Editing Photocathode (section)

==Photocathode materials==
*Ag-O-Cs, also called '''S-1'''. This was the first compound photocathode material, developed in 1929. Sensitivity from 300&nbsp;nm to 1200&nbsp;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 (physics)|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&nbsp;°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), also called '''S-20'''.  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&nbsp;nm by a special photocathode activation processing. With the broadened response, this is sometimes referred to as '''S-25'''.
*GaAs ([[gallium(II) arsenide]]). This photocathode material covers a wider spectral response range than multialkali, from ultraviolet to 930&nbsp;nm. GaAs photocathodes are also used in [[Particle accelerator|accelerator]] facilities where polarized electrons are required.<ref>{{cite journal |last1=Pierce |first1=D. T. |last2=Celotta |first2=R. J. |last3=Wang |first3=G.-C. |last4=Unertl |first4=W. N. |last5=Galejs |first5=A. |last6=Kuyatt |first6=C. E. |last7=Mielczarek |first7=S. R. |title=The GaAs spin polarized electron source |journal=Review of Scientific Instruments |date=April 1980 |volume=51 |issue=4 |pages=478–499 |doi=10.1063/1.1136250 |bibcode=1980RScI...51..478P |language=en |issn=0034-6748}}</ref> One of the important property of GaAs photocathode is, it can achieve Negative [[Electron affinity|Electron Affinity]] due to Cs deposition on the surface.<ref>{{Cite journal|title=The optimization of (Cs,O) activation of NEA photocathode - IEEE Conference Publication|language=en-US|doi=10.1109/IVESC.2004.1414231|s2cid=25911728}}</ref> However GaAs is very delicate and loses Quantum Efficiency(QE) due to couple of damage mechanism. Ion Back Bombardment is one of the main cause of GaAs cathode QE decay.<ref>{{Cite journal|last1=Grames|first1=J.|last2=Suleiman|first2=R.|last3=Adderley|first3=P. A.|last4=Clark|first4=J.|last5=Hansknecht|first5=J.|last6=Machie|first6=D.|last7=Poelker|first7=M.|last8=Stutzman|first8=M. L.|date=2011-04-20|title=Charge and fluence lifetime measurements of a dc high voltage GaAs photogun at high average current|journal=Physical Review Special Topics: Accelerators and Beams|volume=14|issue=4|pages=043501|doi=10.1103/physrevstab.14.043501|bibcode=2011PhRvS..14d3501G|issn=1098-4402|doi-access=free}}</ref>
*InGaAs ([[indium gallium arsenide]]). Extended sensitivity in the infrared range compared to GaAs. Moreover, in the range between 900&nbsp;nm and 1000&nbsp;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&nbsp;nm. 
*Cs-Te, Cs-I ([[caesium]]-[[telluride (chemistry)|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&nbsp;nm, and Cs-I to those longer than 200&nbsp;nm.