Testing CsI photocathodes in Liquid Xenon

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Testing CsI photocathodes in Liquid Xenon

• Marsela Jorgolli
• XENON Project
• Summer REU 2005

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Outline of the talk

• The dark matter problem
• Searching for WIMPs
• The XENON experiment
• Using CsI photocathodes in a Liquid Xenon chamber
• Testing and results

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What is the Universe made of?

• Various cosmological observations point to a
  Concordance Model of the Universe. We know that
• Only 5 of the Universe is known.
• 95 stays hidden from view. Of this 22 is an
  exotic form of matter we call Dark Matter

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Evidence for dark matter comes from astronomical
studies Rotational curves of spiral galaxies
Bergstrom/hep-ph/0002126
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Estimating masses of clusters using gravitational
lensing
http//hubblesite.org
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What is Dark Matter?

• Two different types of dark matter are predicted
  
• Baryonic dark matter
• Black Holes
• Brown Dwarfs
• Non-baryonic dark matter
• Hot Dark Matter ? Particles traveling at
  relativistic velocities (neutrino)
• Cold Dark Matter ? Particles traveling at
  sub-relativistic velocities (WIMPs)

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Weakly Interacting Massive Particles (WIMPs) in a
galactic halo (artistic representation)
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• A SuperSymmetric solution?
• SuperSymmetry offers a candidate in the lightest
  SuperSymmetric particle ? the neutralino. The
  neutralino has favorable characteristics such as
• Stable and neutral.
• Weakly interacting not star-forming.
• Massive 20 1000 GeV/c2
• ? Candidate WIMP
• WIMPs may make up most of the dark matter in the
  Universe

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How is Dark Matter detected?

• Two complimentary methods are used for detection
  of dark matter
• Indirect detection ? detecting the annihilation
  products of dark matter
• Direct detection ? measuring the energy deposited
  by elastic scattering of a WIMP in a terrestrial
  target

WIMP-Nucleus Scattering
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Various Direct Detection Techniques

• DRIFT

• CDMS
• EDELWEISS

  100 detected energy relatively slow
requires cryogenic detectors

• XENON
• DAMA/LIBRA
• ZEPLIN

  few  detected energy usually fast no
surface effects ?

• CRESST
• ROSEBUD

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XENON

• A next generation Dark Matter Direct Detection
  experiment
• Dual phase Liquid/Gas Xenon Time Projection
  Chamber
• Currently a 10 kg module is being tested and
  will be placed at Gran Sasso Underground
  Laboratory in Italy
• Proposed to be scaled to 1 tone active mass

The LXeTPC module for XENON the 100kg fiducial
target is surrounded by an active LXe shield
enclosed in the Cu vessel.
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Why Liquid XENON?

• It is available in large quantities at a
  reasonable cost (1k/kg)
• Its high density (3g/cm3) and high atomic number
  (Z 54, A 131) allow for a compact and
  self-shielded detector geometry.
• As a detector material LXe has excellent
  ionization and scintillation properties
• High photon yield
• Fast time response
• Good stopping time
• It can be purified to achieve long distance drift
  of ionization electrons.
• Additional processing can reduce the traces of
  radioactive elements 85Kr, 42Ar, Ra to the low
  level required.

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Signal Detection and Discrimination between
Nuclear and Electron recoils with LXe

• Two signals are detected from each event
• Prompt Scintillation ? S1
• Proportional Scintillation from direct ionization
  ? S2
• Nuclear Recoils ? Slow, i.e. strong columnar
  recombination
• WIMPs, Neutrons
• Scintillation, weak ionization
• Electron Recoils ? Fast, i.e. weak columnar
  recombination
• ?,e-,?
• Scintillation, substantial ionization

High (99.5) event by event discrimination for e
/ n recoils possible by the distinct S2/S1 ratio
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Adding the CsI Photocathode

• The addition of CsI photocathode at the bottom of
  the chamber will generate a new signal since a
  substantial amount of light travels downward due
  to TIR in the gas/liquid interface
• ? from absorbing primary photons
• drifting the produced photoelectrons into the gXe
  
• ? detecting the proportional scintillation as a
  tertiary signal

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CsI Photocathode vs. other light detectors (PMT,
PD)

• Uniform response
• Reflective CsI photocathodes work well in the
  liquid rare gases
• Very low intrinsic radiation
• Can be made in large sizes at a low cost
• High sensitivity in Vacuum Ultra Violet photon
  detection
• Efficient electron extraction at
• Room temperature
• One atmosphere or low-pressure gas media

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My Summer Projectwith Dr. Singh and C. Macanka
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Making a photocathode

• High Vacuum
• Deposition
• Chamber for
• the
• Production of
• Photocathodes

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Steps of Making a photocathode

• CsI is placed inside the boat (Molybdenum)
• A polished stainless steel plate of the wanted
  dimensions is placed inside the deposition
  chamber
• The Chamber is tightly sealed
• Out gassing of the chamber is made by applying a
  current of 50Amps making sure not to boil the
  CsI
• The Chamber is left under high vaccum (10-6) for
  3 days for baking (out gassing is very
  important)
• The chamber and the plate are ready for deposition

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Making a photocathode

• Parameters controlled during deposition
• Temperature inside the chamber
• Vacuum
• Rate of Deposition (should be kept as constant
  as possible of uniform deposition)
• Thickness of CsI on the SS plate
• Current Applied

Data from July, 8th
Vacuum of the chamber Rate of Deposition Current Applied Evaporation Temperature Thickness of CsI on the plate
3.3 x 10-6 torr 0.5 1 Nm / sec 90 100 Amps 62 67 C 600 nm
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Testing a Photocathode

• Parallel plate Ionization Chamber
• ? - source (5.5 MeV) from anode (241Am)
• 3.5 mm between plates
• Reflective CsI photocathode placed on the bottom
  (facing up) connected to the Charge Sensitive
  Amplifier
• ?-particles collide with the Xe molecules ?
  scintillation and ionization
• Photons hit the CsI photocathode ? Photoelectrons
  are emitted through photoemission
• Signal collection by applying High Voltage
  connected to the anode
• () H.V. ? Light Collection
• (-) H.V. ? Charge Collection

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Better photoelectron extraction

• The electron affinities of LXe has been measured
  to be negative V0(LXe) -0.67 eV V0(CsI) -
  0.1 ev ? the CsI photocathode has a positive
  electron affinity in Lxe ? Photoelectrons will
  see a potential well ? photoelectron extraction
  is greatly enhanced
• The strong E-Field bends the the band structure
  of the CsI favoring the transport of conduction
  electrons in the CsI film towards the CsI-liquid
  interface
• Strong electric field also prevents the back
  diffusion of the electrons

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Testing Chamber while being cooled with LN2 ?
liquefying gXe
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Experimental Setup
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Experimental Techniques

• Photocathodes of two different sizes and of two
  different thickness were tested
• Size
• 6 cm in diameter
• 12 cm in diameter
• Thickness
• 5000 C
• 6000 C
• Test Chamber baked externally at
• 150 C
• for more than 24hrs
• 10-6 10-7 torr vacuum
• Xenon was purified once through getter and
  constantly afterwards

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Xenon Purification System
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Charge Calibration

• Charge calibration done by sending a test pulse
  coupled with a known capacitor to the input
• Typical offset pulse height
• (4 different offsets are used for calibration)

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Results with ?-particles

• Electronic noise determined by the test-pulse
  peak
• Typical pulse height spectra of the scintillation
  light from 241Am 5.5MeV ?-particles
• (07/22/2005)

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Results with ?-particles

• Photoelectron Yield and Direct Ionization Yield
  vs. Electric Field
• As the E-field increases the yield of both
  photoelectrons and ionization electrons increases

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Formulas used to do the calculations

• QE L / L0
• L ? Photoelectrons extracted from the
  photocathode
• L0 ? Photons hitting the photocathode
• Q / Q0 Charge collection
• Q ? Ionization charge that reaches the
  photocathode
• Q0 ? Ionization charge from the collisions
• Q or L (a peak) offset (test voltage) /
  (test peak) C / e
• Q0 Ea / Wcharge
• 5.5 MeV / 15.6 eV
• 3.74 x 105
• L0 Ea / Wlight LQ O/4p
• 1.35 x 105
• C 1 fC e 1.6 x 10-19 C LQ (Light Quench)
  0.9-1.0 O/4p 0.4

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Results with ?-particles

• Quantum
• Efficiency (QE)
• Increases with
• increasing
• Electric Field
• July 22, 2005

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Results with ?-particles

• Summary of tests from different dates and setups

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Comparison with published results
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Goals for the future

• Achieve optimal conditions for the production of
  CsI photocathodes on-site
• Make CsI photocathodes with high Quantum
  Efficiencies.
• Further testing to match published results of QEs