Photocathode

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In a photomultiplier or photodiode tube, a photocathode is a negatively charged electrode coated with a photosensitive compound. When 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 metals with very low work functions.

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Photocathode Materials

  1. 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.
  2. Sb-Cs Antimony-Cesium has a spectral response from UV to visible range and is mainly used in reflection-mode photocathodes.
  3. Bialkali (Antimony-Rubidium-Cesium Sb-Rb-Cs, Antimony-Potassium-Cesium 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 counters.
  4. High temperature bialkali or low noise bialkali (Sodium-Potassium-Antimon Na-K-Sb). This material is often used in 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.
  5. Multialkali (Sodium-Potassium-Antimon-Cesium 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 spectrophotometers and photon counting applications. The long wavelength response can be extended to 930 nm by a special photocathode activation processing.
  6. GaAs Gallium Arsenide This photocathode material covers a wider spectral response range than multialkali, from ultraviolet to 930 nm.
  7. InGaAs Indium gallium arsenide Extended sensitivity in the infrared range compared to GaAs. Moreover, in the range between900 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 1700nm.
  8. Cs-Te, Cs-I Cesium-Telluride, Cesium-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.
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References

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