![]() The effectiveness of a photocathode is commonly expressed as quantum efficiency, that being the ratio of emitted electrons vs. A variation is the double reflection type, where the metal base is mirror-like, causing light that passed through the photocathode without causing emission to be bounced back for a second try. A reflective type is typically formed on an opaque metal electrode base, where the light enters and the electrons exit from the same side. A transmission type is typically a coating upon a glass window in which the light strikes one surface and electrons exit from the opposite surface. Photocathodes divide into two broad groups transmission and reflective. By using a substrate with matched lattice parameters, crystalline photocathodes can be made and electron beams can come out from the same position in lattice's Brillouin zone to get high brightness electron beams. Molecular beam epitaxy is broadly applied in today's manufacturing of photocathode. In operation the photocathode requires an electric field with a nearby positive anode to assure electron emission. Most cathodes are sensitive to air the construction of photocathodes typically occurs after the enclosure has been evacuated. Photocathodes operate in a vacuum, so their design parallels vacuum tube technology. The more recent development of solid state optical devices such as photodiodes has reduced the use of photocathodes to cases where they still remain superior to semiconductor devices. Phototubes have been used for years in movie projectors to read the sound tracks on the edge of movie film. Simple phototubes were used for motion detectors and counters. It was therefore the key element in opto-electronic devices, such as TV camera tubes like the orthicon and vidicon, and in image tubes such as intensifiers, converters, and dissectors. As such it tends to function as a form of 'electric film' and shared many characteristics of photography. Uses įor many years the photocathode was the only practical method for converting light to an electron current. Lifetime is then the time constant of the exponential. Cathode death is modeled as a decaying exponential as a function of either time or emitted charge. These effects are quantified by the lifetime of the photocathode. Additionally, using the photocathodes in high current applications will slowly damage the compounds as they are exposed to ion back-bombardment. Many photocathodes require excellent vacuum conditions to function and will become "poisoned" when exposed to contaminates. This is important for applications such as image intensifiers, wavelength converters, and the now obsolete image tubes. Outside of accelerator physics, MTE and thermal emittance play a role in the resolution of proximity-focused imaging devices that use photocathodes. An ideal band structure for low MTEs is one that does not allow photoemission from large transverse momentum states. ĭue to conservation of transverse momentum and energy in the photoemission process, the MTE of a clean, atomically-ordered, single crystalline photocathode is determined by the material's band structure. Quantum efficiency may be calculated from photocurrent ( I is the temperature of electrons in the solid. For many applications, QE is the most important property as the photocathodes are used solely for converting photons into an electrical signal. This property depends on the wavelength of light being used to illuminate the photocathode. It is the ratio of the number of electrons emitted to the number of incident photons. Quantum efficiency is a unitless number that measures the sensitivity of the photocathode to light. Important Properties Quantum Efficiency (QE) Photocathodes are also commonly used as the negatively charged electrode in a light detection device such as a photomultiplier, phototube and image intensifier. Electron beams generated with photocathodes are commonly used for free electron lasers and for ultrafast electron diffraction. Photocathodes are important in accelerator physics where they are utilised in a photoinjector to generate high brightness electron beams. Cs-K-Sb photocathode centered on a Molybdenum plug (a) after growth in the preparation chamber and (b) after transfer into the photoinjectorĪ photocathode is a surface engineered to convert light ( photons) into electrons using the photoelectric effect.
0 Comments
Leave a Reply. |