The European Future of Dark Matter Searches with Cryogenic Detectors

H KrausUniversity of Oxford

for the EURECA collaboration

EURECA

• Based on CRESST and EDELWEISS expertise, with additional groups joining.

• Target materials: Ge, CaWO4, etc (A dependence)

• Mass: above 100 kg towards 1 ton

• 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

Russia

DLNP Dubna

CRESST + EDELWEISS + new groups

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

σ = 10−6 pb:

~1 event/kg/day~0.1 now reached

σ = 10−8 pb:

~3 events/kg/yearAims of phase II experiments

σ = 10−10 pb:

~30 events/ton/yearNext generation requires further x100 improvement!

Dark Matter Signatures

1.Recoil energy spectrum Energy resolution exponential, similar to background.

2.Nuclear (not electron) recoils Discrimination is really required now.

3.Coherence: A dependence Multi-target essential once first signal is identified.

4.Absence of multiple interactions Large Array removes some fraction of background.

5.Diurnal modulation nice, but needs low-pressure gaseous target.

6.Annual modulation (requires many events) tricky; most events are close to threshold, small effect.

Detection Techniques

Ar, XeArDM, WARP, XENON, ZEPLIN

NaIDAMALIBRAANAISNAIAD

GeHDMSGenius-TFIGEXDRIFT

Si, GeCDMSEDELWEISS

Al2O3

CRESST I

CaWO4

CRESST IIROSEBUD

Cryogenic Techniques

Initial recoil energy

Displace-ments,

Vibrations

Athermalphonons

Ionization(~30 %)

Thermal phonons(Heat)

Scintillation

(~3 %)

Combination of 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 ionisation and phonon signals. Clean γ-calibration data: no event below Q = 0.7.

CRESST – Detectorsheat bath

thermal link

thermometer(W-film)

absorbercrystal

Particle interaction in absorber creates a temperature rise in thermometer which is proportional to energy deposition in absorber

Temperature pulse (~6keV)

Resis

tance [mΩ

]

normal-conducting

super-conducting

δδδδT

δδδδR

Width of transition: ~1mKSignals: few µ KStablity: ~ µ K

Phonon – ScintillationDiscrimination 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 i

n l

igh

t c

ha

nn

el

ke

Vee]

Energy in phonon channel [keV]

high rejection:99.7% > 15 keV99.9% > 20 keV

α – β/γ Discrimination

Danewich et al – Kiev(116CdWO4 – 330 g, 2975 hrs)

CRESST – LNGS(CaWO4 – 300 g, 633 hrs)

A ‘standard’ scintillator(pulse shape discrim.)

Scintillation part of a phonon – scintillation detector (CRESST)

α – β/γ Discrimination

Decay Identification

EURECA Strategy

Some aspects covered by ILIAS working groups.

Main thrust: demonstrate current experiments.

EDELWEISS: new (larger) cryostat; improved

shielding; 8kg Ge in 2006; 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 in 2006; ongoing improvement of scintillation signals. Up to ~10kg target possible in present cryostat.

EURECA Tasks

Detector Development: improving rejection, target materials, 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.

EDELWEISS II Ge / NTD Detectors

♦ Amorphous Ge and Si sub-layer: better charge collection for surface events

♦ Optimized NTD size and operating temp. (16-18 mK) : sub keV resolution

♦ New holder and connectors (Teflon and copper only)

Multi-target Approach

300 350 400 450 500 550 600 650 700 7500

5

10

15

20

25

Lu

min

esce

nce

in

tensity, a

.u.

Wavelength, nm

CaWO4

ZnWO4

CaMoO4

CdMoO4

MgMoO4

Excitation: 3.2keV Temperature: 8K

0 50 100 150 200 250 300 3500

20

40

60

80

100

120

CaWO4

ZnWO4

CdMoO4

CaMoO4In

ten

sity,

a.u

.Temperature, K

Germanium and Scintillators:

many materials scintillate…

Cryostat Designs

Cryo / Electronics InfastructureEDELWEISS: up to ~30kg

• Read out electronics:66 SQUIDs (CRESST), 100 high-impedance readout (EDELWEISS)

• Shielding / n-moderator / µ-veto:

Cu/Pb, 50 cm polyethylen, plastic scintillators, radon box, etc.

• Future cryogenic facility: Modular design, “hot-swap“, uptime, balance traditional with modern, address cabling (heat load) issue, separate: detector volume – cryostat, separate: cooling functions – readout

CRESST: up to ~10kg

The 66 channel SQUID readout system

• 66 chan. 33 detector modules with up to 10 kg of target material

• Will need 103 – 104

channels

• Need factor 5 – 10 denser packing

• More automatic tuning,

logging and control

Example:Electronics

EDELWEISS-II Electronics

♦ Electronics (by CEA and IN2P3) directly on the cryostat

♦ Heat modulation and charge bias by programmable DACs

♦ Continuous 200 kHz digitization, output by optical fibers

♦ Filtering, demodulation and trigger by soft in the acquisition system

Numeric

(DACs and

ADCs

Heat

amplifier

Charge

amplifiers

Power supply

To the cryostat

(cold electronics and

bolometers)

Outputs

EURECA Near Future

• R&D through EDELWEISS and CRESST.

• ILIAS working groups.

• EU – FP7 Design Study for Detector Module Layout, Shielding, Underground Infrastructure, etc.

• Apply to national funding agencies.

• Building on EDELWEISS and CRESST expertise to build tonne-scale cryogenic experiment.

Summary

CRESST and EDELWEISS on track; improvements w.r.t. present achievements will have to be shown.

Cryogenic Detector Technology with nuclear recoil identification has the potential required for these improvements.

EURECA builds on CRESST and EDELWEISS expertise aiming towards a European multi-target array for direct dark matter searches.