sendai 34
Front-end electronics
� Requirements:� Low noise, low radioactivity, low power
consumption, operational at 87 K
� Monolithic JFET semi-integrated CSA currently used for prototype testing
� 2 R&D programs for ASIC CMOS chips� Characterization and
testing ongoing
September 14, 2007 H. Simgen, TAUP 2007 / Sendai 35
Monte Carlo Simulations
� Joint Gerda/Majoranacode “MaGe” based on GEANT4
� Extensive physics validation program (most test setups are implemented).
September 14, 2007 H. Simgen, TAUP 2007 / Sendai 36
Muon-induced background I:Prompt background
Segmentation
(phase II): 3�10-4
Energy (keV)
Co
un
ts/k
g/k
eV
/y
goal
no cut: 10-2
No µ-veto!
Anticoincidence(phase I): 10-3
� 75% effective muon-veto is sufficient to achieve 10-4 counts/kg/keV/y
September 14, 2007 H. Simgen, TAUP 2007 / Sendai 37
Muon-induced background II:Delayed background
5 · 10-5Others
1.1 · 10-477,77mGe
Background in LAr [cts/(kg·keV·y)]
�77Ge produced from 76Ge by n-capture.
� Significant reduction possible by delayed coincidence cut (muon, γ-rays, β-decay).
September 14, 2007 H. Simgen, TAUP 2007 / Sendai 38
Schedule
� June 2007: GERDA safety concept officially approved by Gran Sasso
� Water tank installation started continued after cryostat delivery beginning of 2008
� Next: Construction of lab building, platform, cleanroom and lock (~1 year)
� Meanwhile: Prototype and enriched detector testing is going on
� Commissioning of GERDA ~14 months after cryostat delivery
September 14, 2007 H. Simgen, TAUP 2007 / Sendai 39
Summary� The challenge:
� Reduction of background by ~2 orders of magnitude wih respect to previous 76Ge experiments ⇒ Using bare diodes
� The status:� Construction of cryostat and water-tank started� Good understanding of bare detector handling� Reprocessing of existing enriched diodes almost finished� New 76Ge for phase II available� Different new background reduction strategies for phase II
and beyond under investigation
� The future:� Start data taking in 2009