Dan Akerib
Case Western Reserve University
7 July 2001
Snowmass, Colorado
E6.2 Working Group
The CDMS I & II Experiments:
Challenges Met, Challenges Faced
Snowmass 2001, E6.2 Dark Matter Detection: CDMS D. Akerib, CWRU 2
The Cryogenic Dark Matter Search Collaboration
Case Western Reserve UniversityD.S. Akerib,D. Driscoll, S. Kamat, T.A. Perera, R.W. Schnee, G.Wang
Fermi National Accelerator LaboratoryM.B. Crisler, R. Dixon,
D. Holmgren
Lawrence Berkeley National LabR.J. McDonald, R.R. Ross
A. Smith
Nat’l Institute of Standards & Tech.J. Martinis
Princeton UniversityT. Shutt
Santa Clara UniversityB.A. Young
Stanford UniversityD. Abrams, L. Baudis, P.L. Brink,
B. Cabrera, C. Chang, T. Saab
University of California, BerkeleyS. Armel, V. Mandic, P. Meunier,
M. Perillo-Isaac, W. Rau, B. Sadoulet, A.L. Spadafora
University of California, Santa BarbaraD.A. Bauer, R. Bunker,
D.O. Caldwell, C. Maloney,
H. Nelson, J. Sander, S. Yellin
University of Colorado at DenverM. E. Huber
University College London/Brown Univ.R.J. Gaitskell
Snowmass 2001, E6.2 Dark Matter Detection: CDMS D. Akerib, CWRU 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 ~ 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
Snowmass 2001, E6.2 Dark Matter Detection: CDMS D. Akerib, CWRU 4
Direct Detection in the Galactic Halo
• 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 modulationWIMP detector
~10 keV energy nuclear recoil
WIMP-Nucleus Scattering
• 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
Snowmass 2001, E6.2 Dark Matter Detection: CDMS D. Akerib, CWRU 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
DAMA 100kg NaICDMS
CDMS Stanford
CDMS Soudan
CRESST
Genius Ge 100kg 12 m tank
Snowmass 2001, E6.2 Dark Matter Detection: CDMS D. Akerib, CWRU 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
polyethyleneouter moderator
detectors inner Pbshield
dilutionrefrigerator
Iceboxouter Pb shieldscintillator
veto
Ethermal
E char
ge
Background
Signal
Snowmass 2001, E6.2 Dark Matter Detection: CDMS D. Akerib, CWRU 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
Ethermal
E char
ge Background
Signal (Y =
Cha
rge
Yiel
d)
external gamma source
external neutron source
(blip
det
ecto
r)> 99.8% gamma rejection
Snowmass 2001, E6.2 Dark Matter Detection: CDMS D. Akerib, CWRU 8
Rb
NTD Gethermistors
ionizationmeasurementcircuit
ionizationcollectionelectrodes
lockin-amplifier
165 g p-type Ge
QinnerQouter
(electronics not shown)
Vpb
Vqb
• 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
Inner Ionization Electrode
Outer IonizationElectrode
Passive Ge shielding
(NTD-Ge thermistors on underside)
Tower• Wiring• heat sinking• holds cold FETs for
amplifiers
Berkeley Large Ionization- and Phonon-mediated Detectors
Germanium BLIP Detectors
Snowmass 2001, E6.2 Dark Matter Detection: CDMS D. Akerib, CWRU 9
ZIP Ionization & Phonon Detectors
Q inner
Q outer
A
B
D
C
Rbias
I bias
SQUID array Phonon D
Rfeedback
Vqbias
ZIP: At end of fabrication steps involving µm photolithography at Stanford Nanofabrication Facility
Fast athermal phonon technology Superconducting thin films of W/Al Stable Electrothermal Feedback
configuration Aluminum Quasiparticle Traps give
area coverage
Snowmass 2001, E6.2 Dark Matter Detection: CDMS D. Akerib, CWRU 10
collimator
X
y
Time delay
• Internal backgrounds Tends to surfaces or edges
• Wimps Uniform throughout bulk
(zip detector)
Position Sensitivity: fast phonon sensors
Snowmass 2001, E6.2 Dark Matter Detection: CDMS D. Akerib, CWRU 11
• 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
Rejection History
Improve electrode structure Fast phonon signal risetime
Snowmass 2001, E6.2 Dark Matter Detection: CDMS D. Akerib, CWRU 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
electron events
0 10 20 30 40 5010-1
100
101
102
103
PRE MUON VETO
Cu Fluorescence x-rays
Ga activation x-rays (640 eV fwhm)
Tritium Contamination (Fit shown)POST MUON VETO
Events in NUCLEARRECOIL BAND
Recoil Energy [keV]
9708241417HistR1516Post muon veto
Snowmass 2001, E6.2 Dark Matter Detection: CDMS D. Akerib, CWRU 13
• 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
Rejection History
Improve electrode structure Fast phonon signal risetime
Snowmass 2001, E6.2 Dark Matter Detection: CDMS D. Akerib, CWRU 14
• Electron Source (14C) probes charge collection at surface directly• Conventional p-type implanted contact shows ~30% collection
• Significant improvement with new blocking contact
Improved Charge Collection for Surface Events
Snowmass 2001, E6.2 Dark Matter Detection: CDMS D. Akerib, CWRU 15
• 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
Surface-Event Discrimination in BLIPs
616 Neutrons (external source)
1334 Photons (external source)
Ionization Threshold
233 Electrons (tagged contamination)
1999 SUF run
Snowmass 2001, E6.2 Dark Matter Detection: CDMS D. Akerib, CWRU 16
• 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
Rejection History
Improve electrode structure Fast phonon signal risetime
Snowmass 2001, E6.2 Dark Matter Detection: CDMS D. Akerib, CWRU 17
Surface-Event Discrimination in ZIPs: Risetimegammas
neutrons
Neutrons (low y, slow tr)
electrons
electrons
surf
ace
bu
lkR
ise
time Bulk events well
separated in charge yield…
Charge yield, y
…surface events not.
Snowmass 2001, E6.2 Dark Matter Detection: CDMS D. Akerib, CWRU 18
Summary of gamma/beta rejection history
• Steady improvement of rejection factors Can we continue trend to next generation?
Goals for CryoArray, see R.Gaitskell’s talk in E6, 9 July
(Background fraction that leaks through)
Snowmass 2001, E6.2 Dark Matter Detection: CDMS D. Akerib, CWRU 19
• 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
Rejection History
Improve electrode structure Fast phonon signal risetime
Succeeded with 1999 Data Set… see below
Snowmass 2001, E6.2 Dark Matter Detection: CDMS D. Akerib, CWRU 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
Single scatters
Nuclear recoils
Snowmass 2001, E6.2 Dark Matter Detection: CDMS D. Akerib, CWRU 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
Ioni
zatio
n Yi
eld
B6
Ionization Yield B4
photons
neutronneutrons
Ioni
zatio
n Yi
eld
B5,
6
Ionization Yield B4,5
surfaceelectrons
photons
Neighbor interaction
B4
B3
B5
B6
Non-Neighbor interaction
Neighbors Non-Neighbors
Snowmass 2001, E6.2 Dark Matter Detection: CDMS D. Akerib, CWRU 22
mostly neutrons
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
bulk events NR candidates
1998 CDMS Si Data (ZIP)
Snowmass 2001, E6.2 Dark Matter Detection: CDMS D. Akerib, CWRU 23
Dark Matter Limit from CDMS I
CDMS 1999
DAMA 3
DAMA 2
DAMA 1996
Ge ioniza
tion
Gondolo et alBottino et al
• 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)
Snowmass 2001, E6.2 Dark Matter Detection: CDMS D. Akerib, CWRU 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
CDMS bkg subtractedBest simultaneous fit to CDMS and DAMA predicts too little annual modulation in DAMA, too many events in CDMS
DAMA residual spectrum
Snowmass 2001, E6.2 Dark Matter Detection: CDMS D. Akerib, CWRU 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
Soudan Mine, Northern Minnesota
2300’ depth
CDMS IISoudan II
MINOS Genius Ge 100kg 12 m tankCDMS Soudan
CDMS StanfordDAMA 100kg NaI
CDMS (Latest)
CRESST
CDMS II
Snowmass 2001, E6.2 Dark Matter Detection: CDMS D. Akerib, CWRU 26
•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
CDMS II Detector Deployment
Snowmass 2001, E6.2 Dark Matter Detection: CDMS D. Akerib, CWRU 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)
Fridge
Outer polyethylene
Active Muon Veto
Inner polyethylene
lead
detectors
Top View
Snowmass 2001, E6.2 Dark Matter Detection: CDMS D. Akerib, CWRU 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
T1 S
T1 SUF
T1-4 S
2000 2001 2002 2003 2004 2005
Full Science Running
T1-7 S
T1-2 SSoudan ready
1 towerin Soudan
2 tower2in Soudan
30%4 tower2in Soudan
60%
Begin Science
ENGR
Snowmass 2001, E6.2 Dark Matter Detection: CDMS D. Akerib, CWRU 29
CDMS II goals @ Soudan (2070 mwe depth)
• Goal: 0.01 evt/kg/day= 0.0003 evt/kg/keV/dayBackground source Shielded Muon
Veto
After detector
rejection
Background
subtracted
Systematics
’s , external radioactivity 0.01 0.01 0.000 05
’s , cosmics in shield 0.002 5 0.000 025 0.000 000 2
’s, internal single scatters 0.25 0.25 0.001 3
Total ’s 0.26 0.26 0.001 4 0.000 22 0.000 07
’s, surface contamination 0.02 0.02 0.001 0 0.000 18 0.000 10
n’s, external radioactivity 0.000 005 0.000 005
n’s, cosmics in shield 0.000 5 0.000 005
n’s, cosmics in rock 0.000 1 0.000 1
Total neutrons 0.000 6 0.000 11 0.000 09 0.000 01
Total background 0.28 0.28 0.002 4 0.000 30 0.000 12
Units: /kg/keV/day at 15 keV(5kg Ge, 2kg Si - 2500 kg-days in Ge) ~1 per 0.25-kg detector per year
0.01 /kg /day
99.5% rejection
95% rejection
Snowmass 2001, E6.2 Dark Matter Detection: CDMS D. Akerib, CWRU 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
Snowmass 2001, E6.2 Dark Matter Detection: CDMS D. Akerib, CWRU 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
DAMA 100kg NaICDMS
CDMS Stanford
CDMS Soudan
CRESST
Genius Ge 100kg 12 m tank
Snowmass 2001, E6.2 Dark Matter Detection: CDMS D. Akerib, CWRU 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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