The XENON Project

1
The XENON Project

• A 1 tonne Liquid Xenon experiment for a sensitive
  Dark Matter Search
• Elena Aprile
• Columbia University

2
The XENON Collaboration

• Columbia University E. Aprile (Principal
  Investigator)
• T. Baltz, A. Curioni (graduate student), K-L.
  Giboni, C. Hailey, L. Hui,
• M. Kobayashi and K. Ni (graduate student)
• Brown University R. Gaitskell
• Princeton University T.Shutt
• Rice University U. Oberlack
• LLNL W. Craig

3
Current and Projected Limits of WIMP Searches

• Projection for CDMS Soudan (7kg GeSi) is 1
  event / kg / yr. Similar limits projected for
  competing experiments in Europe.
• It will take a substantial increase in target
  mass and superior background discrimination power
  to reach a sensitivity of 1 event / 100kg / yr.
• For a Xe target with lt10 keV recoil energy
  threshold this rate corresponds to a WIMP-nucleon
  s of 10-46 cm2
• A 1 tonne XENON experiment approaches this
  sensitivity, assuming 3.9x 10-5 cts /kg /d /keV
  background rate, 99.5 discrimination and 10
• keV recoil energy threshold .

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Liquid Xenon for a Dark Matter Detector

• Many favorable properties, from high Z (54) and
  density (3g/cc) for a compact instrument of
  flexible design, to high ionization (60,000 e/
  MeV) and scintillation yields if highly
  purified, to only stable isotopes ...etc but a
  low energy threshold is essential for a sensitive
  WIMP detector.

Expected rates of WIMP interactions in Xe and
other targets as a function of recoil energy
threshold for a 100 GeV WIMP with a s 10-9 pb
(from R. Schnee).
5
Ionization and Scintillation in Liquid Xenon
I/S (electron) gtgt I/S (non relativistic particle)
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The XENON Experiment Design Overview

• The XENON design is modular.
• An array of 10 independent 3D position
  sensitive LXeTPC modules, each with a 100 kg
  active Xe mass, is used to make the 1-tonne
  scale experiment.
• The TPC fiducial LXe volume is self-shielded by
  a few cm thick layer of additional LXe. The
  active scintillator shield is very effective for
  charged and neutral background rejection.
• One common vessel of 60 cm diameter and 60 cm
  height is used to house the TPC teflon and copper
  rings structure filled with the 100 kg Xe target
  and the 50 kg Xe for shielding.

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The XENON TPC Principle of Operation

• 30 cm drift gap to maximize active target ? long
  electron lifetime in LXe demonstrated
• 5 kV/cm drift field to detect small charge from
  nuclear recoils ? internal HV multiplier
  (Cockroft Walton type)
• Electrons extraction into gas phase to detect
  charge via proportional scintillation (1000 UV
  g/e/cm)? demonstrated
• Internal CsI photocathode with QE31 (Aprile et
  al. NIMA 338,1994) to enhance direct light
  signal and thus lower threshold ? demonstrated
• PMTs readout inside the TPC for direct and
  secondary light ? need PMTs with low activity
  from U/Th/K

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The XENON TPC Signals Nuclear Recoil
Discrimination

• Redundant information from charge (secondaryl
  light) signal (S2) and primary scintillation
  light (S1) signal from PMTs and CsI photocathode
• Background (g,e,a) produce electron recoils with
  S2/S1 gtgt0
• WIMPs (and neutrons) produce nuclear recoils with
  S2/S10
• Depth of interaction Z from timing and XY from
  c.o.g of PMT signals.
• 3D event localization for effective background
  rejection via fiducial volume cuts

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The Columbia LXeTPC for Gamma-Ray Astrophysics
30 kg active Xe mass 20 x 20 cm2 active area 8
cm drift gap with 4 kV/cm Charge and Light
readout for calorimetry and 3D imaging 1st
LXeTPC demonstration in near space
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Compton Imaging of MeV g-ray Sources with a LXeTPC
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Background Considerations for XENON

• ? and ? induced background
• 85Kr (?1/210.7y) 85Kr/Kr ? 2 x 10-11 in air
  giving 1Bq/m3
• Standard Xe gas contains 10ppm of Kr??10 Hz
  from 85Kr decays in 1 liter of LXe.
• Allowing lt1 85Kr decay/day i n XENON energy
  band ? lt1 ppb level of Kr in Xe
• 136Xe 2??? decay (?1/28 x 1021y) with Q 2.48
  MeV expected rate in
• XENON is 1 x 10-6 cts/kg/d/keV before any
  rejection
• Neutron induced background
• Muon induced neutrons spallation of 136Xe and
  134Xe ? take 10 mb and Homestake 4.4 kmwe?
  estimate 6 x 10-5 cts/kg/d before any rejection
• ? reduce by muon veto with 99 efficiency
• (?,n) neutrons from rock ?1000/n/m2/d from (?,n)
  reactions from U/Th of rock
• ? appropriate shield reduces this background to
  ?1 x 10-6 cts/kg/d/keV
• Neutrons from U/Th of detector materials within
  shield, neutrons from U/Th of
• detector components and vessel give?? 5 x 10-5
  cts/kg/d/keV
• ? lower it by x10 with materials selection

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Background Considerations for XENON

• ? -rays from U/Th/K contamination in PMTs and
  detector components dominate the background rate.
  For the PMTs contribution we have assumed a low
  activity version of the Hamamatsu R6041 ( ? 100
  cts/d ) consistent with recent measurements in
  Japan with a Hamamatsu R7281Q developed for the
  XMASS group (Moriyama et al., Xenon01 Workshop).

Numbers are based on Homestake location and
reflect 99.5 background rejection but no
reduction due to 3D imaging and active LXe
shield.
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Charge readout with GEMs a promising alternative

• High gain in pure Xe with 3GEMs demonstrated
• Coating of GEMs with CsI
• 2D readout for mm resolution

See Bondar et al.,Vienna01
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Proposed Strategy for XENON

• 10 kg prototype with all design parameters of
  100 kg module.
• Demonstrate electron drift over 30 cm and
  charge/light readout under high field operation.
  Test with electron/nuclear recoils.
• Study Kr removal techniques (distillation,
  cryopumping).
• Develop and test low activity PMTs (Hamamatsu
  metal channel and/or Burle MCP based).
• Test multi GEMs charge readout in LXe.
• Studies of low activity detector materials.
• Finalize the design of a 100 kg LXeTPC after
  2yrs of RD.
• Construction phase of 1st XENON module starts in
  3rd year.
• Enlarge collaboration and start underground
  operation by 2005.

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Summary

• Liquid Xenon is an excellent detector material
  well suited for the large target mass required
  for a sensitive Dark Matter experiment.
• The XENON experiment is proposed as an array of
  ten independent, self shielded, 3D position
  sensitive LXeTPCs each with 100 kg active mass.
• The detector design, largely based on established
  technology and gt10 yrs experience with LXe
  detectors development at Columbia, maximizes the
  fiducial volume and the signal information useful
  to distinguish the rare WIMP events from the
  large background.
• With a total mass of 1-tonne, a nuclear recoil
  discrimination gt 99.5 and
• a threshold of 10 keV, the projected
  sensitivity for XENON is ? 0.0001 events/kg/day
  in 3 yrs operation, covering most SUSY
  predictions.