1LAPD Team Meeting 10/14/09

O.H.W. Siegmund, Experimental Astrophysics Group,

Space Sciences Laboratory,

U. California at Berkeley

Bialkali Photocathode Development at SSL

GALEX M31

2LAPD Team Meeting 10/14/09

““Blue” Photocathode OverviewBlue” Photocathode Overview

Semi-transparent cathodes (thin film on entrance window) are commonly used in the visible/NIR (night vision, etc).

Opaque cathodes are often used in the UV (astronomy) for large area detectors and are deposited onto microchannel plate surface.

Substrate

Cathode

e-e-

Opaquemode

Semitransparentmode

Photocathode Wavelength QE Environment

Multi-alkali 150-1000nm high Ultra high vacuum (UHV)

Basic standard is Bialkali for our wavelength range,which is most often a combination of Na, K, and Sb

3LAPD Team Meeting 10/14/09

Photocathode Configuration Photocathode Configuration

Numerous processes affect the QE

Bialkali is a few 100Å thick,and is compatiblewith depositionas semitransparenton the window,but is very difficultto achieve as anopaque cathode on the MCPsurface.

4LAPD Team Meeting 10/14/09

Photocathodes, 100nm - 600nmPhotocathodes, 100nm - 600nm

Emission spectrum of Cherenkov inwater compared with bialkali response.

Kapusta, IEEE 2007

Comparison of high efficiency Bialkali PMT cathodes, processing not compatible with LAPD!

5LAPD Team Meeting 10/14/09

Photocathode SummaryPhotocathode Summary

Comparison of Photonis cathodesComparison of various cathodes

6LAPD Team Meeting 10/14/09

Multialkali Photocathode QEMultialkali Photocathode QE

Hamamatsu Bialkali photocathodeComparison of multialkali Photocathodes.

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.

7LAPD Team Meeting 10/14/09

Bialkali Photocathode QEBialkali Photocathode QE

Examples of bialkali photocathode depositions with different wavelength optimizations (on fiber optics).

0

0.2

0.4

0.6

0.8

1

1.2

350 400 450 500 550 600 650

AST (t)AST (d)STL02 (d)Cherenkov

Wavelength (nm)

8LAPD Team Meeting 10/14/09

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

9LAPD Team Meeting 10/14/09

4” Tube and Cathode Process Tank System

Has been used to processalkali cathodes and tubesof small area (<4”). Systemof choice for test cathodeprocesses.- 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

10LAPD Team Meeting 10/14/09

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.

11LAPD Team Meeting 10/14/09

Small Cathode Process Tank System

Detector with MCPs and Indium seal ring.

Fused silica window,metalized, in holder.

Window can be movedfrom cathode processingstation onto tube usingexternal manipulator.

12LAPD Team Meeting 10/14/09

Window Seal Development and Window Seal Development and Cathode Test “Diodes”Cathode Test “Diodes”

• Can metalize window and produce an Indium seal on a frame to Can metalize window and produce an Indium seal on a frame to test leak tightness of the seal.test leak tightness of the seal.

• Can also make a cathode and produce an Indium sealed “diode” Can also make a cathode and produce an Indium sealed “diode” on a simple ceramic spacer with metal anode to test stability.on a simple ceramic spacer with metal anode to test stability.

13LAPD Team Meeting 10/14/09

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

14LAPD Team Meeting 10/14/09

8” Tube and Cathode Process Tank System

UHV 16” processChamber:-Initial use for cathode and seal development.Eventually use for complete tubeprocess task.

Borofloat 338.66” windows- large area cathodes- uniformity tests- QE optimization

“Smokey Joe” tube process station- or - BBQ/meat smoker

15LAPD Team Meeting 10/14/09

Cathode Development SummaryCathode Development Summary

• Bialkali cathodes are still the standard for the wavelength Bialkali cathodes are still the standard for the wavelength regimeregime

• Establish Semitransparent Bialkali processes and window Establish Semitransparent Bialkali processes and window treatments on small 1.2” substratestreatments on small 1.2” substrates

• Test intermediate window size (4”) to confirm Test intermediate window size (4”) to confirm scaleability for larger sizes while the large station is scaleability for larger sizes while the large station is being commissionedbeing commissioned

• Confirm that bialkalis can be deposited on large windowsConfirm that bialkalis can be deposited on large windowsand sealed into leak tight enclosuresand sealed into leak tight enclosures

• Make trial full up LAPD detector(s)Make trial full up LAPD detector(s)