Vacuum problems of miniaturization of vacuum electronic components: a new generation of compact photomultipliers

Recently, a new generation of compact photomultipliers exhibiting high sensitivity, fast time response, extremely low background and extended dynamic range has been introduced. The electron multiplication is enabled by a high-gain and extremely low-background, closed-end, channel electron multiplier serving also as a body of the new mini electron tube. By preventing ultra-fine leaks by the introduction of ultra-sensitive leak detection, and incorporating a high performance getter material, the new channel photomultiplier becomes a very small volume static vacuum system with a relatively large inner surface, evolving gases with different outgassing rates. For an extended channel photomultiplier lifetime, it is of great importance to maintain extreme vacuum conditions and therefore the tube bodies have to be preconditioned by a thorough UHV baking followed by an effective electron and UV light scrubbing. Semitransparent photocathodes are remotely processed, onto many faceplates in one cycle, in order to maximize quantum efficiency at the chosen wavelength, minimize photocathode dark current and ensure excellent uniformity. The faceplates are hermetically sealed to the tube bodies during the course of a vacuum transfer technique. The tube shelf life is mainly dependent on the photocathode stability in an alkali-free environment inside the tube while the tube operational life is dependent on the intensity of ion bombardment of the photocathode reflecting both in the ejection of secondary electrons followed by the formation of spurious pulses and photocathode surface sputtering. Possible sources of chemically inert gases, which can affect the correct functioning of the tubes are discussed. r 2001 Elsevier Science Ltd. All rights reserved.