The EDELWEISS-II experiment
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Transcript of The EDELWEISS-II experiment
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The EDELWEISS-II experiment
Silvia SCORZA Université Claude Bernard- Institut de Physique nucléaire de Lyon
CEA-Saclay DAPNIA/DRECAM (FRANCE), CNRS/CRTBT Grenoble (FRANCE),CNRS/IN2P3/CSNSM Orsay (FRANCE), CNRS/IN2P3/IPN Lyon (FRANCE), CNRS/INSU/IAP Paris (FRANCE), CNRS-CEA/Laboratoire Souterrain de Modane (FRANCE), JINR Dubna (RUSSIA), FZK/Universtat Karlsruhe (GERMANY)
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Direct Search PrincipleDetection of the
energy deposited due to elastic scattering off target nuclei
• Event Rate : < 1 ev /kg/week• Recoil Energy : 1 – 100 keV
• Low energy threshold• Large detector mass• Low background Radio – purity Active/passive shielding Deep underground sites
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EDELWEISS @ LSM
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EDW–II set-up Radiopurity
dedicated HPGe detectors for systematic checks of all materials
Strict control of bkg: material selection/cleaning procedure/Environment
Shielding 20 cm Pb shielding 50 cm PE and better coverage active μ veto (> 98% coverage)
Up to 110 detectors -> 40kg Ge detector
Ge/NTD Develop new ID detectors
Goal: EDW-I × 100 σW-n ≤ 10-8 pb<0.003 evts/kg/day (Er>10keV)
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µ-veto
Candidates for coincident veto-bolometer events
Accidental coincidences
Expected from Geant4 simulation:
~0.03 events/kg.dMeasured:
0.04 events/kg.d
Pb
Polyethylene
Veto
roc
k
p,π,α
´
n Cu(Cryostat)
Ge
<300GeV>
nVeto
Pb
Gd-loaded scintillator
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Ge Heat-Ionization Detectors
• Simultaneous measurements:• Ionization @ few V/cm with
Al electrodes• Heat @ 20 mK with Ge/NTD
sensor
• Different Ionization/Heat energy ratio for nuclear and electronic recoils (dominate bkg)
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Event by event background rejection
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Edw-I limiting backgroundPRD71, 122002 (2005)
Problem:Surface electron recoils
Interpretation: Bad charge collection
(trapping and recombination) Indications of 210Pb
contamination (exposition to Radon):
α rate ~ e rate ~ 4 /kg.day
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206Pbβ
210Pb
210Po
210Po
210Pb
210Pb source
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GeNTD data: improved bkg Gamma background:
reduction of x3 relative to EDELWEISS-I
210Pb-chain background: reduction of x2 relative to EDELWEISS-I
EDW-I~4 /kgd
EDW-II~2 /kgd
5 kgd
95 kgd
preliminary
Full volume
Further bkg reductions after fiducial+ coincidence cuts
Conclusion: Reduction of background from EDW-I to EDW-II
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Physics run: GeNTD• 11 detectors with <30 keV
threshold• Threshold chosen before
start of run(EDW-I results expected
bkg)• 93.5 kg.day• 3 events observed in nuclear
recoil band• 31, 31 and 42 keV
• Evidence for events with deficient charge collection from 210Pb
Preliminary
EDELWEISS93.5 kgd
Question:How to reach < 10-8 pb ?
NEED >1000 kgd at 15 keV threshold
>105 rejection for gammas to reject expected >4000 from 210Pb
IDea:Develop detectors with surface event rejection
using interleaved electrode design (ID)
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A A A A AB B B B
C C C C CD D D D
G
H
GuardFiducial volume
→E
→E
→E
→E
‘A’ electrodes: +2V
ID detector
Radius (cm)
Z (c
m)
guard ‘G’: + 1V
guard ‘H’: - 1V
‘C’ electrodes:-2V ‘D’ electrodes: -1V
Electron trajectoriesholetrajectories
‘A&C’ bulk event
‘A&B’ near- surface event
‘A,B&C’ event in low-field area
NTD heat sensor• E-field modified near surface with interleaved electrodes• B+D signals -> vetos % surface• 1x200g + 3x400g tested in2008• 10x400g running‘B’ electrodes:
+1V
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ID detector rejection Gamma rejection of 400g
~1 month calibrations Beta rejection of 200g
-equivalentto 3x104 kgd
6x104
6x104210Bi
6x104210Po
0 events
-equivalentto ~103 kgd
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Physics run: ID detectors 1 x 400g + 1 x 350g
detectors, 86 live days
<15 keV threshold achieved for exposure of 18.6 kg.days
50% efficiency at 10 keV
No events in (or around) nuclear
recoil band
Conclusion: This is the good technology for 10-8 pb and beyond
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Limits 93.5 kgd GeNTD
• 11 detectors x 4 months• 30 keV threshold• 3 events observed in
nuclear recoil band 18.3 kgd ID
• 2 detectors x 4 months• 15 keV threshold• No nuclear recoils• No evts outside band
2009: 10 ID detectors• improvement in
sensitivity: 4x10-8 pb• More detectors build in
2009
Preliminary
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Conclusions/Outlook• Significant reduction in , β and γ backgrounds relative to
EDELWEISS-I• Improved understanding of backgrounds and of response of
detectors to backgrounds• Improved limit relative to EDELWEISS-I• Passive background reduction alone not sufficient to reach
< 10-8 pb• ID detectors have the surface rejection needed to reach this
goal Running in 2009 with 10 x 400g detectors Prototype of ID detectors with larger fiducial volumes currently in test EURECA = 1 ton scale experiment (CRESST, EDELWEISS, CERN, …) @ LSM extension
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This is the end…
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Decay Chain
210Po
206Pb
5.3 MeV100 keV
Al 70nm
amGe 70nm
Ge 2cm
206Pb
e-
10keV - 100keV - 1MeV
Cu few mm
5 mm
~50 nm
350 nm
20 m
700 m
210Bie- 1.16 MeV max
210Pbe- 61 keV max
46.5 keV (4%)
46 keV
3 mm
NO ionisation
22 yr
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EDELWEISS-II 210Pb source calibration
Confirms interpretation of EDW-I bkg as 210Pb surface . Response of detectors to this important background
210Pb
206Pb
β 210Po
E=5.3 MeVQ~0.3
Edw-Idata
EDELWEISS-II210Pb source
210Pb
210Po
206Pb recoilscoincident with 210Po
210Bi
210Bi
Edw-II
Edw-II
Edw-II
Implantation depth of 210Pb
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2020
EURECA - II
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EURECA-I
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France-Italy Fréjus tunnel :new safety gallery planned
Existing road tunnel
Existing lab
s ?
2 projectsLimited extensionVery large cavity
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2424G Gerbier EURECA ULISSE meeting- Lyon july 2008
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Implantation of new lab
First drawings nov 2006 (Lombardi eng company)
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Compatibility DAMA other experiments SI
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Compatibility DAMA other experiments SD
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Present limits
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Number of events in WIMP region: q< 0.5, 25 < E < 60 кэВregistered23 events
Experimental data
Alpha rate 2.5 events
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N alphas in calibration spectrum = 3040 eventsNumber of events in WIMP region: q< 0.5, 25 < E < 60 кэВregistered = 460 events
460 events
Expected number events from calibration with 210Pb =37 events
Estimation of background from 210Pb