Conventional Photocathodes Research Articles

Carbon nanotube based photocathodes

This paper describes a novel photocathode which is an array of vertically alignedmulti-walled carbon nanotubes (MWCNTs), each MWCNT being associatedwith one p–i–n photodiode. Unlike conventional photocathodes, the functionsof photon–electron conversion and subsequent electron emission are physicallyseparated. Photon–electron conversion is achieved with p–i–n photodiodes and theelectron emission occurs from the MWCNTs. The current modulation is highlyefficient as it uses an optically controlled reconfiguration of the electric field at theMWCNT locations. Such devices are compatible with high frequency and very largebandwidth operation and could lead to their application in compact, light and efficientmicrowave amplifiers for satellite telecommunication. To demonstrate this newphotocathode concept, we have fabricated the first carbon nanotube based photocathodeusing silicon p–i–n photodiodes and MWCNT bunches. Using a green laser, thisphotocathode delivers 0.5 mA with an internal quantum efficiency of 10% and anION/IOFF ratio of 30.

Negative-electron-affinity photoemitters

The operation of negative-electron-affinity photoemitters is discussed briefly and some of the important design considerations are listed. The spectral response and dark current of the new emitters are compared to those of conventional photoemitters. It is shown that in either property more than an order of magnitude improvement over conventional photocathodes is achieved in some representative negative-electron-affinity photoemitters.

3-5 compound photocathodes: A new family of photoemitters with greatly improved performance

Recently developed 3-5 compound semiconductor photocathodes show promise of substantial advantages over conventional photocathodes in increased yield and longer wavelength response. This paper reviews the physical principles of operation, preparation, and activation of these new types of photocathode. Also discussed are current fabrication techniques, problem areas, practical difficulties, and operating characteristics of experimental cathodes.

New Rugged High-Temperature Photomultipliers

A new photomultiplier has been designed which, up to temperatures of 150°C, is suitable for scintillation counting of the natural radioactivity of sediments traversed by an oil well. It uses a special head-on photocathode which is a compromise of stability, sensitivity and noise. The tube envelope is made of a succession of parallel Kovar and glass rings fused together. Venetian-blind dynodes of the Lallemand type are welded to the inner side of the Kovar rings and bleeder resistors to the outer side. This structure combines excellent mechanical properties (ruggedness, freedom from microphonics) with the very high electrical insulation needed, even at elevated temperatures. Taking advantage of the great versatility of the multiplier structure, several other types of tubes were built with conventional photocathodes (S-11 and S-17) and 14 and 18 stages. They are intended for applications requiring good performance associated with ruggedness.