Photomultiplier

A photomultiplier is a cold cathode device. A single photon ejects an electron from a photocathode coated with cesium oxide. That electron is accelerated across a positive voltage to a second cathode, where it ejects several electrons. The tube has a cascade of ten or more cathodes. The final emission is in the picoamp range and can be measured with a sensitive current meter. In operation they may be cooled with liquid air and are left on for long periods of time (days) before use, to reduce noise levels.

Photomultiplier tubes respond to single photons and are an order of magnitude more sensitive that the human eye. > More