t. frank for the cresst collaboration laboratori nazionali del gran sasso c. bucci...
TRANSCRIPT
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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
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Outline
• Introduction• WIMPs• Cryo detectors• Background discrimination
• Detector development• Light detector• Phonon detector• Prototype module
• Summary and Future
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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
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Setup
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Setup
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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
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138h live time run
Results limited by residual background
-> active background rejection
Sapphire results
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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
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• 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
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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
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•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
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•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
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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
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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]
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Diffusive reflector end cap (sintered Teflon)
300g CaWO4 in specular reflector(reflecting plastic foil)
Diffusive reflector end cap with20x20 mm2 light detector
300g module
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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
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• 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