Edelweiss-II : status and first results A new generation of - - PowerPoint PPT Presentation

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Edelweiss-II : status and first results A new generation of - - PowerPoint PPT Presentation

Feb 13, 2024 42 likes •205 views

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 ~

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Edelweiss-II : status and first results

A new generation of background-free bolometers for WIMP search

Eric Armengaud - CEA / IRFU 6 June - SUSY 09

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Direct search for WIMPs

astro-ph/08054705

KIMS EDELWEISS-I CRESST-II CDMS-II XENON

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…

Detector challenges :

  • Low energy threshold
  • Large mass
  • Low background
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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)

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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

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The Edelweiss-II setup

Operated at the Underground Laboratory of Modane (4μ/day/m2)

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

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Neutron rejection : the muon veto

Ionization/recoil ratio

  • 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

  • The ionization yield distribution of

coincidences is consistent with muon- induced events

2007-2008 data (280 kg.d - no cut)

spectra

  • In addition: several neutron flux

measurements carried out near the experiment

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The issue of surface interactions

210Pb 210Po

α

206Pb

β

EDELWEISS-II 93.5 kgd Preliminary

Beta radioactivity at the surface of detectors

  • Incomplete charge collection
  • « Leakage » of such events down to the

nuclear recoil region 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

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Surface event rejection with phonon measurements

The CDMS way

  • Athermal phonon measurement

(transition-edge sensors on one side

  • f the cristal)
  • Measurement of phonon risetime &

delay ⇒ surface e- rejected Early EDW « NbSi » R&D

  • 2 NbSi films measuring athermal

phonons + ionization signals

  • Surface event rejection correct
  • Pbs of threshold / reproducibility

200 150 100 50 80 60 40 20 x10

  • 3

200 150 100 50 80 60 40 20 x10

  • 3

Transient Thermal Heat signal of thermometer NbSiA : Surface event Bulk event Thermometer Thermometer NbSi NbSi B B Thermometer Thermometer NbSi NbSi A A Thermometer Thermometer NbSi NbSi A A Thermometer Thermometer NbSi NbSi B B

Time (ms)

200 150 100 50 80 60 40 20 x10

  • 3

200 150 100 50 80 60 40 20 x10

  • 3

Transient Thermal Heat signal of thermometer NbSiA : Surface event Bulk event Thermometer Thermometer NbSi NbSi B B Thermometer Thermometer NbSi NbSi A A Thermometer Thermometer NbSi NbSi A A Thermometer Thermometer NbSi NbSi B B

200 150 100 50 80 60 40 20 x10

  • 3

200 150 100 50 80 60 40 20 x10

  • 3

Transient Thermal Heat signal of thermometer NbSiA : Surface event Bulk event Thermometer Thermometer NbSi NbSi B B Thermometer Thermometer NbSi NbSi A A Thermometer Thermometer NbSi NbSi A A Thermometer Thermometer NbSi NbSi B B

Time (ms)

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Rejecting surface events with interleaved electrodes

‘a’ electrodes (+4V) collecting

‘b’ electrodes (-1.5V) field shapping

‘c’ (-4V) ‘d’ (+1.5V)

  • Keep the EDW-I NTD

phonon detector

  • Modify the E field near the

surfaces with interleaved electrodes

  • Use ‘b’ and ‘d’ signals as

vetos against surface events First detector built 2007 1x200g + 3x400g tested in 2008 10x400g running since beginning 2009

the « ID » (interdigit) detector

fiducial volume

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An outstanding surface event discrimination with IDs

E1 = energy of top collecting electrode E2 = bottom collecting electrode

Beta calibration (210Po) 200g detector

« single-side » surface events : E1=0 « 3-electrode » surface events 46 keV gamma-ray line fiducial volume events : E1 = E2

  • A combination of cuts is made
  • n 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 !

  • From β calibration : overall

rejection ~ 1/105

2

PRELIMINARY

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ID fiducial volume

400g detector, background run

Fiducial volume cut

Cosmogenic lines:

  • 68Ge and 65Zn isotope lines

at ~10keV

  • Background electron recoil

events

  • Homogeneously distributed

in the volume of the cristal

  • Fiducial volume measurement

~ 160g, primarily limited by the guard regions

  • Result confirmed with neutron

measurements

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Ionization resolution of IDs

8.98 keV 10.37 keV σion = 300 eV

  • Ionization resolution

important to get a good recoil threshold

  • Approx. ~ 20 kg.d
  • f background data

with two 400g detectors (2008 data)

  • Background

dominated by the cosmogenic lines at ~10keV

  • Good and stable

energy resolution

Background ionization spectrum, ID3+ID401 After fiducial cuts

PRELIMINARY

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IDs : overall background rejection performances

0 events

γ-equivalent to ~103 kgd

β-equivalent to 3x104 kgd arXiv:0905.0753

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Physics run with IDs

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 »)

Background free !

WIMP mass (GeV) cross section

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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 m2 extension

  • f present LSM (4 µ/m2/d), to be dig

in 2011-2012

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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 (~1/105)

Good fiducial volume fraction - with improvments ongoing

 ID detectors are well-fitted for future 100kg / ton-scale

bolometric dark matter searches