Testing CsI photocathodes
in Liquid Xenon
Marsela Jorgolli
XENON Project
Summer REU 2005
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
Universe is known.
the
 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
WIMP-Nucleus Scattering
WIMP
Direct detection 
measuring the energy
deposited by elastic
scattering of a WIMP in a
terrestrial target
Marsela Jorgolli, Summer REU 2005
detector
energy transferred appears in
the recoiling nucleus
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Various Direct Detection
Techniques
Ge
Ge, Si
 DRIFT
Ionization
Liquid Xe
 CDMS
 EDELWEISS
Heat
• ≈ 100% detected energy
• relatively slow
• requires cryogenic
detectors
 XENON
 DAMA/LIBRA
 ZEPLIN
Light
NaI, Xe
• ≈ few % detected energy
• usually fast
• no surface effects ?
Al2O3, LiF
CaWO4, BGO
 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:
o High photon yield
o Fast time response
o 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 Project
with 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
~ 600
nm
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°C
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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 = [(α peak) + offset] * (test voltage) / (test peak) * C / e
Q0 = Eα / Wcharge
= 5.5 MeV / 15.6 eV
= 3.74 x 105
L0 = Eα / Wlight * LQ * Ω/4π
= 1.35 x 105
C = 1 fC; e = 1.6 x 10-19 C; LQ (Light Quench) = 0.9-1.0; Ω/4π = 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
QE vs. Electric Field
Quantum Efficiency (%)
25
31-May
20
3-Jun
15
13-Jun
22-Jun
10
7-Jul
12-Jul
5
22-Jul
0
0
2
4
6
8
Electric Field (kV/cm)
10
12
14
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
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