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Dan Akerib Case Western Reserve University 7 July 2001 Snowmass, Colorado E6.2 Working Group

The CDMS I & II Experiments: Challenges Met, Challenges Faced. Dan Akerib Case Western Reserve University 7 July 2001 Snowmass, Colorado E6.2 Working Group. Case Western Reserve University D.S. Akerib,D. Driscoll, S. Kamat, T.A. Perera, R.W. Schnee, G.Wang

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Dan Akerib Case Western Reserve University 7 July 2001 Snowmass, Colorado E6.2 Working Group

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  1. The CDMS I & II Experiments: Challenges Met, Challenges Faced Dan Akerib Case Western Reserve University 7 July 2001 Snowmass, Colorado E6.2 Working Group

  2. Case Western Reserve University D.S. Akerib,D. Driscoll, S. Kamat, T.A. Perera, R.W. Schnee, G.Wang Fermi National Accelerator Laboratory M.B. Crisler, R. Dixon, D. Holmgren Lawrence Berkeley National Lab R.J. McDonald, R.R. Ross A. Smith Nat’l Institute of Standards & Tech. J. Martinis Princeton University T. Shutt Santa Clara University B.A. Young Stanford University D. Abrams, L. Baudis, P.L. Brink, B. Cabrera, C. Chang, T. Saab University of California, Berkeley S. Armel, V. Mandic, P. Meunier, M. Perillo-Isaac, W. Rau, B. Sadoulet, A.L. Spadafora University of California, Santa Barbara D.A. Bauer, R. Bunker, D.O. Caldwell, C. Maloney, H. Nelson, J. Sander, S. Yellin University of Colorado at Denver M. E. Huber University College London/Brown Univ. R.J. Gaitskell The Cryogenic Dark Matter Search Collaboration

  3. WIMPs and Dark Matter • Non-Baryonic dark matter • Dynamical measurements of clusters m = 0.3  0.1 • Corroborated by CMB + SNe Ia: m ~ 0.3 L ~ 0.7 • BBN baryon density b = 0.05  0.005 • Structure formation requires Cold dark matter • WIMPs: EW-scale couplings and 10 – 1000 GeV mass range • Thermally produced • Non-relativistic freeze-out • SUSY/LSP a natural candidate

  4. Galactic halo ~20% Machos 8 – 50% @ 95%C.L. Basic paradigm intact Direct detection scattering experiment Few keV recoil energy < 1 event/kg/d Background suppression/rejection Low energy threshold Signal modulation WIMP detector ~10 keV energy nuclear recoil WIMP-Nucleus Scattering Direct Detection in the Galactic Halo • Importance of threshold and high quenching factor • I/Xe a 50 keV true nuclear recoil threshold is equivalent to about 5 keV electron equivalent recoil

  5. Selected results & goals • CDMS I – best limit to date and first example of cryogenic detectors to surpass sensitivity of conventional detectors (HPGe, NaI) • CDMS II – at Soudan to be 100x more sensitive CDMS DAMA 100kg NaI CDMS Stanford CRESST CDMS Soudan Genius Ge 100kg 12 m tank

  6. outer Pb shield scintillator veto Icebox polyethylene outer moderator Background detectors inner Pb shield dilution refrigerator Echarge Signal Ethermal CDMS Strategy Lines of defense • Underground site: hadrons,  • Muon veto: cosmogenic , , n • Pb shield: ,  • Poly shield: n • Recoil type: ,  • Multiple-scatters: n • Position sensitive A D C B

  7. Photon and electron backgrounds give more-ionizing electron recoils WIMPs and neutrons give less-ionizing nuclear recoils Plot as ratio: “Charge Yield” Erecoil = Ethermal – Ethermal Y = Echarge/Erecoil Background Echarge Signal Ethermal Two Signals: Reject the Background > 99.8% gamma rejection external gamma source (blip detector) (Y = Charge Yield) external neutron source

  8. Germanium BLIP Detectors BerkeleyLarge Ionization- and Phonon-mediated Detectors • Tower • Wiring • heat sinking • holds cold FETs for amplifiers Inner Ionization Electrode • Four 165 g Ge detectors, for total massof 0.66 kg during 1999 Run • Calorimetric measurement of total energy • Energy resolution: sub-keV FWHM in phonons and ionization Outer Ionization Electrode Passive Ge shielding (NTD-Ge thermistors on underside)

  9. ZIP Ionization & Phonon Detectors Fast athermal phonon technology • Superconducting thin films of W/Al • Stable Electrothermal Feedback configuration • Aluminum Quasiparticle Traps give area coverage ZIP: At end of fabrication steps involving µm photolithography at Stanford Nanofabrication Facility

  10. Position Sensitivity: fast phonon sensors y A D collimator X A D B C C B Time delay • Internal backgrounds • Tends to surfaces or edges • Wimps • Uniform throughout bulk (zip detector)

  11. Rejection History • Basic simultaneous charge/ionization 1992 ~90% -rejection • Suspected charge trapping at edges limits effectiveness • Evolution from segmented electrode to “edgeless design” 1993-1994 gives 99% -rejection • Early Stanford runs (1995-1997): reveals low-energy electrons • Electrons 10 - 100 keV stop in surface layer = “dead layer” • Reduced charge yield due to trapping defeats rejection of electron recoils • Sources: • Tritium background traced to NTDs and eliminated in bakeout procedure • Surface contamination – especially in earlier prototypes (too much handling) • Limits rejection to ~50% @ 10 – 20 keV • Need ~factor 10 reduction to equal gammas/neutrons • 4-part strategy (also applies to new ZIP detectors for CDMS II) • Cleanliness • Close-pack array • Improve electrode structure • Fast phonon signal risetime

  12. ElectronBackgrounds • Continuum beta contamination, problematic up to ~ 100 keV on thermal phonon-mediated Ge detectors • Tritium contamination below 20 keV in Ge • Eliminated through bakeout procedure Post muon veto electron events

  13. Rejection History • Basic simultaneous charge/ionization 1992 ~90% -rejection • Suspected charge trapping at edges limits effectiveness • Evolution from segmented electrode to “edgeless design” 1993-1994 gives 99% -rejection • Early Stanford runs (1995-1997): reveals low-energy electrons • Electrons 10 - 100 keV stop in surface layer = “dead layer” • Reduced charge yield due to trapping defeats rejection of electron recoils • Sources: • Tritium background traced to NTDs and eliminated in bakeout procedure • Surface contamination – especially in earlier prototypes (too much handling) • Limits rejection to ~50% @ 10 – 20 keV • Need ~factor 10 reduction to equal gammas/neutrons • 4-part strategy (also applies to new ZIP detectors for CDMS II) • Cleanliness • Close-pack array • Improve electrode structure • Fast phonon signal risetime

  14. Improved Charge Collection for Surface Events • Electron Source (14C) probes charge collection at surface directly • Conventional p-type implanted contact shows ~30% collection • Significant improvement with new blocking contact

  15. Surface-Event Discrimination in BLIPs • Beta contamination in top detector in stack of four • Serendipitous population of tagged electron events • New electrodes of 1999 BLIP minimize “dead layer” and amount of charge lost during ionization measurement • >95% event-by-event rejection of surface electron-recoil backgrounds 1999 SUF run 1334 Photons (external source) 233 Electrons (tagged contamination) 616 Neutrons (external source) Ionization Threshold

  16. Rejection History • Basic simultaneous charge/ionization 1992 ~90% -rejection • Suspected charge trapping at edges limits effectiveness • Evolution from segmented electrode to “edgeless design” 1993-1994 gives 99% -rejection • Early Stanford runs (1995-1997): reveals low-energy electrons • Electrons 10 - 100 keV stop in surface layer = “dead layer” • Reduced charge yield due to trapping defeats rejection of electron recoils • Sources: • Tritium background traced to NTDs and eliminated in bakeout procedure • Surface contamination – especially in earlier prototypes (too much handling) • Limits rejection to ~50% @ 10 – 20 keV • Need ~factor 10 reduction to equal gammas/neutrons • 4-part strategy (also applies to new ZIP detectors for CDMS II) • Cleanliness • Close-pack array • Improve electrode structure • Fast phonon signal risetime

  17. Surface-Event Discrimination in ZIPs: Risetime gammas Neutrons (low y, slow tr) Bulk events well separated in charge yield… surface bulk Rise time neutrons …surface events not. electrons Charge yield, y electrons

  18. Summary of gamma/beta rejection history • Steady improvement of rejection factors • Can we continue trend to next generation? (Background fraction that leaks through) Goals for CryoArray, see R.Gaitskell’s talk in E6, 9 July

  19. Succeeded with 1999 Data Set… see below Rejection History • Basic simultaneous charge/ionization 1992 ~90% -rejection • Suspected charge trapping at edges limits effectiveness • Evolution from segmented electrode to “edgeless design” 1993-1994 gives 99% -rejection • Early Stanford runs (1995-1997): reveals low-energy electrons • Electrons 10 - 100 keV stop in surface layer = “dead layer” • Reduced charge yield due to trapping defeats rejection of electron recoils • Sources: • Tritium background traced to NTDs and eliminated in bakeout procedure • Surface contamination – especially in earlier prototypes (too much handling) • Limits rejection to ~50% @ 10 – 20 keV • Need ~factor 10 reduction to equal gammas/neutrons • 4-part strategy (also applies to new ZIP detectors for CDMS II) • Cleanliness • Close-pack array • Improve electrode structure • Fast phonon signal risetime

  20. Combined data set from 3 BLIPs Muon anti-coincident 45 Live days – 10.6 kg-d exposure Well-separated , , nuclear recoils above 10 keV threshold 13 single-scatters consistent with residual neutron background 4 nuclear-recoil multiple-scatter events Singles to multiples ratio established by MC 4 nuclear recoils in silicon Standard halo assumptions used to set limit Single scatters Nuclear recoils 1999 CDMS Ge Data (BLIP)

  21. Non-Neighbor interaction B3 B4 B5 Neighbor interaction B6 Neutron Multiple Scatters Observe 4 neutron multiple scatters in 10-100 keV multiple events • 3 neighbors, 1 non-neighbor • Calibration indicates negligible contamination by electron multiples Neighbors Non-Neighbors surface electrons photons photons Ionization Yield B5,6 Ionization Yield B6 neutron neutrons Ionization Yield B4,5 Ionization Yield B4

  22. 1998 CDMS Si Data (ZIP) bulk events NR candidates Si ZIP measured external neutron background • For neutrons 50 keV - 10 MeV, Si has ~2x higher interaction rate per kg than Ge • Not WIMPs: Si cross-section too low (~6x lower rate per kg than Ge) • Electron-recoil leakage into nuclear recoil (NR) band small • upper limit on electron-recoil leakage determined by electron, photon calibrations • in 1998 Run data set:<0.26 events in 20-100 keV range at 90% CL mostly neutrons

  23. Excludes new parameter space Better than expected based on Ge singles 1 mulitple expected, 4 observed Worse agreement 6% of the time Likely to improve in new analysis with increased fiducial volume Bottom of DAMA NaI/1-2 2- contour excluded at 89% Bottom of DAMA NaI/1-4 3- contour excluded at 75% Simultaneous fit ruled out at > 99.8% CL PRL 84, 19 June 2000 astro-ph/0002471 Detailed PRD in preparation with increased fiducial mass (2x) DAMA 3 Gondolo et al Bottino et al DAMA 2 Dark Matter Limit from CDMS I Ge ionization DAMA 1996 CDMS 1999

  24. Compatibility of CDMS and DAMA • Estimate DAMA Likelihood function based on “Figure 2” data (left) • Simultatneous best fit to CDMS + DAMA • “standard” halo • A2 scaling • Ruled out at > 99.8% CL • Accommodation? • Halo parameters? • Direct test with NaIAD DAMA residual spectrum CDMS bkg subtracted Best simultaneous fit to CDMS and DAMA predicts too little annual modulation in DAMA, too many events in CDMS

  25. CDMS II – 100x improvement over present limits Larger array & longer exposure Second generation detectors with event positions Ge (WIMP + n) and Si (WIMP/10 + n) (per unit volume) Deeper site for further reduction in cosmic-ray background DAMA 100kg NaI CDMS (Latest) CDMS Stanford CRESST CDMS Soudan Genius Ge 100kg 12 m tank CDMS II Soudan Mine, Northern Minnesota 2300’ depth MINOS CDMS II Soudan II

  26. CDMS II Detector Deployment • Already demonstrated discrimination to < 10 event / kg / year • >99.9% rejection of photons >10 keV (~0.5 events/keV/kg/day) • >99% rejection of surface-electrons >15 keV (~0.05 events/keV/kg/day) • Identical Icebox, but no internal lead/poly, so fits seven Towers each with three Ge & three Si ZIP detectors • Total mass of Ge = 7 X 3 X 0.25 kg > 5 kg • Total mass of Si = 7 X 3 X 0.10 kg > 2 kg

  27. 2000-2005: CDMS II at Soudan • Reduce neutron background from ~1 / kg / day to ~1 / kg / year • Soudan: Depth 713 m (2000 mwe) • First detectors in Jan 2001 • Use layered polyethylene - lead - polyethylene shield (moderate the neutrons trapped inside the lead) Inner polyethylene detectors lead Outer polyethylene Active Muon Veto Top View Fridge

  28. 2002 2000 2003 2004 2005 2001 T1 SUF ENGR T1 S T1-2 S Soudan ready 1 tower in Soudan T1-4 S T1-7 S 2 tower2 in Soudan 30% 4 tower2 in Soudan 60% Begin Science Full Science Running CDMSII Deployment/Exposure Schedule • Scenario 1-2-4-7 tower deployments • Factor of ~10 improvement in ~1.5 years • Factor of ~2 improvement each subsequent year

  29. CDMS II goals @ Soudan (2070 mwe depth) • Goal: 0.01 evt/kg/day= 0.0003 evt/kg/keV/day 99.5%  rejection 95%  rejection 0.01 /kg /day Units: /kg/keV/day at 15 keV (5kg Ge, 2kg Si - 2500 kg-days in Ge) ~1 per 0.25-kg detector per year

  30. Sensitivity: CDMS II projections • Based on exposure versus time and expected backgrounds • 90% CL event-rate upper limit S90 • WIMP-nucleon cross section upper limit Wn(90) at M = 40 GeV

  31. Selected results & goals • CDMS I – best limit to date and first example of cryogenic detectors to surpass sensitivity of conventional detectors (HPGe, NaI) • CDMS II – at Soudan to be 100x more sensitive CDMS DAMA 100kg NaI CDMS Stanford CRESST CDMS Soudan Genius Ge 100kg 12 m tank

  32. Conclusion • Challenges met: technology is in hand • Challenges ahead • Fabrication/yield: control of tungsten Tc understood • More of the same re cleanliness & screening • Radon reduction/minimization • Activation of materials • Operating complex cryogenic experiment at remote deep site • If that weren’t hard enough… CryoArray: See R. Gaitskell’s talk in E6 on Mon 9 July • Description and goals for a 1000-kg experiment based on CDMS detectors • Goal of 100 event sample at 10-46 cm2, with <100 background events

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