The XENON10 dark matter search

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The XENON10 dark matter search

• T. Shutt
• Case Western Reserve University

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The XENON10 Collaboration
Columbia University Elena Aprile (PI),
Karl-Ludwig Giboni, Sharmila Kamat,
Maria Elena Monzani, , Guillaume Plante, and
Masaki Yamashita Brown University Richard
Gaitskell, Simon Fiorucci, Peter Sorensen, Luiz
DeViveiros University of Florida Laura Baudis,
Jesse Angle, Joerg Orboeck, Aaron Manalaysay
Lawrence Livermore National Laboratory Adam
Bernstein, Norm Madden and Celeste Winant Case
Western Reserve University Tom Shutt, Adam
Bradley, Paul Brusov, Eric Dahl, John Kwong and
Alexander Bolozdynya Rice University Uwe
Oberlack , Roman Gomez and Peter Shagin Yale
University Daniel McKinsey, Richard Hasty, Angel
Manzur, Kaixuan Ni LNGS Francesco Arneodo,
Alfredo Ferella Coimbra University Jose Matias
Lopes, Luis Coelho, Joaquim Santos
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Promise of liquid Xenon.

• Good WIMP target.

• Readily purified (except 85Kr)
• Self-shielding - high density, high Z.
• Can separate spin, no spin isotopes
• 129Xe, 130Xe, 131Xe, 132Xe, 134Xe, 136Xe

• Low-background PMTs available
• Rich detection media
• Scintillation
• Ionization
• Recombination discriminates between electron
  (backgrounds) and nuclear (WIMPs, neutrons)
  recoils

Scalable to large masses
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XENON Dual Phase, LXe TPC Calorimeter

• Very good 3D event location.
• Background discrimination based on recombination

• XENON Overview
• Modular design 1 ton in ten 100 kg modules.
• XENON10 Phase 15 kg active target in Gran Sasso
  Lab as of March, 2006. Shield under
  construction. Physics runs start June 2006.
• XENON100 Phase design/construction in FY07 and
  FY08 (2M construction). Commission and
  undeground start physics run with 2008.

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Scintillation Efficiency of Nuclear Recoils
Columbia and Yale
Columbia RARAF 2.4 MeV neutrons
p(t,3He)n
Borated Polyethylene
Lead
LXe
L 20 cm
?
Use pulse shape discrimination and ToF to
identify n-recoils
BC501A
Aprile et al., Phys. Rev. D 72 (2005) 072006
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Nuclear and electron recoils in LXe
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Charge and light yields
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Recombination fluctuations

• Recombination independent energy E W0 ? (ne-
  ng)
• Improves energy resolution
• Restores linearity.
• Recombination fluctuations fundamental issue for
  discrimination.
• New energy definition itself cannot improve
  discrimination

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Discrimination at low energy

• Charge yield increase for BOTH nuclear recoils
  and electron recoils at low energy.
• Egt 20 keVr recombination fluctuations dominate.
• Monte Carlo
• gt99 discrimination at 10 keVr. This is value
  used in XENON10/100/1T proposals

See T.Shutt, et al., astro-ph/0608137
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Some comments on Ar and Xe atomic physics
surprises

• Xe
• Drop in recombination for low energy nuclear
  recoils
• Energy independence of nuclear recoil
  recombination.
• Drop in recombination for very low energy
  electron recoils

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XY Position Reconstruction in 3 kg prototype

• Chisquare estimate from Monte Carlo - generated
  S2 map

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XENON10 Cryostat Assembly
Pulse tube cryocooler
Re-condenser
PMTs (top 48)
Gas Region
LXe Active
PMTs (bottom 41)
Vacuum Cryostat
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XENON10 Detector Assembly
89 Hamamatsu R5900 (1 square) 20 cm diameter, 15
cm drift length 22 kg LXe total 15 kg LXe active
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Summary XENON10 Backgrounds
Monte Carlo studies of Radioactivity (Background
Events) from Gamma / Electron ? Gammas inside
Pb Shield PMT (K/U/Th/Co) Vessel Stainless
Steel (Co) Contributions from Other
Components ? Xe Intrinsic Backgrounds (incl.
85Kr) ? External Gammas - Pb Shield ? Rn
exclusion ? Detector Performance/Design Gamma
Discrimination Requirements Use of xyz cuts
instead of LXe Outer Veto Neutron
Backgrounds ? Internal Sources PMT
(a,n) ? External Rock (a,n) Muons in
Shield ? Punch-through neutrons Generated by
muons in rock NOTE Active Muon Shield Not
Required for XENON10 _at_ LNGS ? Neutron flux from
muon interaction in Pb shield ltlt Target Level
Background Modeling U. FLORIDA / BROWN/COLUMBIA
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XENON10 Shield Construction - LNGS
Red-Shield Dimension Blue-Ex-LUNA Box Dimension
Brown Design / LNGS Engineering 40 Tonne Pb /
3.5 Tonne Poly Low-Activity (210Pb 30 Bq/kg)
inner Pb Normal Activity (210Pb 500 Bq/kg)
Outer Pb Construction Underway Contractor
COMASUD Mid May Expect Completion of
Installation LNGS (Ex-LUNA) Box Dimensions are
critical constraints for shield - expansion of
shield to accommodate much larger detector
difficult Inner Space for XENON10 detector 900
x 900 x 1075(h) mm
Crane Hook
2630 mm
2410 mm
3500 mm
200 mm
4400 mm
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XENON10 Punch-Through Neutron Backgrounds

• High Energy Neutrons from Muons in Rock
• Poly in shield is not efficient in moderating
  High Energy Muon-Induced Neutrons
• Depth is standard way to reduce high energy
  neutron flux (LNGS effective depth is 3050 mwe)
• Brown Monte Carlos show that
• Goal WIMP 1.3 evts/10 kg/mth LNGS depth
  comfortably achieves goal
• Goal WIMP 1.3 evts/100 kg/mth HE Neutrons evts
  1/6 rate of dark matter. Much reduced comfort
  margin. (Continue to study mitigation strategies)

Ratio HE Neutron BG / WIMP Signal
DM Goal(Rates for Current XENON10 Shield Design) NR Signal Rate Xe _at_ 16 keVr Soudan2.0 kmwe Gran Sasso3.0 kmwe Home-stake 4.3 kmwe
High Energy Neutron Relative Flux (from muons) x6.5 x1 x1/5
1.3 evts/10kg/mths 2x10-44 cm2 300 µdru x1/10 x1/60 x1/300
1.3 evts/100kg/mths 2x10-45 cm2 30 µdru x1.1 x1/6 x1/30
1.3 evts/1000kg/mths 2x10-46 cm2 3 µdru x11 x2 x1/3
Mei and Hime, astro-ph/0512125, calculate HE
neutron/dm 1/21/3 for Ge detectors at Gran
Sasso Depth
Additional margin could be achieved using
anti-coincidence signal from muon veto (none
currently in place for XENON10) around shield
(c.f. CDMSII simulations indicate factor 15
reduction may be possible by tagging pions also
generated in muon showers that generate HE
neutrons). However, it will be important to
verify that such a strategy works before relying
on it. (CDMSII Reisetter, U. Minn. / R
Hennings-Yeomans, CWRU)
For 2x10-46 cm2 also evaluating water/active
shielding wrt all types of background
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Kr removal

• 85Kr - beta decay, 687 keV endpoint.
• Goals for 10, 100, 1000 kg detectors Kr/Xe lt
  1000, 100, 10 ppt.
• Commercial Xe (SpectraGas, NJ) 5 ppb (XMASS)
• Chromatographic separation on charcoal column

25 Kg purifed to lt 10 ppt
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XENON10 expected background

• Dominant background Stainless Steel Cryostat
  PMTs
• Stainless Steel 100 mBq/kg 60Co
• 4x higher than originally assumed, but faster
  assembly
• PMTs - 89 x 1x1 sq Hamamatsu 8520
• 17.2/lt3.5/12.7/lt3.9 mBq/kg, U/Th/K/Co
• Increased Bg from high number of PMTs / trade off
  with increased position info. Bg diagnostic
• Expected background 0.4 cnts/kg/keV/day
  (before discr.)

• Analytical estimate
• Single, low-energy Compton scattering
• Very forward peaked.
• Probability of n scatters while traversing
  distance L

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XENON10 Underground at LNGS
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Test Mounting Of Detector (June 22)
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XENON10 Example Low Energy Event

• Low Energy Compton Scattering EventS115.4 phe
  6 keVeeDrift Time 38 µs 76 mm(Max depth 150
  mm)
• Bulk gamma calib shows avg S12.3 phe/keVee0.9
  phe/keVr
• Trigger ngt4 in 80 ns window
• Able to trigger on S1 for 10 keVr with gt90 eff
• Also catching S2 triggers (with pretrigger
  look-back)
• Noise on separate PMT chans ltlt0.1 phe equiv

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Status of XENON10

• 15 kg detector (8 kg fiducial) now operational
  in Gran Sasso
• First low background operations in shield started
  8/12.
• Calibrations ongoing
• Activated Xe soon
• 164 236 keV lines
• 80 keV beta.
• Neutron
• External gammas
• Background close to expectation
• Results soon.

SUSY TheoryModels
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Scaling LXe Detector Fiducial BG Reduction /1

• Compare LXe Detectors (factor 2 linear scale up
  each time)15 kg (ø21 cm x 15 cm) -gt 118 kg
  (ø42 cm x 30 cm) -gt 1041 kg (ø84 cm x 60 cm)
• Monte Carlos simply assume external activity
  scales with area (from PMTs and cryostat) using
  XENON10 values from screening

Low energy rate in FV before any ER vs NR
rejection /keVee/kg/day
15 kg
Gross Mass
118 kg
1041 kg
x2 linear
x2 linear
x10 reduction
gt102 reduction
(Brown)
Fiducial Mass
dru cts/keVee/kg/day
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Scaling LXe Detector Fiducial BG Reduction /2

• Assuming ER are rejected at 99 for 50
  acceptance of NR
• Diagonal dashed lines show background x exposure
  giving 1 event leakage
• If rej. 99 -gt 99.5 and acc. 50 -gt 100 then
  all s are better by 4x

Low energy rate in FV before any ER vs NR
rejection /keVee/kg/day
15 kg
Gross Mass
118 kg
1041 kg
x2 linear
x2 linear
s 4 10-43 cm2
XENON10 - 400 mdruee7 live-days x 8 kg fid
x10 reduction
1 leakage event (rej 99)
s 4 10-44 cm2
118 kg - 40 mdruee30 live-days x 20 kg fid
gt102 reduction
1 week
1 month
s 2 10-46 cm2
1041 kg - 0.2 mdruee1200 live-days x 100 kg fid
1 year
4 years
Reference Current CDMS II 90 CL s 2 10-43
cm2 for 100 GeV WIMP
Fiducial Mass
(Brown)
dru cts/keVee/kg/day