1. EDELWEISS-II : Status and future
Véronique SANGLARD
CNRS/IN2P3/IPNLyon

2. Outline Context EDELWEISS-I EDELWEISS-II Conclusion
Run with heat trigger
Final results
Background studies
EDELWEISS-II
Improvements
Present and future runs
Conclusion
Dark Matter 2006 – Marina del Rey, California – 02/24/ « EDELWEISS-II : Status and future » Véronique SANGLARD

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

4. Heat and Ionization Ge detectors
Center electrode
Guard ring
7 cm
Reference electrode
Thermometer
(Ge NTD)
Fiducial volume(≈ 57%)
Ge crystal
Guard
Electrodes
Center electrode
Ionization
guard
Simultaneous measurement of
17 mK with Ge/NTD sensor
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 recoils
EI/ER = 1 for electronic recoils
m=320g
Ionization
center
Ionization
threshold
Neutrons 73Ge(n,n',γ) Gammas
Dark Matter 2006 – Marina del Rey, California – 02/24/ « EDELWEISS-II : Status and future » Véronique SANGLARD

5. 2003 data taking with heat trigger
Several runs between 2000 and 2003
Last run = data taking with trigger on heat signal
Improved efficiency at low energy (50 % at 11 keV)
Stable behavior over 4 months of the detectors
Fiducial exposure: 22 kg.d
18 nuclear recoil candidates > 15 keV
Possible backgrounds
Residual neutron flux
1 n-n coincidence observed
2 single expected by MC
Surface electron recoils
Miscollected charge events at low energy
Leak of events down to the nuclear recoil band not visible in coincidence events
Further, studies concerning the possible origins for these backgrounds
Dark Matter 2006 – Marina del Rey, California – 02/24/ « EDELWEISS-II : Status and future » Véronique SANGLARD

6. Final results of EDELWEISS-I (1)
Final results: 62 kg.d (fid. exp.)
Overall 15 keV = 50%
40 nuclear recoil candidates > 15 keV
Only 3 with 30 <ER< 100 keV  strong constraint on an eventual WIMP rate
Unknown background
Used method developed by S. Yellin to derive exclusion limits (as CDMS) *(PRD 66, (2002))
No background subtraction
V.Sanglard et al., PRD 71, (2005)
Experiment stopped in March 2004
Dark Matter 2006 – Marina del Rey, California – 02/24/ « EDELWEISS-II : Status and future » Véronique SANGLARD

7. Exclusion limit – Spin Independent
New exclusion consistent with our previous limits
Best sensitivity : 1.5x GeV/c²
EDELWEISS starts to explore some optimistic SUSY models
EDELWEISS-II
EURECA
Need a gain in sensitivity of a factor 100 – (EDW-II, EURECA*) to explore more interesting SUSY models
This gain depends on improvements
On background discrimination
On mass
(* See Hans Krauss’s talk)
Dark Matter 2006 – Marina del Rey, California – 02/24/ « EDELWEISS-II : Status and future » Véronique SANGLARD

8. Exclusion limit – Spin Dependent
In some models, the spin dependent interactions dominate
7.8 % of 73Ge (J=9/2) in natural Ge
Spin 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
Optimisitic SUSY models are a factor 100 below
Direct detection experiments not yet competitive with indirect detection (SK)
Dark Matter 2006 – Marina del Rey, California – 02/24/ « EDELWEISS-II : Status and future » Véronique SANGLARD

9. g-ray background in EDELWEISS-I
Before nuclear recoil selection, rate in detectors is ~1.5 event/kg/day/keV at low energy
At high energy, spectrum shape and rate consistent with simulations of the measured U/Th contamination in the bulk of the Cu shielding
Room for improvement in EDELWEISS-II, where this Cu is not used
Dark Matter 2006 – Marina del Rey, California – 02/24/ « EDELWEISS-II : Status and future » Véronique SANGLARD

10. Neutron background in EDELWEISS-I
Recent estimate of the neutron flux in the lab with E >1 MeV :  0.1(stat) 10-6 n/cm²/s
Good 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.d
Not a strong constraint on the single neutron rate in the data
Expected ratio double/single ~1/10
1 n-n  1 – 40 singles with ER>15 90% C.L.
Indistinguishable from the miscollected events in the nuclear recoil band
Dark Matter 2006 – Marina del Rey, California – 02/24/ « EDELWEISS-II : Status and future » Véronique SANGLARD

11. Surface backgrounds in EDELWEISS-I
Peak at E=5.3 MeV
a’s from 210Po?
Q=0.3  a decays near surfaces
Rate ~ 400 /m²/d
As expected, non-fiducial part more exposed to a flux
Very likely due to 210Pb on Cu or Ge surfaces
No 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 b’s expected in 210Pb hypothesis
(but does not exclude possible contribution from 14C)
By removing Cu covers between detectors
Events should disappear
Better identification by coincidences
E = 5.3 MeV, Q = 0.3 1
Dark Matter 2006 – Marina del Rey, California – 02/24/ « EDELWEISS-II : Status and future » Véronique SANGLARD

12. EDELWEISS-II in few words
Installation in the LSM started summer 2004
1rst 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 2006
Commissioning run with 8 bolometers
New 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)
Dark Matter 2006 – Marina del Rey, California – 02/24/ « EDELWEISS-II : Status and future » Véronique SANGLARD

13. EDELWEISS-II improvements – Cryostat
Reversed geometry
Nitrogen free : 3 Pulse tube (50K and 80K screens) and 1 He cold vapor reliquefier (consumption  0)
Large volume 50l  Self shielding
Up to  120 detectors  More statistics
Compact and hexagonal arrangement  More coincidence (n bkg)
Dark Matter 2006 – Marina del Rey, California – 02/24/ « EDELWEISS-II : Status and future » Véronique SANGLARD

14. EDELWEISS-II improvements – Backgrounds
Radiopurity
Dedicated HPGe detectors for Systematic checks off all materials
Clean Room (class 100 around the cryostat, class for the full shielding
Deradonized air (from NEMO3) (0.1 Bq/kg)
20 cm Pb shielding
Neutron Shielding
EDW-I : 30cm paraffin
EDW-II : 50 cm PE and better coverage
µ veto (99% coverage)
Dark Matter 2006 – Marina del Rey, California – 02/24/ « EDELWEISS-II : Status and future » Véronique SANGLARD

15. EDELWEISS-II improvements – Ge/NTD detectors
Developed by CEA Saclay and Camberra-Eurisys
Amorphous Ge and Si sublayer (better charge collection for surface events)
Optimized NTD size and homogeneous working T (16-18 mK) : sub keV resolution
New holder and connectors (Teflon and copper only)
Dark Matter 2006 – Marina del Rey, California – 02/24/ « EDELWEISS-II : Status and future » Véronique SANGLARD

16. EDELWEISS-II improvements – surface events
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, regeneration
Physics of the Ge and Si amorphous sublayer
Detectors with thick electrodes
Active rejection : identification of the surface events
Pulse 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
Dark Matter 2006 – Marina del Rey, California – 02/24/ « EDELWEISS-II : Status and future » Véronique SANGLARD

17. Identification of surface events with Ge/NbSi detector
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
Tests with 200g prototype in EDELWEISS-I
Improvement of a factor 20 of the rejection
Fiducial volume reduction of 10 %
Before rejection
After rejection (1mm cut)
Dark Matter 2006 – Marina del Rey, California – 02/24/ « EDELWEISS-II : Status and future » Véronique SANGLARD

18. EDELWEISS-II present status
Commissioning run with 8 bolometers :
2*320g Ge/NTD with EDW-I holder
2*320g Ge/NTD with EDW-II holder and teflon clamp
2*320g Ge/NTD with EDW-II holder and Cu springs
1*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…
Dark Matter 2006 – Marina del Rey, California – 02/24/ « EDELWEISS-II : Status and future » Véronique SANGLARD

19. EDELWEISS-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 : evt/kg/year
EURECA : 10 evt/kg/year
Dark Matter 2006 – Marina del Rey, California – 02/24/ « EDELWEISS-II : Status and future » Véronique SANGLARD