Wednesday, April 4, 2012


PMT - Photomultiplier Tube, a vacuum tube light sensor in which input photons cause a cathode to emit electrons that are amplified through a chain of electron amplifiers called a multiplier. Invented in 1934, PMTs are still in use. They offer high gain, low noise, reasonably fast response, and a large collecting area, all important for detecting faint signals.

First observed in 1887 by Heinrich Hertz, the light-induced emission of electrons became an important puzzle because classical physics could not explain why electron emission occurred only for wavelengths shorter than a threshold value rather than depending on light intensity. Albert Einstein won the Nobel prize in 1921 for showing that the photoelectric effect was due to photons needing to have a threshold energy to free electrons from atoms.

The first photoemissive detectors were vacuum photodiodes, in which light illuminated a metal cathode, freeing electrons collected by an anode when a voltage was applied across the tube. Alkali metal cathodes were used to detect visible light. In the days before semiconductor electronics, these devices were called photodiodes or photocells. They were too insensitive for use in early electronic television cameras, so engineers added amplification stages, called dynodes, inside the tube; electrons collided with the dynodes, producing additional or secondary electrons. A series of acceleration stages and dynodes multiplied the photocurrent, thus earning the name photomultiplier. Developed in the 1930s, PMTs became the detectors of choice for applications demanding high sensitivity, low noise, and high speed.

This high performance has made the PMT a remarkably durable technology, one of the last vacuum tubes that is still a standard product in the age of solid-state photonics. Continuing refinements in design and packaging have adapted PMTs for modern applications such as single-photon counting. PMTs continue to new challenges, such as silicon photomultipliers, containing a hundred to several thousand tiny avalanche photodiodes (APDs) connected in parallel for single-photon detection, also called multichannel APDs. But PMTs just keep plugging along.

Modern metal-channel dynode PMT, shown in cutaway. (Image courtesy of Hamamatsu)

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