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Background Reduction in Cryogenic Detectors
Dan Bauer, Fermilab
LRT2004, Sudbury, December 13, 2004
Detector
Shielding
Veto
U/Th/K/Rn
,nU/Th/K/Rn
x
RockRock
n
LRT 2004 Dan Bauer
Cryogenic Dark Matter Search - CDMS
•Dark Matter Search Goal is direct detection of a few
WIMPS/year• Signature is nuclear recoil with
E<100 KeV
•Cryogenic Cool very pure Ge and Si crystals
to < 50 mK
•Active Background Rejection Detect both heat (phonons) and
charge• Nuclear recoils produce less
charge for the same heat as electron recoils
•Deep Underground (Soudan)• Fewer cosmic rays to produce
neutrons • Neutrons produce nuclear recoils
Detector Tower
DilutionRefrigerator
Shield/Muon Veto
Electronics and Data Acquisition
•Shielding (Pb, polyethylene, Cu)Reduce backgrounds from radioactivityActive scintillator veto against cosmic rays
LRT 2004 Dan Bauer
CDMS Background Rejection Strategy
Detector Rejection of Backgrounds
y
x
Phon
on ti
min
g
Y = Charge/phonons
Y =
Cha
rge/
phon
ons
Erecoil (keV)
gamma cal.
Charge yield: , Phonon timing: surface events ()
Multiple-scatters: n(also Si vs Ge rates) Position information: locate discrete sources
LRT 2004 Dan Bauer
CDMS Background Reduction Strategy
Layered shielding (reduce , , neutrons)~1 cm Cu walls of cold volume (cleanest material)Thin “mu-metal” magnetic shield (for SQUIDs)10 cm inner polyethylene (further neutron moderation)22.5 cm Pb, inner 5 cm is “ancient” (low in 210Pb)40 cm outer polyethylene (main neutron moderator)All materials near detectors screened for U/Th/K
Active Veto (reject events associated with cosmics)Hermetic, 2” thick plastic scintillator veto wrapped around shieldReject residual cosmic-ray induced eventsInformation stored as time history before detector triggersExpect > 99.99% efficiency for all , > 99% for interacting MC indicates > 60% efficiency for -induced showers from rock
LRT 2004 Dan Bauer
The Radon Problem• Radon levels high, vary seasonally at Soudan (200-700 Bq/m^3)
Decays include energetic gammas which can penetrate to detectors, and eject betas from Compton scatters (‘ejectrons’)
Need to displace Radon from region inside Pb shield Six purge tubes along stem shield penetrations
• Purge gas is medical grade breathing air ‘aged’ in metal cylinders for at least 2 weeks to allow decay of 90% of 222Rn
Radon variation at Soudan
0
100
200
300
400
500
600
700
800
Jun-01 Jan-02 Jul-02 Feb-03 Aug-03 Mar-04 Oct-04 Apr-05
LRT 2004 Dan Bauer
Measured Gamma Backgrounds•Typically “bulk” events
High ionization yield in detector bulk
Rejection 99.9999% at 70% nuclear recoil efficiency
•Sources Residual contamination in the
Pb, polyethylene and copper Environmental radon
• Three event classes Compton scatters from nearby
passive materials have low solid-angle for hitting detectors
Compton scatters from nearest neighbor can be vetoed
Dominant component is 1 in ~30000 gammas interacting in dead layer: expect <0.1 events in CDMSII (after timing cuts)
• Comparison of data and MC: Gammas from U/Th/K in Pb, Poly, Cu at
assayed level Radon between purged volume & Pb
• Fit concentration to data in summed spectra• 35 Bq/m3 compared with ambient ~500 Bq/m3
Fair agreement but actual radon level may be slightly lower based on:
• 609 keV 214-Bi line lower in data• 1765 keV 214-Bi line agrees
L. Baudis, UFL
U/Th/K: ~1/4 total rate
Radon: fit to data
LRT 2004 Dan Bauer
Measured Beta Backgrounds
• Typically surface events: rejected at 99.4% in present analysis
Timing 97% Ionization yield 80%
• Sources Residual contamination on detector and
nearby surfaces: “intrinsic” betas Soft x-rays Pb-210, K-40, C-14 primary focus
• Identification in situ direct counting
• Correlate with gammas and alphas surface science techniques
• Auger, SIMS, RBS+PIXE
• Rates Observe ~0.4/det/day on inner detectors Expect ~7 Events in CDMS-II for present
analysis and rate Modest improvements will keep us background free
• Important to ID and characterize these backgrounds for CDMSII
Robust leakage estimates• Convolve source spectrum in Monte
Carlo to model charge collection• Confirm with calibration/TF data
— Charge side— Phonon side
Depth (um)
Cha
rge
Eff
icie
ncy
J.-P. Thompson, Brown
LRT 2004 Dan Bauer
Sources of residual beta background
• Pb-210 — from airborne radon daughters Could be dominant source — further analysis needed Complex decay chain with numerous alphas and betas expect and observe
roughly equal numbers• Detailed simulations to check relative detection efficiency in progress
char
ge
Recoil Energy (keV) Recoil Energy (keV)
Eve
nts
J. Cooley-Sekula, Stanford
LRT 2004 Dan Bauer
Sources of residual beta background
• K-40 — from natural potassium Direct upper limit less than half observed rate 1460 keV gamma: lack of observed photopeak or compton edge sets
upper limit of 0.15 betas/det/day RBS+PIXE surface probe for natK and assumption that 40K is in standard
cosmogenic abundance limits rate to 0.04 betas/det/day
• C-14 — from natural carbon Auger spectroscopy and RBS indicate 2-3 monolayers of “adventitious”
carbon 0.3 betas/det/day to 156-keV endpoint 0.05 betas/det/day in 15-45
keV
• Work is ongoing Complete Pb-210 analysis Broaden scope to more possible isotopes Just beginning use of new technique: ICP-MS
• Inductively coupled plasma mass spectroscopy• Antimony found on test wafer - normalization not known yet
R. Schnee, D. Grant, Case; P. Cushman, A. Reisetter, U Minn
LRT 2004 Dan Bauer
Reduction of EM Backgrounds
• Reduce beta contamination via active screening/cleaning Observed alpha rate indicates dominated by 210Pb on detectors
• Improved radon purge should help, if this is correct Materials surface analysis (PIXE/RBS/SIMS/Auger) (in progress)
• Try to pinpoint source(s) of beta contamination Developing multiwire proportional chamber or cloud chamber as
dedicated alpha/beta screener (Tom Shutt talk)• Necessary for 17 beta emitters that have no screenable
gammas/alphas
• Reduce photon background via improved shielding Active (inexpensive) ionization
“endcap” detectors to shield against betas, identify multiple-scatters
Add inner ‘clean’ Pb shielding Improved gamma screening (Rick
Gaitskell talk)
LRT 2004 Dan Bauer
Neutron Backgrounds
• Predictions based on neutron propagation from rock and shield, normalized to Soudan muon flux
Expected <0.05 unvetoed neutrons in first data set - none observed Expected 1.9 vetoed neutrons - none observed (agrees at 85% CL)
• Should see ~ 5 vetoed neutrons in second data set Will allow normalization of Monte Carlos Observe one muon-coincident multiple-scatter nuclear recoil so far
• Ongoing work to refine estimates Direct measure of muon flux from veto Throw primary muon spectrum in Fluka + Geant4
• Hadron production• Correlations of particles from same parent muon• Simulate vetoed fraction of externally produced events• Predict 60% of “punch through” (>50 MeV) are vetoed by outer scintillator
Expect <0.2 unvetoed neutrons in full CDMS-II exposure• Will reach ‘natural’ neutron background limit at Soudan in a few years
S. Kamat, R. Hennings-Yeomans, Case; A. Reisetter, U Minn; J. Sander, H. Nelson, UCSB
LRT 2004 Dan Bauer
Neutron Reduction Strategies
Depth (meters water equivalent)
Mu
on
Flu
x (
m-2s-
1)
Super CDMS @ SNOLABAvoid the problem by reducing muon flux by
500x
CDMS II @ SoudanCould add inner neutron veto
LRT 2004 Dan Bauer
CDMS GoalMaintain Zero Background as MT increases
CDMS II Goal 1998
Tower 1: Fall 03Expected CDMSII end 2005
Expected Tower 1+2 Summer 04
Zero background 58% efficiency
Blue points illustrate random fluctuation from experiment to experiment
04/04/14
Currently 45% Z 2,3,5 > 10keV90% CL upper limit 0.005
Improvement linear until background events appearThen degrades as √MT until systematics dominate
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