Proportional Light in a

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Proportional Light in a Dual Phase Xenon Chamber


Elena Aprile for the XENON Collaboration Physics
Department and Columbia Astrophysics Laboratory
Columbia University, New York
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The XENON Collaboration
Columbia University Elena Aprile
(PI),Karl-Ludwig Giboni ,Chuck Hailey ,Pawel
Majewski, Kaixuan Ni and Masaki Yamashita Rice
University Uwe Oberlack ,Omar Vargas Yale
University Daniel McKinsey Princeton University
John Kwong, Tom Hartmann, Kirk McDonald,
Nathaniel Ross, Tom Shutt Brown University
Richard Gaitskell, Peter Sorensen, Luiz
DeViveiros Lawrence Livermore National
Laboratory William Craig, Norm Madden University
of Florida Laura Baudis
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The XENON Dark Matter Experiment

• Dual Phase Liquid/Gas Xe
• The XENON design is modular
• Multiple 3D position sensitive LXeTPC modules,
  each with a 100 kg active Xe mass --gt 1-tonne
  scale experiment.
• The 100 kg fiducial LXe volume of each module is
  shielded by additional 50 kg LXe. Active shield
  very effective for charged and neutral
  background rejection
• Proposed Sept. 2001.
• Funded Sept. 2002.
• Currently - RD towards 10 kg prototype.
• Proposal for XENON100 submitted Oct. 2003

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XENON Dark Matter Sensitivity
http//dmtools.berkeley.edu
Edelweiss (June 2002)
0.25 event/kg/d
1 event/kg/yr
1 event/100 kg/yr
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Liquid Xenon for Dark Matter WIMPs

• High mass Xe nucleus (A 131) good for WIMPs S.I.
  Int. ( s A2 )
• Odd Isotopes with large spin-dependent
  enhancement factors
• High atomic number (Z54) and density (r3g/cc)
  of liquid state good for compact and flexible
  detector geometry
• Production and purification of Xe with ltlt 1ppb O2
  in large quantities for tonne scale experiment.
  Easy cryogenics at 100 C.
• Excellent ionizer and scintillator with distinct
  charge/light ratio for electron/nuclear energy
  deposits for background rejection
• No long-lived radioactive isotopes. 85Kr
  reducible to ppt level

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Electron vs Nuclear Recoil Discrimination in
XENON
Measure both direct scintillation(S1) and charge
(proportional scintillation) (S2)

• Nuclear recoil from
• WIMP
• Neutron
• Electron recoil from
• gamma
• Electron
• Alpha

Gas
1µs
anode
Proportional scintillation depends on type of
recoil and applied electric field. electron
recoil ? S2 gtgt S1 nuclear recoil ? S2 lt S1 but
detectable if E large
grid
Drift Time
e-
E
Liquid
40ns
cathode
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Outline

• Operation of a single phase xenon chamber with
  PMT in LXe
• Chamber description, experiment setups
• Charge collection and electron lifetime
• Light and charge correlation
• Light collection improvement with PTFE
• Operation of a dual phase xenon chamber
• Operation technique, chamber parameters
• Direct and proportional light waveforms, method
  of analysis
• Proportional light spectrum
• Properties of electron emission and proportional
  scintillation
• Electron emission yield with extraction field
• Proportional light yield as a function of field
  and pressure
• Ratio between direct and proportional light

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Single phase LXe Detector with Charge and Light
PMT
Q
Anode
Vg
Grid
LXe
E
Bi-207
Vc
Cathode

• 1cm between Grid and Cathode, 5mm between Anode
  and Grid.
• Gamma rays (570keV and 1064keV) and electrons
  (554keV and 976keV) from Bi-207 deposited on the
  center of Cathode.
• Direct scintillation light read out by PMT
  (Hamamatsu R6041) immersed in LXe.
• Ionization electrons read out by
  charge-sensitive pre-amplifier.

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Charge collection
570keV

• Charge collection calculated assuming W- value
  of 15.6 eV for LXe.
• With PMT and its HV divider in the LXe, a good
  charge collection was achieved after several
  cycles of purification and baking of chamber.

1064keV
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Electron lifetime
1064 keV gamma ray
976keV electron
554keV electron
570keV gamma ray

• Event drift time defined with respect to the
  scintillation light trigger.
• A linear fit of the 570keV Gamma ray line shows
  a lifetime of about 1.5ms.
• Electron drift velocity at 1kV/cm is about 2mm/µs

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Light and charge correlation

• There is clear anti-correlation between
  ionization (charge) and scintillation (light) in
  liquid xenon.
• Energy resolution can be improved by combining
  charge and light signals.

570keV Gamma Rays
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Light collection improvement with PTFE

• Adding PTFE wall and PTFE piece on the bottom
  improved the light collection efficiency.
• The purity level of liquid xenon is not affected
  by PTFE.
• Light spectrum of Bi-207 at zero field was
  obtained with the PTFE structure.

Light Spectrum, Bi-207
570keV
PTFE
1064keV
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Operation of a dual phase xenon chamber

• Liquid level is below the Grid
• No ionization electron can escape from LXe to
  GXe at low extraction field (0.5kV/cm) gt No
  proportional light is produced

4kV/cm
GXe
Vg
E
0.5kV/cm
LXe
Bi-207
Vc
Proportional light

• Liquid level is above the Grid and below the
  Anode
• Ionization electron can be extracted from LXe to
  GXe at high extraction field (4kV/cm) gt
  proportional light is abundantly produced

GXe
4kV/cm
Vg
LXe
E
0.5kV/cm
Bi-207
Vc
GXe

• Liquid level is above the Anode
• Ionization electrons are collected by the Anode,
  no GXe gt no proportional light is produced

4kV/cm
Vg
LXe
E
0.5kV/cm
Bi-207
Vc
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Method of analysis of waveforms
Low energy x-ray from Bi-207
Bi-207 energy spectrum, readout from proportional
light signals
Direct light
Proportional light
Bi-207 energy spectrum, readout from charge
signals directly.

• Event waveform shows both direct and
  proportional light signals
• The area of the proportional light pulse is
  proportional to the ionization electrons.
  Spectrum of Bi-207 from the proportional light is
  compared with charge spectrum in a single phase
  operation (right)
• Lower energy threshold can be reached from
  proportional light

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Electron emission and proportional scintillation

• The negative ground state energy of quasi-free
  electron in liquid xenon requires an electric
  field to extract electron from LXe to GXe.
• For a full extraction of ionization electrons to
  gas phase, a field of 10kV/cm in the gas xenon is
  needed from our data.

• The proportional light yield is related to the
  field in the gas, the gas gap and the gas
  pressure Bolozdynya, NIM A 99
• The proportional light yield has been measured
  as a function of reduced field (field/pressure)

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Ratio between direct and proportional light

• The ratio between proportional and direct light
  for Bi-207 is measured to be about 500 at 1kV/cm
  drift field and 4kV/cm in the gas phase. The gas
  pressure is around 2atm.
• With improved geometry and control of the liquid
  level, we will improve light collection
  efficiency and energy resolution, and will lower
  the energy threshold.
• The ratio of proportional and direct light will
  be measured for low energy electron recoils and
  nuclear recoils with improved chamber.

570keV