O.H.W. Siegmund

1
Window Transmission and Reflection Considerations
for LAPD Detectors

• O.H.W. Siegmund
• Experimental Astrophysics Group,
• Space Sciences Laboratory, U. California at
  Berkeley

GALEX M31
2
Window/Photocathode Configuration

• Windows for large area devices need to perform
  several functions
• Support vacuum
• Have high transmittance/ low reflection in the
  required bandpass
• Be a good substrate for photocathodes

AR coating
ITO coating
3
Standard Windows
Acceptable cutoff range
4
Standard Night Vision Windows
A. Smith NIM 2002
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Borofloat 33 General Parameters
Composition
Refractive index _at_400nm Borofloat 33
1.47 Air 1.0 Water 1.32
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Transmittance and Reflectance of Borofloat 33
Standard AR coating is bad for LAPD - need
re-optimization or dont use AR coating??
Transmittance is typical for borosilicate glasses
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Graded Film, Porous Borosilicate, AR
Minot, JOSA 1977
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Anti-Reflectance Coatings
Si
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Anti-Reflectance Coatings
Sample AR optimization for NaI(Tl) scintillator
10
Conductive Layer Properties for Windows
ITO is a good choice for the conductive underlayer
Braem, NIMA 2003
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Photocathode Reflectance
GaAs
Red cathode
S20 photocathodes
Blue cathode
12
Window Treatment Summary
Prospects for large windows - borofloat 33 looks
good AR coatings can be very important, but may
be unnecessary? Photocathode reflectivity is
potentially a big issue Have to have a conductive
underlayer, ITO good candidate