Photocathode 488159 198825160 2008-03-17T10:09:42Z Velela 136926 sp as per [[WP:SULF]] In a [[photomultiplier]] or [[Phototube]], a '''photocathode''' is a negatively charged [[electrode]] coated with a [[photosensitive]] compound. When this is struck by light, electrons are emitted due to the [[photoelectric effect]]. Although a plain metallic cathode will exhibit photoelectric properties, the specialized coating greatly increases the effect. A photocathode usually consists of [[alkali metal]]s with very low [[work function]]s. The coating releases electrons much more readily than the underlying metal, allowing it to detect the low-energy photons in infrared radiation. The lens transmits the radiation from the object being viewed to a layer of coated glass. The photons strike the metal surface and transfer electrons to its rear side. The freed electrons are then collected to produce the final image. ==Photocathode materials== #'''Ag-O-Cs''', also called S-1. This was the first compound photocathode material, developed in 1929. Sensitivity from 300 mm to 1200 nm. Since Ag-O-Cs has a higher dark current than more modern materials [[photomultiplier]] tubes with this photocathode material are nowadays used only in the infrared region with cooling. #'''Sb-Cs''' ([[antimony]]-[[caesium]]) has a spectral response from [[UV]] to [[optical spectrum|visible]] and is mainly used in reflection-mode photocathodes. #'''Bialkali''' ([[antimony]]-[[rubidium]]-[[caesium]] Sb-Rb-Cs, [[antimony]]-[[potassium]]-[[caesium]] Sb-K-Cs). Spectral response range similar to the Sb-Cs photocathode, but with higher sensitivity and lower [[dark current]] than Sb-Cs. They have sensitivity well matched to the most common [[scintillator]] materials and so are frequently used for [[ionizing radiation]] measurement in [[scintillation counter]]s. #'''High temperature bialkali or low noise bialkali''' ([[sodium]]-[[potassium]]-[[antimony]], Na-K-Sb). This material is often used in [[well logging|oil well logging]] since it can withstand temperatures up to 175 °C. At room temperatures, this photocathode operates with very low dark current, making it ideal for use in [[photon counting]] applications. #'''Multialkali''' ([[sodium]]-[[potassium]]-[[antimony]]-[[caesium]], Na-K-Sb-Cs). The multialkali photocathode has a wide spectral response from the ultraviolet to near infrared region. It is widely used for broad-band [[spectrophotometer]]s and [[photon counting]] applications. The long wavelength response can be extended to 930 nm by a special photocathode activation processing. #'''GaAs''' ([[gallium(II) arsenide]]). This photocathode material covers a wider spectral response range than multialkali, from ultraviolet to 930 nm. #'''InGaAs''' ([[indium gallium arsenide]]). Extended sensitivity in the infrared range compared to GaAs. Moreover, in the range between 900 mm and 1000 nm, InGaAs has a much better [[signal to noise ratio]] than Ag-O-Cs. With special manufacturing techniques this photocathode can operate up to 1700 nm. #'''Cs-Te, Cs-I''' ([[caesium]]-[[telluride]], [[caesium]]-[[iodide]]). These materials are sensitive to [[vacuum UV]] and UV rays but not to visible light and are therefore referred to as solar blind. Cs-Te is insensitive to wavelengths longer than 320 nm, and Cs-I to those longer than 200 nm. ==References== *[http://sales.hamamatsu.com/assets/applications/ETD/pmt_handbook_complete.pdf Photomultiplier Tubes] Basics and Applications from [[Hamamatsu Photonics K.K.]] [[Category:Electrochemistry]] *[http://www.photonis.com/products/photomultiplier-tubes/application_book PHOTOMULTIPLIER TUBES : Principles & applications] from PHOTONIS [[de:Photokathode]] [[pl:Fotokatoda]]