Dan Akerib

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
The Cryogenic Dark Matter Search Collaboration

• 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
• Natl 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

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
Direct Detection in the Galactic Halo

• Galactic halo 20 Machos
• 8 50 _at_ 95C.L.
• Basic paradigm intact
• Direct detection scattering experiment
• Few keV recoil energy
• lt 1 event/kg/d
• Background suppression/rejection
• Low energy threshold
• Signal modulation

• 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
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
Two Signals Reject the Background

• 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

gt 99.8 gamma rejection
external gamma source
(blip detector)
(Y Charge Yield)
external neutron source
8
Germanium BLIP Detectors
Berkeley Large 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

• 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 _at_ 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
Electron Backgrounds

• 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 _at_ 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
• gt95 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 _at_ 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.Gaitskells talk in
E6, 9 July
19
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 _at_ 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
1999 CDMS Ge Data (BLIP)

• 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

21
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)

• 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 setlt 0.26 events in 20-100
  keV range at 90 CL

mostly neutrons
23
Dark Matter Limit from CDMS I

• 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
• gt 99.8 CL
• PRL 84, 19 June 2000
• astro-ph/0002471
• Detailed PRD in preparation with increased
  fiducial mass (2x)

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 gt 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

• 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

Soudan Mine, Northern Minnesota 2300 depth
MINOS
CDMS II
Soudan II
26
CDMS II Detector Deployment

• Already demonstrated discrimination to lt 10 event
  / kg / year
• gt99.9 rejection of photons gt10 keV (0.5
  events/keV/kg/day)
• gt99 rejection of surface-electrons gt15 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 gt 5 kg
• Total mass of Si 7 X 3 X 0.10 kg gt 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
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 _at_ 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 werent hard enough CryoArray See R.
  Gaitskells 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 lt100
  background events