Edelweiss-II : status and first results A new generation of background-free bolometers for WIMP search Eric Armengaud - CEA / IRFU 6 June - SUSY 09 1
Direct search for WIMPs WIMPs forming our Galactic halo • 10 - 1000 GeV mass • v ~ 200 km/s ⇒ 10-50 keV nuclear recoils (~ exponential spectrum) SUSY neutralino ( σ prediction) • 1 collision / kg / month or 1 / ton / yr… astro-ph/08054705 Detector challenges : KIMS EDELWEISS-I CRESST-II • Low energy threshold XENON • Large mass CDMS-II • Low background 2
Backgrounds in direct detection experiments Backgrounds of importance: γ radioactivity (electron recoils) β radioactivity ( surface electron recoils) Neutron scatterings (nuclear recoils) « Philosophy » : Difference with accelerator physics : large uncertainties on background, detector imperfection effects Probing ultra-low background levels ( ! tails of distributions ! ) Strategy: as soon a background is “sufficiently” understood, it pays more to redesign and rebuild the detector to get rid of it Passive rejection (eg. underground site & shieldings) Active rejection (eg. discrimination between nuclear and electronic recoils) 3
Heat-ionization sensors « Edelweiss-I » detectors Ionization measurement @ few V/cm Heat measurement (NTD sensor) @ 20 mK Discriminating variable between electronic and nuclear recoils : « Q » ~ ionization/heat 4
The Edelweiss-II setup Operated at the Underground Laboratory of Modane (4 μ /day/m 2 ) Cryogenic installation (18 mK) : Reversed geometry cryostat, pulse tubes One of the coldest places in the Universe Shieldings : Clean room + deradonized air Active muon veto (>98% coverage) PE shield Lead shield ⇒ γ background reduced by ~2 wrt EDW1 Facilities : Remotely controlled sources for calibrations + regenerations Detector storage & repair within the clean room 12 cool-downs already operated 5
Neutron rejection : the muon veto spectra • Interactions in detectors due to muon- induced neutrons inside the shields : • Geant4 - expected : ~0.03 evts / kg.d • Mostly nuclear recoils below 50 keV • Measured bolometer - muon veto coincidence rate : ~ 0.04 evts/kg.d Ionization/recoil ratio • The ionization yield distribution of coincidences is consistent with muon- induced events • In addition: several neutron flux 2007-2008 data (280 kg.d - no cut) measurements carried out near the experiment 6
The issue of surface interactions Beta radioactivity at the surface of detectors • Incomplete charge collection • « Leakage » of such events down to the nuclear recoil region EDELWEISS-II 93.5 kgd 210 Pb Preliminary 210 Po β 206 Pb α EDW-II 2008 data : • 11 detectors with < 30keV thresholds • 94 kg.d after cuts in 4 months • Threshold chosen a priori • 3 events in nuclear recoil band • Beta bkg reduced wrt EDW-I but still insufficient to reach 10 -8 pb 7
Surface event rejection with phonon measurements Early EDW « NbSi » R&D The CDMS way • 2 NbSi films measuring athermal • Athermal phonon measurement phonons + ionization signals (transition-edge sensors on one side • Surface event rejection correct of the cristal) • Pbs of threshold / reproducibility • Measurement of phonon risetime & delay ⇒ surface e- rejected Thermometer NbSi Thermometer NbSi Thermometer NbSi NbSi A NbSi A NbSi A A A A Thermometer NbSi Thermometer NbSi Thermometer NbSi Thermometer Thermometer Thermometer NbSi A NbSi A NbSi A A A A Thermometer Thermometer Thermometer Thermometer NbSi Thermometer Thermometer Thermometer NbSi Thermometer Thermometer NbSi NbSi B NbSi B NbSi B B B B Thermometer NbSi Thermometer Thermometer NbSi Thermometer Thermometer NbSi Thermometer NbSi B NbSi B NbSi B B B B Heat signal of thermometer NbSiA : Heat signal of thermometer NbSiA : Heat signal of thermometer NbSiA : 200 200 200 200 200 200 Surface event Surface event Surface event Bulk event Bulk event Bulk event 150 150 150 150 150 150 100 100 100 Transient Transient Transient 100 100 100 Thermal Thermal Thermal 50 50 50 50 50 50 0 0 0 0 0 0 0 0 0 0 0 0 20 20 20 20 20 20 40 40 40 40 40 40 60 60 60 60 60 60 80 80 80 80 80 80 -3 -3 -3 -3 -3 -3 x10 x10 x10 x10 x10 x10 Time (ms) Time (ms) 8
Rejecting surface events with interleaved electrodes the « ID » (interdigit) detector ‘a’ electrodes (+4V) collecting ‘b’ electrodes (-1.5V) field shapping fiducial volume • Keep the EDW-I NTD phonon detector ‘c’ (-4V) ‘d’ (+1.5V) • Modify the E field near the surfaces with interleaved electrodes First detector built 2007 • Use ‘b’ and ‘d’ signals as 1x200g + 3x400g tested in 2008 vetos against surface events 10x400g running since beginning 2009 9
An outstanding surface event discrimination with IDs E1 = energy of top collecting electrode « single-side » surface events : E1=0 E2 = bottom collecting electrode Beta calibration « 3-electrode » surface events ( 210 Po) 46 keV gamma-ray line 200g detector fiducial volume events : E1 = E2 • A combination of cuts is made on the 4 signals of electrodes • Example presented here : cut on the difference of signals between 2 collecting electrodes • Surface and volume events are completely separated ! PRELIMINARY • From β calibration : overall rejection ~ 1/10 5 2 10
ID fiducial volume 400g detector, background run Cosmogenic lines: - 68 Ge and 65 Zn isotope lines at ~10keV - Background electron recoil events - Homogeneously distributed in the volume of the cristal • Fiducial volume measurement ~ 160g, primarily limited by the Fiducial volume cut guard regions • Result confirmed with neutron measurements 11
Ionization resolution of IDs • Ionization resolution Background ionization spectrum, ID3+ID401 important to get a After fiducial cuts good recoil threshold 10.37 keV • Approx. ~ 20 kg.d σ ion = 300 eV 8.98 keV of background data with two 400g detectors (2008 data) • Background dominated by the PRELIMINARY cosmogenic lines at ~10keV • Good and stable energy resolution 12
IDs : overall background rejection performances arXiv:0905.0753 γ -equivalent to ~10 3 kgd β -equivalent 0 events to 3x10 4 kgd 13
Physics run with IDs cross section Background free ! WIMP mass (GeV) 2008 : • 86 live days / 4 months / 2x400g detectors • 18.3 kg.d with < 15 keV threshold, ~50% efficiency at 10keV 2009 : physics run ongoing • 10 detectors running • x 20 improvement in 8 months : 4x10 -8 pb • Additional new detectors with increased fiducial volume (« FIDs ») 14
The future : EURECA EURECA: beyond 10 -9 pb, major efforts in background control and detector development Joint effort from teams from EDELWEISS, CRESST, ROSEBUD, CERN, +others… >>100 kg cryogenic experiment, multi-target Part of ILIAS/ASPERA European Roadmap Prefered site: 60 000 m 2 extension of present LSM (4 µ /m 2 /d), to be dig in 2011-2012 15
Edelweiss summary Edelweiss-II now aiming at ~ 10 -8 pb : Neutron shield and muon veto designed for < 10 -8 New generation of « ID » detectors have proven Robust fabrication and operation Thresholds <15 keV Excellent electron recoil rejection and surface event rejection (~1/10 5 ) Good fiducial volume fraction - with improvments ongoing ID detectors are well-fitted for future 100kg / ton-scale bolometric dark matter searches 16
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