T. Frank for the CRESST collaboration

Laboratori Nazionali del Gran SassoC. Bucci

 Max-Planck-Institut für Physik

M. Altmann, M. Bruckmayer, C. Cozzini, P. Di Stefano,

T. Frank, D.Hauff, F. Pröbst, W. Seidel, L. Stodolsky 

Technische Universität MünchenF.v.Feilitzsch, T.Jageman, J.Jochum, M. Stark,

H. Wulandari 

University of OxfordG. Angloher, N. Bazin, S.Cooper, R.Keeling,

H.Kraus, Y.Ramachers 

Development of 300g scintillating calorimeters for WIMP searches

Outline

• Introduction• WIMPs• Cryo detectors• Background discrimination

• Detector development• Light detector• Phonon detector• Prototype module

• Summary and Future

WIMP direct detection

• Density: 0.3 GeV/cm3

• Velocity: 230 km/s• Mass: GeV range• Interaction via elastic scattering on

nuclei• Very low event rates (< 1event /kg/keV/day)• Transfered energy few keV

Very sensitive detector necessary Very good shielding of background

Low temperature detectors & underground setup

Setup

Setup

Particle interacts in the absorber

Temperature rise in the thermometer proportional to deposited energy

Superconducting to normal transition: small T

=> relatively large R

Principle of low temperature calorimeters

138h live time run

Results limited by residual background

-> active background rejection

Sapphire results

H eat det ect orT her momet er

L ight det ect or

Par t ic le

• Simultaneous measurement of phonons and light

• Scintillating absorber crystal (CaWO4, PbWO4, BaF, BGO) with thermometer to detect phonons

• Very sensitive detector close by to detect light

-> ratio of detected phonon signal versus light signal allows identification of interaction

Phonon and Light Principle

Reflector

• 6g CaWO4 crystal with glued W thermometer

• Sapphire light detector with Si coating

• Al-mirrors

•Rejection 99.9% for E>20keV

99.7% for E>15keV

•0.8% of total energy in light channel

Proof of Principle

No dead layer !

n

Goals:

• Same or better light collection as with 6g test module

• Threshold of module <10keV

-> discrimination with >99% down to 10keV

Challenges:

• Scintillation light only small fraction of total energy

• Large reflector surface requires very high reflectivity

• Large sensitive light detector with good absorption needed

Scale up to 300g

•Thin sapphire/silicon substrates

•W-thermometer

•Sputtered Si absorption layer (sapphire substrates)

•Special surface treatment of silicon wafers to reduce reflectivity

Requirements:

•High sensitivity (< 100eV light)

•Large area

•Good absorption of emitted light

Light detector

X-ray hit in light detector

Scintillation light from CaWO4

•Test in scintillation holder to check light collection

•Test with 6 keV source impinging on different spots to check threshold and uniformity

40x40x0.4mm³ light detector

Large sizes possible with good sensitivity & uniformity

Light detector tests

Scintillation

6 KeV

300g CaWO4 @ 45mK threshold ~10keV

Problem:

relatively high transition temperature decreases sensitivity

-> interdiffusion barrier between crystal and W-film

Background spectrum

Extenal readout circuit

External heatercircuit

Thermal connectionto holder

External readout circuit

Externalheater circuit

Heater for temperature stabilization & detector calibration

Phonon detector

0 50 100 150 2000

10

20

30

40

50

Holder:

• bilayer of polymeric and Al foil as reflector

• fragile CaWO4 require shrinkage compensation

•light detector should be fully exposed for maximum sensitivity

• parasitic absorptions of light should be kept at a minimum

Scintillation:

• 425nm FWHM 90nm

• 1.5 fold increase of

light on cooling from room temperature to 4.2K

Light yield strongly dependent on crystal quality

Holder & Crystals

[nm]

Diffusive reflector end cap (sintered Teflon) 

300g CaWO4 in specular reflector(reflecting plastic foil)

Diffusive reflector end cap with20x20 mm2 light detector

300g module

  detected

No reflector reflectivity light detector light*

________________________________________________

I Al-mirrors ----- sapphire 200mm2

0.8%

II teflon ----- sapphire 130mm2 1.2%

1 teflon 97.5% sapphire 130mm2

0.33%

2 teflon 97.5% sapphire 200mm2

0.52%

3 teflon&foil 98.7% sapphire 200mm2

0.68%

4 foil bilayer 97.0% sapphire 400mm2 0.5%

5 foil bilayer 97.0% silicon 400mm2 0.55%

6 foil bilayer 97.0% silicon 400mm2 etched0.7%

7 foil bilayer 97.0% silicon 900mm2 1.3%

-> better than proof of principle

 

I & II with 6g CaWO4 1-7 with 300g CaWO4

* In percent of deposited energy in CaWO4

Silicon light detectors have better light absorption but stronger spatial dependence of response

Results

• new absorption layer for sapphire light detector

• improved layout and bonding scheme for light detectors

•test of new 300g detector module at Gran Sasso

Projected sensitivity with 30kg years of data

Next steps