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photocathode(foh-toh-kath -ohd) An electrode in an electronic device, such as a photocell, photomultiplier, or image tube, that emits electrons when a beam of electromagnetic radiation strikes the surface. By a suitable choice of photocathode material, a reasonable response may be obtained from near-infrared wavelengths to low-energy X-ray wavelengths. The electrons result from the photoelectric effect. As many as 30% of the incident photons can liberate electrons, although the percentage is usually lower when taken over a wide spectral region. The current of resulting electrons increases linearly with radiation intensity over a wide range of intensities.
a cathode in certain vacuum-tube devices that emits electrons when exposed to light. Photocathodes are usually made of substances based on compounds consisting of elements from groups I and V or groups I and VI of the periodic system of elements.
The most widely used types of photocathodes are cesium oxide-silver, cesium antimonide (Cs3Sb), and trialkali photocathodes. Cesium oxide-silver photocathodes consist of Cs2O containing free cesium and free silver. Trialkali photocathodes are made of Sb-Cs, Sb-K, and Sb-Na compounds. The emissive material is deposited as a monomolecular layer on a metal or glass substrate. A photocathode may be either opaque or semitransparent. An opaque photocathode is exposed to light through the vacuum; a semitransparent photocathode is exposed through the substrate.
The main parameter characterizing the efficiency of a photocathode is the luminous sensitivity, which is equal to the ratio of the photoelectric current and the luminous flux that produces the current. For example, the luminous sensitivity of opaque cesium oxide-silver and cesium antimonide photocathodes is 100–120 microamperes per lumen (µA/lm); the luminous sensitivity of opaque trialkali photocathodes may be as high as 1,000 µA/lm, and that of semitransparent trialkali photocathodes is 600 µA/lm.
A new type of photocathode, called the negative-electron-affinity (NEA) photocathode, was developed in the 1960’s (seeELECTRON AFFINITY). NEA photocathodes include photocathodes made of III-V compounds—for example, GaAs photocathodes, which are sensitive to visible light, and InAsP and InGaAs photocathodes, which are sensitive to visible light and to infrared radiation at wavelengths of up to 1.5 micrometers. The luminous sensitivity of opaque NEA photocathodes may be as high as or even exceed 1,500 µA/lm. The luminous sensitivity of semitransparent NEA photocathodes is relatively low. Thus, the luminous sensitivity of GaAs photocathodes with a film thickness of 1–2 micrometers does not exceed 400(µA/lm; that is, it is lower than the luminous sensitivity of semitransparent trialkali photocathodes.
The production technology for NEA photocathodes is considerably more complex than that for conventional photocathodes. Hence, NEA photocathodes are not widely used.
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