T. Frank for the CRESST collaboration Laboratori Nazionali del Gran Sasso C. 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 München F.v.Feilitzsch, T.Jageman, J.Jochum, M. Stark, H. Wulandari University of Oxford G. 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/cm 3 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

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

Simultaneous measurement of phonons and light Scintillating absorber crystal (CaWO 4, PbWO 4, 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 CaWO 4 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 CaWO 4

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 CaWO 45mK threshold ~10keV Problem: relatively high transition temperature decreases sensitivity -> interdiffusion barrier between crystal and W-film Background spectrum Heater for temperature stabilization & detector calibration Phonon detector

Holder: bilayer of polymeric and Al foil as reflector fragile CaWO 4 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 CaWO 4 in specular reflector (reflecting plastic foil) Diffusive reflector end cap with 20x20 mm 2 light detector 300g module

detected No reflector reflectivity light detector light* ________________________________________________ I Al-mirrors sapphire 200mm 2 0.8% II teflon sapphire 130mm 2 1.2% 1 teflon 97.5% sapphire 130mm % 2 teflon 97.5% sapphire 200mm % 3 teflon&foil 98.7% sapphire 200mm % 4 foil bilayer 97.0%sapphire 400mm 2 0.5% 5 foil bilayer 97.0%silicon 400mm % 6 foil bilayer 97.0%silicon 400mm 2 etched0.7% 7 foil bilayer 97.0%silicon 900mm 2 1.3% -> better than proof of principle I & II with 6g CaWO with 300g CaWO 4 * In percent of deposited energy in CaWO 4 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