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===Photocathode materials=== The photocathodes can be made of a variety of materials, with different properties. Typically the materials have low [[work function]] and are therefore prone to [[thermionic emission]], causing noise and dark current, especially the materials sensitive in infrared; cooling the photocathode lowers this thermal noise. The most common photocathode materials are<ref name="pmt">[https://www.chem.uci.edu/~unicorn/243/handouts/pmt.pdf Photomultiplier Tubes. Construction and Operating Characteristics. Connections to External Circuits], Hamamatsu</ref> Ag-O-Cs (also called S1) transmission-mode, sensitive from 300β1200 nm. High dark current; used mainly in near-infrared, with the photocathode cooled; GaAs:Cs, [[caesium]]-[[Activator (phosphor)|activated]] [[gallium arsenide]], flat response from 300 to 850 nm, fading towards ultraviolet and to 930 nm; InGaAs:Cs, caesium-activated [[indium gallium arsenide]], higher infrared sensitivity than GaAs:Cs, between 900β1000 nm much higher [[signal-to-noise ratio]] than Ag-O-Cs; Sb-Cs, (also called S11) caesium-activated [[antimony]], used for reflective mode photocathodes; response range from ultraviolet to visible, widely used; bialkali (Sb-K-Cs, Sb-Rb-Cs), caesium-activated antimony-rubidium or antimony-potassium alloy, similar to Sb:Cs, with higher sensitivity and lower noise. can be used for transmission-mode; favorable response to a NaI:Tl [[scintillator]] flashes makes them widely used in [[gamma spectroscopy]] and radiation detection; high-temperature bialkali (Na-K-Sb), can operate up to 175 Β°C, used in [[well logging]], low dark current at room temperature; multialkali (Na-K-Sb-Cs), (also called S20), wide spectral response from ultraviolet to near-infrared, special cathode processing can extend range to 930 nm, used in broadband [[spectrophotometer]]s; [[Solar-blind technology|solar-blind]] (Cs-Te, Cs-I), sensitive to vacuum-UV and ultraviolet, insensitive to visible light and infrared (Cs-Te has cutoff at 320 nm, Cs-I at 200 nm).
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