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A photocathode is a negatively charged electrode in a light detection device such as the input screen in an image intensifier (II) that is coated with a photosensitive compound. When this is struck by light photons, the absorbed energy causes electron emission due to the photoelectric (PE) effect.
In an image intensifier, the photocathode is a thin layer of an alloy of antimony and cesium (SbCs3). The photocathode converts the light photons emitted by the entrance phosphor to photoelectrons via the PE effect:
- electrons are produced in direct proportion to the brightness of the input screen
- 10-20% conversion efficiency, i.e. approx. 400 electrons are released from the photocathode for each 60 keV x-ray photon absorbed by the phosphor
In order to preserve the image faithfully, the input phosphor and photocathode must be in intimate contact with each other, separated only by a thin transparent wafer which prevents chemical interaction between the two layers.
The light spectrum of the input phosphor should also match the sensitivity profile of the photocathode, to maximize the conversion efficiency from light photons to photoelectrons.
- 1. Bushberg JT, Siebert JA, Leidholdt EM & Boone JM. The Essential Physics of Medical Imaging, Lippincott, Williams & Wilkins, 3rd Edition (2011). p233