Bialkali Photocathode Development
at SSL
O.H.W. Siegmund,
Experimental Astrophysics Group,
Space Sciences Laboratory,
U. California at Berkeley
GALEX
M31
LAPD Team Meeting 10/14/09
1
“Blue” Photocathode Overview
Semi-transparent cathodes
(thin film on entrance window) are
commonly used in the visible/NIR (night
vision, etc).
Semit ransparent
mo de
Subst rat e
Opaque cathodes are often used in
the UV (astronomy) for large area
detectors and are deposited onto
microchannel plate surface.
Photocathode
Multi-alkali
Wavelength
150-1000nm
Cat hode
e-
eOp aque
mo de
QE
high
Environment
Ultra high vacuum (UHV)
Basic standard is Bialkali for our wavelength range,
which is most often a combination of Na, K, and Sb
LAPD Team Meeting 10/14/09
2
Photocathode Configuration
Numerous processes affect the QE
Bialkali is a
few 100Å thick,
and is compatible
with deposition
as semitransparent
on the window,
but is very difficult
to achieve as an
opaque cathode
on the MCP
surface.
LAPD Team Meeting 10/14/09
3
Photocathodes, 100nm - 600nm
Kapusta, IEEE 2007
Comparison of high efficiency Bialkali PMT
Emission spectrum of Cherenkov in
water compared with bialkali response. cathodes, processing not compatible with LAPD!
LAPD Team Meeting 10/14/09
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Photocathode Summary
Comparison of Photonis cathodes
Comparison of various cathodes
LAPD Team Meeting 10/14/09
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Multialkali Photocathode QE
Most photocathodes in this arena are semitransparent, deposited on the inside of the window.
However opaque photocathodes are generally higher efficiency and less dependent on thickness.
Hamamatsu Bialkali photocathode
Comparison of multialkali
Photocathodes.
LAPD Team Meeting 10/14/09
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Bialkali Photocathode QE
Relative Efficiency/Emission
1.2
AST (t)
AST (d)
STL 02 (d )
Chere nko v
1
0.8
0.6
0.4
0.2
0
350
400
450
500
550
600
650
Wavelength (nm)
Examples of bialkali photocathode depositions with different
wavelength optimizations (on fiber optics).
LAPD Team Meeting 10/14/09
7
4” Tube and Cathode Process Tank System
Small window cathode development, 1.3” samples
- Process samples to optimize QE and bandpass
- Use several substrate materials, verify Borofloat 33
- Apply AR coating/processes and test cathodes
- Test metalization and sealing techniques
Medium size window cathode study, 4” samples
- Study source alkali design for large cathodes
- Test metal/ITOconductor underlayer for cathodes
- Develop techniques to make larger area uniform QE
- Optimize cathode QE levels
- Trial metalization and seals on 4in “frames”
- Seal diode tubes to verify stability
LAPD Team Meeting 10/14/09
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4” Tube and Cathode Process Tank System
Has been used to process
alkali cathodes and tubes
of small area (<4”). System
of choice for test cathode
processes.
- Can take 6 samples/run
- Small sample test runs
- Substrate material tests
- AR coating tests
- Sealing tests to 4”
- Cathodes up to 4”
-Uniformity & QE
LAPD Team Meeting 10/14/09
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1.3” Cathode Substrates
We have cut up one borofloat 33 window to make 30ea 1.3”
test samples, also have 18ea fused silica as control samples.
LAPD Team Meeting 10/14/09
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Small Cathode Process Tank System
Window can be moved
from cathode processing
station onto tube using
external manipulator.
Detector with MCPs and
Indium seal ring.
LAPD Team Meeting 10/14/09
Fused silica window,
metalized, in holder.
11
Window Seal Development and
Cathode Test “Diodes”
• Can metalize window and produce an Indium seal on a frame to
test leak tightness of the seal.
• Can also make a cathode and produce an Indium sealed “diode”
on a simple ceramic spacer with metal anode to test stability.
LAPD Team Meeting 10/14/09
12
8” Tube and Cathode Process Tank System
Large window cathode development, 8.66” square
- Implement alkali source design for large cathode areas
- Develop wet cleaning and plasma cleaning processes
- Establish metalization scheme
- Test metal/ITO conductor underlayer for 8” cathodes
- Implement AR coatings/treatment on both surfaces
- Develop techniques to make 8” area uniform QE
- Optimize cathode QE levels
- Trial seals on 8.66” “frames”
- Then
- Make LAPD 8” tubes
LAPD Team Meeting 10/14/09
13
8” Tube and Cathode Process Tank System
UHV 16” process
Chamber:Initial use for
cathode and seal
development.
Eventually use for
complete tube
process task.
“Smokey Joe” tube process station
- or - BBQ/meat smoker
LAPD Team Meeting 10/14/09
14
Borofloat 33
8.66” windows
- large area cathodes
- uniformity tests
- QE optimization
Cathode Development Summary
• Bialkali cathodes are still the standard for the wavelength
•
•
•
•
regime
Establish Semitransparent Bialkali processes and window
treatments on small 1.2” substrates
Test intermediate window size (4”) to confirm
scaleability for larger sizes while the large station is
being commissioned
Confirm that bialkalis can be deposited on large windows
and sealed into leak tight enclosures
Make trial full up LAPD detector(s)
LAPD Team Meeting 10/14/09
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