EDELWEISS-II : Status and future Véronique SANGLARD CNRS/IN2P3/IPNLyon [email protected]

EDELWEISS-II : EDELWEISS-II : Status and futureStatus and future

Véronique SANGLARDCNRS/IN2P3/IPNLyon

[email protected]://edelweiss.in2p3.fr

mailto:[email protected]





Dark Matter 2006 – Marina del Rey, California – 02/24/06 « EDELWEISS-II : Status and future » Véronique SANGLARD

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OutlineOutline

ContextEDELWEISS-I

Run with heat triggerFinal resultsBackground studies

EDELWEISS-IIImprovementsPresent and future runs

Conclusion


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The EDELWEISS CollaborationThe EDELWEISS Collaboration

CEA-Saclay DAPNIA/DRECAM

CNRS/CRTBT Grenoble

CNRS/IN2P3/CSNSM Orsay

DUBNA (Russia)

FZK/Univ. Karlsruhe (Deutschland)

CNRS/INSU/IAP Paris

CNRS/IN2P3/IPN Lyon

CNRS – CEA/Laboratoire Souterrain de Modane

1700 m depth under the Fréjus tunnel (4800 we) 4 µ/m²/d (106 less than at the surface)

*Expérience pour DEtecter Les WIMPs En SIte Souterrain (Underground experiment to detect WIMP)


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Heat and Ionization Ge detectorsHeat and Ionization Ge detectors

Simultaneous measurement of

Heat @ 17 mK with Ge/NTD sensorIonization @ few V/cm with Al electrodes

Different charge/heat ratio for nuclear and electron recoils (WIMP and neutron have lower charge than γs, βs )Discrimination event-by-event of electron recoils (main background)

EI/ER = 0.3 for nuclear recoilsEI/ER = 1 for electronic recoils Neutrons 73Ge(n,n',γ) Gammas

Fiducial volume(≈ 57%)

Heat

Ionizationguard

Ionization

center

Thermometer(Ge NTD)

Reference electrode

Center electrodeGuard

Electrodes

Ge crystal

Center electrodeGuard ring

7 cmm=320g

Ionization threshold


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2003 data taking with heat trigger2003 data taking with heat triggerSeveral runs between 2000 and 2003Last run = data taking with trigger on heat signalImproved efficiency at low energy (50 % at 11 keV)Stable behavior over 4 months of the detectorsFiducial exposure: 22 kg.d 18 nuclear recoil candidates > 15 keV

Possible backgroundsResidual neutron flux

1 n-n coincidence observed 2 single expected by MC

Surface electron recoilsMiscollected charge events at low energyLeak of events down to the nuclear recoil band not visible in coincidence events

Further, studies concerning the possible origins for these backgrounds


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Final results of EDELWEISS-I (1)Final results of EDELWEISS-I (1)

Final results: 62 kg.d (fid. exp.)Overall efficiency @ 15 keV = 50%40 nuclear recoil candidates > 15 keVOnly 3 with 30 <ER< 100 keV strong constraint on an eventual WIMP rateUnknown background

Used method developed by S. Yellin to derive exclusion limits (as CDMS) *(PRD 66,032005 (2002))

No background subtraction

V.Sanglard et al., PRD 71,122002 (2005)Experiment stopped in March 2004


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Exclusion limit – Spin IndependentExclusion limit – Spin IndependentNew exclusion consistent with our previous limitsBest sensitivity : 1.5x10-6 pb @ 80 GeV/c² EDELWEISS starts to explore some optimistic SUSY models

Need a gain in sensitivity of a factor 100 – 10000 (EDW-II, EURECA*) to explore more interesting SUSY modelsThis gain depends on improvements

On background discriminationOn mass

EDELWEISS-II

EURECA

(* See Hans Krauss’s talk)


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Exclusion limit – Spin DependentExclusion limit – Spin DependentIn some models, the spin dependent interactions dominate7.8 % of 73Ge (J=9/2) in natural GeSpin on neutron or proton

results present in (ap, an) diagram

ap, an = effective coupling between WIMP and proton, neutron

For each WIMP mass, the constraint is 22

2

limlim 24 pFA

n

n

Ap

p

G

aa

Optimisitic SUSY models are a factor 100 belowDirect detection experiments not yet competitive with indirect detection (SK)


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-ray background in EDELWEISS-I-ray background in EDELWEISS-I

Before nuclear recoil selection, rate in detectors is ~1.5 event/kg/day/keV at low energyAt high energy, spectrum shape and rate consistent with simulations of the measured U/Th contamination in the bulk of the Cu shieldingRoom for improvement in EDELWEISS-II, where this Cu is not used


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Neutron background in EDELWEISS-INeutron background in EDELWEISS-I

Recent estimate of the neutron flux in the lab with E >1 MeV : 1.1 0.1(stat) 10-6 n/cm²/sGood understanding of neutron propagation in the setup (agreement between simulated and experimental spectrum for a neutron calibration) Determination of single rate : ~2 nuclear recoil expected in 62 kg.d (ambient radioactivity + U contamination of copper and lead) 1 n-n coincidence observed in 62 kg.dNot a strong constraint on the single neutron rate in the data

Expected ratio double/single ~1/101 n-n 1 – 40 singles with ER>15 keV @ 90% C.L.Indistinguishable from the miscollected events in the nuclear recoil band


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Surface backgrounds in EDELWEISS-ISurface backgrounds in EDELWEISS-I

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E = 5.3 MeV, Q = 0.3

Peak at E=5.3 MeV’s from 210Po?Q=0.3 decays near surfacesRate ~ 400 /m²/dAs expected, non-fiducial part more exposed to flux

Very likely due to 210Pb on Cu or Ge surfacesNo 206Pb recoil peak at 100 keV observed as heat-only events : 210Pb implanted in Cu, not Ge.Rate of 0.3<Q<1.0 events at low energy consistent with surface ’s expected in 210Pb hypothesis

(but does not exclude possible contribution from 14C)

By removing Cu covers between detectors

Events should disappearBetter identification by coincidences


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EDELWEISS-II in few wordsEDELWEISS-II in few wordsInstallation in the LSM started summer 20041rst funded stage : 28 detectors

21*320g optimized Ge/NTD detectors and holders 7*400g Ge/NbSi detectors with active surface events rejections

First cryogenic test with bolometers in january 2006Commissioning run with 8 bolometersNew electronic and acquisition systems: square modulation, continuous digitization close to the readout, optical fibers and numerical trigger

Goal *100 in sensitivity : w-n 10-8 pb

0.002 evt/kg/day (ER>10keV)


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EDELWEISS-II improvements – EDELWEISS-II improvements – Cryostat Cryostat

Reversed geometryNitrogen free : 3 Pulse tube (50K and 80K screens) and 1 He cold vapor reliquefier (consumption 0)Large volume 50l Self shieldingUp to 120 detectors More statisticsCompact and hexagonal arrangement More coincidence (n bkg)


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EDELWEISS-II improvements – BackgroundsEDELWEISS-II improvements – Backgrounds

Radiopurity Dedicated HPGe detectors for Systematic checks off all materialsClean Room (class 100 around the cryostat, class 10000 for the full shieldingDeradonized air (from NEMO3) (0.1 Bq/kg)

20 cm Pb shieldingNeutron Shielding

EDW-I : 30cm paraffinEDW-II : 50 cm PE and better coverageµ veto (99% coverage)


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EDELWEISS-II improvements – Ge/NTD EDELWEISS-II improvements – Ge/NTD detectorsdetectors

Developed by CEA Saclay and Camberra-EurisysAmorphous Ge and Si sublayer (better charge collection for surface events)Optimized NTD size and homogeneous working T (16-18 mK) : sub keV resolutionNew holder and connectors (Teflon and copper only)


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EDELWEISS-II improvements – surface EDELWEISS-II improvements – surface eventsevents

Important limitation to the sensitivity : due to diffusion, trapping and recombination, surface events are miscollected and can mimic nuclear recoils

2 approaches :Passive rejection : improve the charge collection for surface event

Physics of the charge collection, trapping, surface charge, regenerationPhysics of the Ge and Si amorphous sublayerDetectors with thick electrodes

Active rejection : identification of the surface eventsPulse shape analysis of the charge signals (but high bandwidth low noise) localization of the event Interdigitzed electrodes Detectors sensitive to athermal phonons Ge/NbSi detectors


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Identification of surface events with Ge/NbSi Identification of surface events with Ge/NbSi detectordetector

Identification of near surface events using athermal phonon measurement with NbSi thin film thermometers

Heat and ionization Ge detectors

Each signal = thermal + athermal component

For surface events, athermal higher in NbSia

Thermal signals proportionnal to the deposited energy

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H e a t s i g n a l o f t h e r m o m e t e r N b S i A :S u r f a c e e v e n t B u l k e v e n t

T h e r m o m e t e r T h e r m o m e t e r N b S iN b S i BB

T h e r m o m e t e r T h e r m o m e t e r N b S i N b S i AA T h e r m o m e t e r T h e r m o m e t e r N b S i N b S i AA


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T i m e ( m s )

Before rejection After rejection (1mm cut)

Tests with 200g prototype in EDELWEISS-I

Improvement of a factor 20 of the rejectionFiducial volume reduction of 10 %


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EDELWEISS-II present statusEDELWEISS-II present status

Commissioning run with 8 bolometers :

2*320g Ge/NTD with EDW-I holder2*320g Ge/NTD with EDW-II holder and teflon clamp2*320g Ge/NTD with EDW-II holder and Cu springs1*IAS 50g heat and light detector (Al2O3)1*200g Ge/NbSi

Goals : Validation of the microphonics (pulse tube decoupling system), new holders and new comb connectors for Ge/ntd, new electronics scheme, new acquisition system…


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EDELWEISS-II futureEDELWEISS-II future

Feb-march 2006 : continue present run = commissioning

April 2006 : EDELWEISS-II with 28 detectors (21 Ge/NTD and 7 Ge/NbSi) + IAS detector (heat and light)

2006 : Approval of the 2nd stage : 120 detectors

In term of sensibility :EDELWEISS-II : 1 evt/kg/yearEURECA : 10 evt/kg/year

Documents

EDELWEISS-II : Status and future Véronique SANGLARD CNRS/IN2P3/IPNLyon [email protected]