the european future of dark matter searches with cryogenic detectors h kraus university of oxford
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The European Future of Dark Matter Searches with Cryogenic Detectors
H KrausUniversity of Oxford
EURECA
• Based on CRESST and EDELWEISS expertise, with additional groups joining.
• Baseline targets: Ge, CaWO4, etc (A dependence)
• Mass: above 100 kg
• Timescale: after CRESST-II and EDELWEISS-II
• Started 17 March 2005 (meeting in Oxford)
• R&D: demonstrate CRESST/EDELWEISS
European Underground Rare Event Calorimeter Array
United Kingdom
Oxford (H Kraus, coordinator)
Germany
MPI für Physik, Munich
Technische Universität München
Universität Tübingen
Universität Karlsruhe
Forschungszentrum Karlsruhe
CRESST + EDELWEISS + new forces
EURECA Collaboration
France
CEA/DAPNIA Saclay
CEA/DRECAM Saclay
CNRS/CRTBT Grenoble
CNRS/CSNSM Orsay
CNRS/IPNL Lyon
CNRS/IAP Paris
CERN
Experiments – MSSM Predictions = 10−6 pb:
~1 event/kg/day~0.1 now reached
= 10−8 pb: ~1 event/kg/yearCDMS-II, CRESST-II and EDELWEISS-II aims
= 10−10 pb: ~1 event/ton/yearNext generation requires further x100 improvement!
Background Rates
Major challenge: typical radioactivity … human body: ~10+6 decays/(kg day)… well above: ~10−4 events/(kg day)
Substantial and robust discrimination required:tails of distributions; hard to understand, difficult to simulate with high precision.
Entering un-chartered territory:need low event rate in ~keV energy range: atomic x-rays, not MeV as in ν experiments
Detection Techniques
Cryogenic Techniques
Initial recoil energy
Displace-ments,
Vibrations
Athermalphonons
Ionization(~10 %)
Thermal phonons(Heat)
Scintil
lation
(~1 %
)
Discrimination by combining phonon measurement with measurement of ionization or scintillation
Phonon: most precise total energy measurement
Ionization / Scintillation: yield depends on recoiling particle
Nuclear / electron recoil discrimination.
EDELWEISS – Detectors
Target:Cyl. Ge crystal, 320 gØ 70 mm, h = 20 mmPhonon - signal:NTD-Ge (~ 20 mK)Ionisation - signal:Inner disc / outer guard ring
Phonon – Ionisation
252Cf60Co
Excellent resolution in both ionisation and phonon signals. Clean γ-calibration data: no event below Q = 0.7.
EDELWEISS 1 – Data
Data: 22.6 kg.d shown.
Probable surface event contamination at Q<0.7
Challenge: less than perfect charge collection for surface events
Identification of Backgrounds
Germanium Surface Events Example of 3rd population, affecting rejection efficiency.
Quality of Rejection Importance of selection variable having good separation and resolution.
More Rejection Signatures Recoil spectrum, coincidence, charge, scintillation, type of recoiling nucleus, etc.
EDELWEISS
LSM (4800 m.w.e)21×320g Ge with NTD 7×400g Ge with NiSb
EDELWEISS – New Cryostat Up to 120 Detectors
EDELWEISS – Shielding
• 20 cm lead• 50 cm PE• Muon veto
March 2005 March 2005
May 2005 Edelweiss II installation at LSM
May 2005: lead, upper and lower PE shields completed.Start μ-VETO installation.
Summer: installation of cryostat.
Autumn: first pulses.
CRESST – Detectors heat bath
thermal link
thermometer(W-film)
absorbercrystal
•Particle interaction in absorber creates a temperature rise in thermometer which is proportional to energy deposit in absorber
Temperature pulse (~6keV)
Res
ista
nce
[m
] normal-conductin
g
super-conducting
T
R
Width of transition: ~1mKSignals: few K Stablity: ~ K
Phonon – Scintillation Discrimination of nuclear recoils from radioactive backgrounds
(electron recoils) by simultaneous measurement of phonons and scintillation light
separate calorimeter as
light detector
light reflector
W-SPT
W-SPT
300 g CaWO4
proof of principle
En
erg
y in
lig
ht
chan
nel
keV
ee]
Energy in phonon channel [keV]
high rejection:99.7% > 15 keV99.9% > 20 keV
300g Detector Prototype
CRESST II: 33 modules; 66 readout channels
Run 28: Low Energy Distribution No Neutron Shield
90% of oxygen recoils below this line.
Rate=0.870.22 /kg/day compatible with expec-ted neutron background (MC).
10.72 kg days
90% of tungsten recoils Q = 40 below this line.
No events
Upper Limits on Scalar WIMP-Nucleon Cross Section
Cryogenic Detectors only
Expected Recoil Spectrum in GS
Contribution of W recoils negligible for E > 12 keV
σ A2 for WIMPs with spin-independent interaction
WIMPs dominantlyscatter on W (A=184) nuclei
Neutrons mainly on oxygen
MC simulation of dry concrete (Wulandari et al)
Quenching Factor Measurement
PMT
UV Lase
r
W, O, Ca ions
CaWO4 crystal
PTFE reflector
collimator
Ion source
• UV Laser desorbs singly or doubly charged ions from almost any material. Acceleration to 18 keV (or 36 keV for doubly charged).
• Mount CaWO4 crystal on PMT at end of flight tube and record single photon counts with fast digitizer.
Deflection plate for ion type selection
target
Quenching Factors for CaWO4
Upgrade • Read out electronics:66 SQUIDs for 33 detector modules and DAQ ready
• Neutron shield: 50 cm polyethylen (installation complete)
• Muon veto: 20 plastic scintillator pannels outside Cu/Pb shield and radon box. Analog fibre transmission through Faraday cage (ready)
• Detector integration in cold box and wiring (entering fabrication stage)
• Excellent linearity and energy resolution at high energies
• Perfect discrimination of , from s
• Identification of alpha emitters (internal, external)
High Energy Performance
Decay of “stable” Tungsten-180
147Sm
152Gd
144Nd 180W
180W
Results from four runs (28.62 kg days)
Half life T1/2 = (1.8±0.2)×1018 years
Energy Q = (2516.4±1.1 (stat.)±1.2(sys.)) keV
Decay of “stable” Tungsten-180
Evolution of Sensitivity
Signatures 1. Recoil energy spectrum Energy resolution
2. Nuclear (not electron) recoils Discrimination
3. Coherence: μ2A2 dependence Multi-target
4. Absence of multiple interactions Array
5. Uniform rate throughout volume Large Array
6. Annual modulation (requires many events)
EURECA Tasks
Detector Development: improving rejection, optimizing size, mass production issues.
Readout and Electronics: scalability.
Cryogenic Environment: size, radiopurity, uptime.
Neutron and Muon Backgrounds: measurement, simulation and shielding design.
Extreme Low-background Materials: selection of materials, processing, handling, etc.
EURECA Strategy
Some aspects covered by ILIAS working groups.
Main thrust: demonstrate current experiments.
EDELWEISS: new (larger) cryostat; improved shielding; 8kg Ge by end of 2005; reduction of surface events expected from improved radiopurity and/or use of NbSi sensors. Up to ~30kg target possible in cryostat.
CRESST: new (66 channel) SQUID readout system; improved shielding; few kg CaWO4 by end of 2005; continuous improvement scintillation signals. Up to ~10kg target possible in present cryostat.
Summary CRESST and EDELWEISS are on track to reaching
LHC-relevant sensitivity; but major improvements w.r.t. present achievements will have to be shown.
Cryogenic Detector Technology with nuclear recoil identification has the necessary potential for these improvements.
The EURECA collaboration builds on CRESST and EDELWEISS experience aiming towards a European multi-target array for direct dark matter searches.
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