Background issues for the Cryogenic Dark Matter Search

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Background issues for the Cryogenic Dark Matter
Search

• Laura Baudis
• Stanford University

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The Cryogenic Dark Matter Search

• phonon and ionization detectors to measure
  WMP-nucleus elastic scattering
• current location SUF 16 mwe
• future location Soudan 2000 mwe
• Run21 1 tower (4 Ge, 2 Si)
• Soudan total of 6 towers (7 kg Ge, 2 kg Si)

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CDMS detectors
measure phonons and ionization discrimination
between nuclear and electron recoils nuclear
recoils WIMPs, n electron recoils
g,e,a ionization yield Yionization/recoil
energy dependent on type of recoil electron
recoil Y1 nuclear recoil Y1/3
gt 99.8 gamma rejection
external gamma source
external neutron source
phonon trigger threshold
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Electron contamination!
???????????
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CDMS background goals
CDMS background goals
SUF 1 event/ kg d or 0.01 events/kg d
keV Soudan factor 100 improvement
0.01 events/kg d 1 event/100 kg d
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Background sources
Muon induced background internal neutrons n -gt
muon capture and low energy photo-nuclear
reactions in Cu cryostat and inner Pb
shield 100/kg d (Cu), 243/kg d (Pb) (veto
coincident) external neutrons n produced by
muon interactions outside the veto
(veto-anticoincident) Intrinsic radioactivity
of materials Ambient gamma and neutron background
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Layout of the CDMS I shield

• plastic scintill.
• 15 cm Pb
• 25 cm PE
• HPCu-cryostat
• 1cm inner Pb

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Electromagnetic background

• muon coincident 60 events/kg d keV
• muon anticoincident 2 events/kg d keV
• (veto efficiency gt 99.9 gt lt 0.1 events/kg d
  keV)
• residuals non-muon induced!
• radioactivity of materials surrounding the
  crystals
• single scatter photon background 0.5 ev/kg d keV
• with 99.9 rejection efficiency gt 0.0005 ev/kg d
  keV
• (SUF goal 0.01 ev/kg d keV)
• surface electron background 0.3 ev/kg d keV
• rejection efficiency gt 95 (ZIPs 99.7!)
• gt 0.015 ev/kg d keV

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ZIP Risetime Cut
trisegt31 s
gammas
neutrons
neutrons
triselt31 s
60 keV
betas
betas
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Neutrons from Rock
Ice Box, concentric Cu cans, outer radius 30 cm
HE m-nuclear interactions gt HE n n with E gt 50
MeV penetrate PE shield and produce LE sec. n (
lt 20 MeV) gt NR lt 100 keV rate from literature
has x4 uncertainty for 17 m.w.e. MC
simulations of m-induced hadron cascades yields
n-rate x3 higher than observed veto-anticoincident
NR due to vetoing of associated m and hadrons
( 40 rejection from n)?
n
Cold Stem
1 kg Ge Detectors
30 cm Poly Shield
15 cm Pb Shield
5 cm Plastic Scintillator
Dimensions give approximate radial thickness of
layers
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External neutron background

• absolute flux difficult to predict!
• can be measured
• compare NR rates in Si and Ge
• rate of multiple scatters gives a direct
• measurement of n background
• (WIMPs scatter only once!)

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CDMS uses Si and Ge detectors
WIMPs Ge has 6x higher interaction rate per kg
than Si Neutrons Si has 2x higher interaction
rate per kg thanGe Breaks the final degeneracy
in particle discrimination!
neutrons
WIMPS 40 GeV
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Data from 1998 and 1999 Data Runs
1999 4x165g Ge BLIP (10.6 kg d) 13 single
scatter nuclear recoils (1.2/kg/day) 4 multiple
scatter nuclear recoils (0.4/kg/day)
1998 100 g Si ZIP (1.6 kg days) 4 single scatter
nuclear recoils (2.5/kg/day)
all single-scatters nuclear recoil candidates
Analysis threshold (10 keV)
90 acceptance
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Comparison with with MC
Ge multiples and Si singles imply large expected
neutron background with large statistical
uncertainty
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Typical background spectra _at_ SUF
Nuclear recoil efficiency
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CDMS II Soudan
muon flux reduced x 104! 7 towers each with 3 Ge
3 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
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CDMS II background goals
factor 3
factor 15
factor 4 x 104
25 events expected for 7 kg yr exposure
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Is this achievable?
Gammas 99.5 discr. eff. assumed (99.9
reached) understand residual
background Betas 95 discr. eff. assumed
(99.7 for ZIPs) avoid
surface contaminations Neutrons m?flux reduced
by factor 104 _at_ Soudan
internal 99 eff. muon veto sufficient
external 1/3 of total expected
background (MC) (25 events for
7 kg yr exposure) better MC
needed
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MC simulations with FLUKA

• standalone FLUKA (http//fluka.web.cern.ch/fluka)
  
• most complete treatment of physical processes at
• high AND low energies (but not very user
  friendly...)
• simulate muon propagation hadron shower
  generation
• in tunnel save HE neutrons entering the tunnel
  and
• transport them in GEANT and/or in FLUKA
• later requires complete geometry in FLUKA, doable
  with
• help of ALIFE (http//AliSoft.cern.ch/offline/fluk
  a/ALIFE.html)
• better estimation of absolute n-flux
• correlations between n-hits and veto response

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What other backgrounds do we fear?
cosmogenics surface contaminations
(Rn-plateout)
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Cosmogenics
Activation of Si/Ge crystals and other materials
during production and transportation at the
Earths surface A precise calculation
requires cosmic ray spectrum (varies with
geomagnetic latitude) cross sections for the
production of isotopes Problem cross sections!
only few measured production is dominated by
(n,x) reactions 95
(p,x) reactions 5 Existing
programs use semiempirical formulas based on
data to calculate cross sections COSMO (Martoff
et al.) SIGMA (J.
Bockholt et al.)
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Cosmogenics in Ge
30 d exposure at see level, 1 year storage below
ground
COSMO
SIGMA
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Important for CDMS
realistic exposure 4 months above ground
estimations from Run 19 3H 1.34 x
COSMO 68Ge 1.26 x COSMO CDMS goal for gammas
95 /kg yr keV
3H 1.34 x 50 -gt not a problem ! 68Ge 1.26 x 2.5
x 103 !
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Cosmogenics in Si
Martoff, Science87
Modif. Cosmo
3 months at see level, 1 yr below ground
3H 47 ev/kg yr keV for 4 month exposure not a
problem!
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However...

• 3H production already in the right order of
  magnitude
• avoid any further activation
• store Ge/Si crystals and Cu in tunnel C _at_ SUF
• transport detectors via ground 10 h of flight
  125 d exposure!
• install PE shield box at SNF (fabrication site)
• 10 cm of PE reduce n-flux by factor of 30!

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The Radon problem
222Rn -gt 210Pb source 238U chain noble
gas colorless tasteless odorless
plateout adhesion of Rn daughters on surfaces
1 Bq 5 x 105 Rn atoms air 40/10 Bq/m3 (in/out)
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Radon plateout
the long lived 210Pb accumulates on surfaces and
decays b- Emax 63 keV most dangereous -gt
surface electrons amount of 210Pb on surfaces
depends on - exposure time t - Rn concentration
in air A - efficiency 222Rn (air) -gt 210Pb
(surface) p goal for CDMS II 10-4/cm2 keV d
scrubbing low p low t!
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Radon Scrubbing Facility _at_ Stanford
use for cleaning and assembly of ZIP detectors
and towers
inner room lt class 100
foyer
wetbench
antiroom
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Radon Scrubbing Facility
continuous particulate Rn monitoring particulate
s better than class 100 Rn 6 Bq/m3 but
scrubbing not started yet! goal factor 10 better
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Conclusions

• CDMS I
• background goal (1 ev/kg d) reached _at_ SUF
• sensitivity _at_ SUF limited by external
  n-background
• CDMS II
• ZIP technology 99.9 discrimination of bulk
  e-recoils
• 99.7
  discrimination of surface e
• still have to keep track of possible
  background sources!
• reach 100 times better sensitivity 1 event /
  100 kg d

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Neutron Multiple Scatters in Ge BLIPs
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
Ionization Yield B4,5