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1 CRESST Cryogenic Rare Event Search with Superconducting Thermometers Jens Schmaler for the CRESST group at MPI MPI Project Review December 14, 2009
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Outline 1.The CRESST experiment 2.Status and progress of CRESST 3.New CRESST test facility 4.Outlook: Search for new scintillators 2
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Outline 1.The CRESST experiment 2.Status and progress of CRESST 3.New CRESST test facility 4.Outlook: Search for new scintillators 3
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4 Dark Matter Detection goal: detection of WIMPs via elastic scattering off nuclei challenges: –low recoil energies: O(10 keV) –low event rate: < O(10 kg -1 yr -1 ) needs: –low energy threshold –excellent background rejection –large target mass –heavy target nucleus CRESST target: scintillating CaWO 4 crystals –up to 33 crystals (10 kg target mass) Ø 40 mm, h=40 mm, 300 g
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5 Detector Principle detectors operated at ~ 10 mK evaporated tungsten film as thermometer: –stabilized in transition to superconducting state –particle interaction temporarily warms up crystal resistance change –SQUID-based readout electronics energy threshold: ~ 1 keV ∆R ∆T 15 16 17 18 19 20 21 350 300 250 200 150 100 50 0 temperature [mK] film resistance [m ] ΔRΔR ΔTΔT crystal W film thermal coupling
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6 Passive Background Suppression dealing with very low signal event rates < O(10 kg -1 yr -1 ) : 1.Gran Sasso underground lab below 1400 m of rock 2.several layers of passive shielding against ambient radioactivity 3.only very radio-pure materials O(10 4 kg -1 yr -1 ) background events still remain copper (10 t) detectors lead (24 t) radon box PE (10 t) muon veto cryostat active background discrimination needed! 1 m
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Active Background Discrimination event-by-event discrimination of particle type higher light output of crystal better discrimination crystals + separate light detector → “detector module” simultaneous measurement of – energy in crystal: E – scintillation light: L define: 7 Light absorber W film CaWO 4 target W film reflective housing β+γ (LY=1 by def.) O-recoils (neutrons, LY=1/10) α (LY=1/5) W-recoils (WIMPs, LY=1/40) light yield := L / E
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8 CRESST Detectors phonon detector light detector
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Composite Detectors motivation: evaporation of W film on crystal degrades light output reduces background discrimination power solution: –first evaporate film on small carrier crystal –glue carrier to large crystal light output can be increased by ~ 50% 9 crystal glue W film carrier crystal
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Outline 1.The CRESST experiment 2.Status and progress of CRESST 3.New CRESST test facility 4.Outlook: Search for new scintillators 10
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11 Reminder: 2008 Data Taking (1) 9 modules operated between August and December 2008 observation: varying number of no-light events in signal region properties of no-light events: –rate decreases with time –partly: different pulse shape in phonon detector → can be rejected by shape cut Rita/Steven, ~ 7 kgd 90% of W-recoils below this line no light with light phonon detector detector effect rather than particle interactions
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12 2008 Data Taking (2) most likely origin of no-light events: stress relaxation events (at contact surface between clamp and crystal) two possibilities: 1.in crystal („micro cracks“) 2.in clamps clamps were mostly covered with plastic layer more no-light events than with pure metal clamps clamp
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13 CRESST – Current Status (1) consequence of 2008 run: new clamps for all crystals 1.thinner material → softer (less pressure on crystals) 2.material: only metal, no plastic coverage new cooldown started mid of May 2009 –delayed by earthquake in L’Aquila region –fast cool-down (stronger thermal coupling of detectors installed) –10 detector modules operational and taking data since June two composite detectors one ZnWO 4 crystal (alternative target material)
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CRESST - Current Status (2) number of no-light events strongly reduced no-light events with different pulse shape disappeared composite detectors work well –high light-output –together with good light detector: significantly lower discrimination threshold achievable 14 acceptance region standard composite 90% of all W-recoils below this line discrimination threshold prelim. preliminary 10 keV threshold 5 keV threshold
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CRESST – Current Status (3) preliminary analysis: a few candidate events observed in the signal region origin of these events? –still detector effect (cracks,...)? –signal (e.g. inelastic Dark Matter)? more statistics needed to investigate properties 15 aim: continue this run at least until mid 2010
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Inelastic Dark Matter assumes inelastic scattering can reconcile claimed DAMA signal with other experiments CRESST: –tightest limits of all experiments (due to heavy target nucleus) –already excludes most of parameter space 16 CRESST (2007 data only) CDMS δ=130 keV [Schmidt-Hoberg et al. arXiv:0907.3940] current data should bring decision on this scenario
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Outline 1.The CRESST experiment 2.Status and progress of CRESST 3.New CRESST test facility 4.Outlook: Search for new scintillators 17
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CRESST Test Facility (1) motivation: full-sized CRESST detector modules cannot be operated over ground (too high count rate) up to now: many tests of new detector developments only in CRESST cryostat itself –long cooling cycle (typically one run per year) –risk to reduce outcome of CRESST data taking aim: decouple R&D and science runs 18 small test cryostat installed in Gran Sasso underground lab by MPI group in 2009
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CRESST Test Facility (2) small cryostat short cooling cycle (3 days) before: used at MPI (~300 cooldowns) moderate Pb shielding readout for one complete detector module (including new data taking system) ready to be cooled down for testing expected to be useable early 2010 19
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Outline 1.The CRESST experiment 2.Status and progress of CRESST 3.New CRESST test facility 4.Outlook: Search for new scintillators 20
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Outlook: New Scintillators amount of scintillation light is limiting factor: 21 find new scintillating target materials higher light output and/or higher light yield for heavy elements find new scintillating target materials higher light output and/or higher light yield for heavy elements candidates? CdWO 4 CsI 1. discrimination threshold: 2. W-recoils: light yield 1/40 not safely distinguished from no-light events at low energy W-recoils CaWO 4 effect of 2x more light CaWO 4 effect of 2x more light β,γ no-light events W-recoils
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Summary status of CRESST: –data taking with 10 detector modules ongoing –new holding clamps: strongly reduced no-light events –important role of CRESST: constrain inelastic Dark Matter model new CRESST test facility at Gran Sasso –allows for efficient detector R&D under low-background conditions –ready for operation plans for 2010: –continue data-taking to clarify nature of remaining events in the signal region –search for new scintillators as target 22
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23 Backup Slides
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24 CRESST Detectors 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.
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25 Detector Readout SQUID-based readout of W-thermometer:
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26 Background from Surface α-Decays in particular from decay chain after 222 Rn: 210 Po → 206 Pb (104 keV) + α (5.4 MeV) Po can be implanted in surface → Pb-recoil in the relevant energy range (below 40 keV) scintillating surrounding as veto when hit by α-particle crystal Pb reflective housing Po
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27 Commissioning Run – DM Limit [G.Angloher et al., Astropart. Phys. 31 (2009), 270-276] 4.8x10 -7 pb for 50 GeV/c 2 WIMPs
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28 Clamp Discrimination Detector modified phonon detector: –normal crystal + 1cm slice of CaWO 4 –parts glued together –thermometer and clamps: on small part signals from the small part should have different pulse shape possibility to discriminate clamp-induced events thermometer clamps glue
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Comp. Detectors – Pulse Shape 29 target crystal thermometer carrier
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