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Use of structured photomultiplier tube windows to enhance quantum efficiency.

A range of structures have been produced on the inside of the windows of ETL’s standard photomultiplier tubes in order to investigate their effect on the quantum efficiency the devices.

The structures have been produced in two distinctive ways:
- Sol-Gel: The sol-gel process has been used to create precise square-based pyramids in silica glass. The arrays of pyramids could then be incorporated into photomultiplier tubes. Different pyramid sizes and cone angles were tested and compared with the photocathode spectral response from a flat window.

- Laser-Ablation: A selection of windows of different glass types were laser ablated by LZH to produce microstructures (approx. 50micron) on their surfaces (see LZH report for details). These were incorporated into standard photomultiplier tubes and the quantum efficiencies compared between the flat and structured parts of the windows.

The ejection of photoelectrons from structured surfaces has been modelled in 3D using the Simion7 electrostatics program. Pyramids of different sizes and cone-angles were investigated. It was evident that features of the order of 1mm in size facilitated the escape of photoelectrons from the photocathode surface, while keeping the effects on the timing characteristics with acceptable limits.

Both sol-gel and laser-ablated structured cathodes give large increases in cathode quantum efficiency, i.e. up to x1.5 at 400nm, x1.8 at 550nm, x2.2 at 650nm, and x2.5 at 800nm). However, laser-ablation is not commercially viable with the current level of technology.

Relevance of this result:
ETL currently produces photomultiplier tubes with “prismatic” windows that have square–based pyramids, pressed into the window. These give enhancement of up to x1.6 at 550nm (for an S20 photocathode). The pressed structures do not have sharply defined features and it has been seen that enhancement is not uniform. Extra enhancement can be achieved with sol-gel structures due to the ability to for very precise repeatable structures and sharp features.

This technique is relatively simple to implement and works well for all photocathode types. Greater enhancement is observed, due to the extra interaction with the photocathode, at longer wavelengths where absorption of the light by the photocathode is weak.

Any general increase in the quantum efficiency of our products will make them more attractive in a highly competitive world market. A structured window may be used with little impact on the rest of a photomultiplier tube, making easy to incorporate into existing designs.

Current Status:
Laser ablated windows were successful in achieving gains in photomultiplier tube performance, but the technique is not commercially viable to be used in production.

Sol-gel windows were also very successful and ETL are interested in ways to viably incorporate the silica windows into our standard products. ETL are also interested in further possible sol-gel applications in cooperation with Novarra.

Informations connexes

Reported by

Electron Tubes Ltd
Bury Street
HA4 7TA Ruislip - Middx
United Kingdom