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

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


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


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


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


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


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


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


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


collimator

X

y

Time delay

• Internal backgrounds Tends to surfaces or edges

• Wimps Uniform throughout bulk

(zip detector)

Position Sensitivity: fast phonon sensors


• 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


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








Rejection History



• 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


• 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








Rejection History



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.


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)








Rejection History


Succeeded with 1999 Data Set… see below


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


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


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)


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)


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


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


•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


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


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


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


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


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


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

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

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