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.