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Development of 300g scintillating calorimeters for WIMP searches

Development of 300g scintillating calorimeters for WIMP searches. 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

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Development of 300g scintillating calorimeters for WIMP searches

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  1. Development of 300g scintillating calorimeters for WIMP searches 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 TechnischeUniversität 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

  2. Outline • Introduction • WIMPs • Cryo detectors • Background discrimination • Detector development • Light detector • Phonon detector • Prototype module • Summary and Future

  3. 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

  4. Setup

  5. Setup

  6. Principle of low temperature calorimeters Particle interacts in the absorber Temperature rise in the thermometer proportional to deposited energy Superconducting to normal transition: small dT => relatively large dR

  7. Sapphire results 138h live time run Results limited by residual background -> active background rejection

  8. Phonon and Light Principle • 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 Reflector

  9. Proof of Principle • 6g CaWO4 crystal with glued W thermometer • Sapphire light detector with Si coating • Al-mirrors No dead layer ! n • Rejection 99.9% for E>20keV • 99.7% for E>15keV • 0.8% of total energy in light channel

  10. Scale up to 300g 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

  11. Light detector • Requirements: • High sensitivity (< 100eV light) • Large area • Good absorption of emitted light X-ray hit in light detector Scintillation light from CaWO4 • Thin sapphire/silicon substrates • W-thermometer • Sputtered Si absorption layer (sapphire substrates) • Special surface treatment of silicon wafers to reduce reflectivity

  12. Light detector tests • 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 • Test in scintillation holder to check light collection Scintillation 6 KeV

  13. Phonon detector Heater for temperature stabilization & detector calibration Background spectrum 300g CaWO4 @ 45mK threshold ~10keV Problem: relatively high transition temperature decreases sensitivity -> interdiffusion barrier between crystal and W-film

  14. Holder & Crystals • Scintillation: • 425nm FWHM 90nm • 1.5 fold increase of light on cooling from room temperature to 4.2K [nm] Light yield strongly dependent on crystal quality • 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

  15. 300g module Diffusive reflector end cap (sintered Teflon) 300g CaWO4 in specular reflector (reflecting plastic foil) Diffusive reflector end cap with 20x20 mm2 light detector

  16. Results 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 etched 0.7% 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

  17. Next steps • 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

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