CRESST Cryogenic Rare Event Search with Superconducting Thermometers Max-Planck-Institut für Physik University of Oxford Technische Universität München Laboratori Nazionali del Gran Sasso Universität Tübingen CRESST

Outline Direct detection and CRESST detectors Results from commissioning run (Run30) Status of present Run (31) Plans for next year

Dectector Requirements for direct detection Small energy transfers to nucleus Featureless spectrum just above threshold Very low event rate < 0.1/kg/day. Large class of MSSM models predict 0.1 /kg/year to 0.1 /kg/day Typical background rates deep underground: radioaktive β+γ background: some 100/kg/day --> need very efficient nuclear recoil discrimination Neutrons from rock (LNGS): ~ 1 /kg/day --> need neutron moderator High energetic neutrons from muons in Pb/Cu shield: ~0.02 /kg/day need muon veto for σ WIMP-nucleon <10 -8 pb W recoil spectra for various WIMP masses

CRESST type cryogenic Detectors heat bath thermal link thermometer (W-film) ‏ absorber crystal Temperature pulse Resistance [m  ] normal- conducting super- conducting TT RR SQUID based read out circuit Width of transition: ~1mK, keV signals: few μK Longterm stablity: ~ μ K Advantages of technique: - measures deposited energy independent of interaction type - Very low energy threshold and excellent energy resolution - Many materials E

CRESST-II Detectors Discrimination of nuclear recoils from radioactive backgrounds by simultaneous measurement of phonons and scintillation light 300g scintillaing CaWO 4 target crystal W sensor Light detector W sensor Light reflector β+γ α O W WIMPs scatter on W

300 g CRESST-II Detector Module CRESST-II: up to 33 detector modules The phonon detector: 300 g cylindrical CaWO 4 crystal. Evaporated tungsten thermometer with attached heater. The light detector: Ø=40 mm silicon on sapphire wafer. Tungsten thermometer with attached aluminum phonon collectors and thermal link. Part of thermal link used as heater

Commissioning 2007 After major upgrade (new detectors, support structure, 66 channel SQUID system, DAQ, neutron shielding, muon veto,...) ‏ 9 detector modules mounted ► 7 Phonon channels O.K delivering sub keV energy resolution ► Cooling problem due to weak thermal coupling of internal parts of detector holder ► Commissioning run March to Dec 2007with two detectors. ► Results submitted for publication this year (arXiv ) ‏

Data from commissioning run 3 Tungsten recoils in 10 to 40 keV range in two detector modules in 50 kg-days Still wider beta/gamma band compared to previous runs due to residual electronic interference in light detectors Zora/SOS23: 23.8kg-days 90% O recoil below this line 90% W recoils below this line

Spectral features at low Energies Very precise energy calibration Lines down to 3.6 keV identified with excellent energy resolution of 300 eV. Cu K α 8.1 keV keV 41 Ca 3.61 keV keV 210 Pb keV

Exclusion limits from commissioning run 4.8x10 -7 pb for 50 GeV WIMPs

Inelastic Dark Matter Invented to make DAMA compatible with limits of other experiments CRESST-II sets tightest constraint CRESST-II XENON10 J. March-Russel et al. arXiv:

180 W 180 W now listed unstable in tables with 1.8x10 18 years half life

Run31 SQUID control electronics replaced by quiet custom design to reduce sources of em interference disturbing the light detectors (Oxford) ‏ 17 detector modules mounted (total ~5 kg) in May 2008, some with design modifications to explore origin of recoil signals of previous run. Design modification include: -- Improved perfection of coverage of internal surfaces with scintillating material to further optimize rejection of 206 Pb recoils. --Glued W sensors to improve light yield of crystals up to 50 % --New material: ZnWO 4, lower radioactive background, more light, availability 10 modules working after slow cooldown of ~month.

Detectors with glued thermometer No degradation of energy resolution of phonon channel observed, works better than expected 57 Co calibration: Hanna (glued detector)‏ Energy resolution: keV 1 60 keV

Glued Detectors: Absorber and Carrier hits Thermometer Carrier hits may have lower light yield Very different pulse shape allows safe discrimination down to E<5 keV Carrier and Absorber signal Pulse shape discrimination

Dark counts with longer pulse shapes Rita/Steven 7 kg-days Rita/Steven after pulse shape cut New type of pulses in this run: --dark --longer decay time, wide distribution --occur when clamps are covered with scintillating epoxy, fewer or none when polymeric foil is used -- rate decreases with time Shape cut not 100% efficient: Dark counts reduced from 20 to 1 count in 10 to 40 keV range

Clamps covered with polymeric foil Julia/BE14 ~11.8 kg days Daisy/BE13 ~7 kg-days No or much fewer dark counts when crystal held by bare clamps or by clamps covered with scintillating polymeric foil Most dark counts have same shape as particle pulses Two mechanisms ? -- long pulses from stress relaxation in plastic at contact area with crystal -- particle like pulses from stress relaxation in crystal (micro cracks)

Plans Stop run end of this week (was continued to work on SQUID electronics issues) ‏ Fabricate new softer clamps for all detectors Prepare stronger thermal coupling of detector mounting structure to reduce cooldown time and safely reach operating temperature of all detectors with low transition. Replace clamps beginning of 2009 Extended physics run with 17 detectors. Set up a small test cryostat at Gran Sasso ?

Conclusions 10 modules running Glued detectors work well EM interferences on light channels improved Except for dark counts discrimination threshold improved Hope that next run will be a successful physics run

The End

 MeV  keV Backrgound from  Surface Contamination 210 Pb 22.3 y 210 Bi 210 Po 206 Pb 210 Pb is first long lived isotope in chain after 222 Rn, implanted in surface by  decay of parent isotope. Efficient source of „surface“ electrons 103 keV 206 Pb recoil nuclei from alpha decay degraded alphas from structure materials around detectors  5.3 MeV  detector 206 Pb surrounding Material (A) ‏ (B) ‏ recoil nucleus 103 keV A) Energy >103 keV. Not really dangerous for DM searches B) Nuclear recoils keV. Dangerous for DM searches with CaWO 4