Photomultiplier Tubes (Phototubes) are a type of vacuum tubes, are extremely sensitive photodetectors of light in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum. These photodetectors multiply the current produced by incident light by as much as 100 million times (i.e., 160 dB), in multiple dynode stages, enabling individual photons to be detected when the incident flux of light is very low. The combination of high gain, low noise, high frequency response, and large area of collection has earned photomultipliers an essential place in nuclear and particle physics, astronomy, medical diagnostics including blood tests, medical imaging, motion picture film scanning (telecine), and high-end image scanners known as drum scanners.

In construction of Photomultiplier, the basic radiation sensor is the photocathode which is located inside a vacuum envelope. Photoelectrons are emitted and directed by an appropriate electric field to an electrode or dynode within the envelope. A number of secondary electrons are emitted at this dynode for each impinging primary photoelectron. These secondary electrons in turn are directed to a second dynode and so on until a final gain of perhaps 106 is achieved. The electrons from the last dynode are collected by an anode which provides the signal current that is read out. 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.

For a large number of applications, the photomultiplier is the most practical or sensitive detector available. The basic reason for the superiority of the photomultiplier is the secondary-emission amplification that makes it possible for the tube to approach “ideal” device performance limited only by the statistics of photoemission. Amplifications ranging from 103 to as much as 108 provide output signal levels that are compatible with auxiliary electronic equipment without need for additional signal amplification. Extremely fast time response with rise times as short as a fraction of a nanosecond provides a measurement capability in special applications that is unmatched by other radiation detectors.

Semiconductor devices, particularly avalanche photodiodes, are alternatives to photomultipliers; however, photomultipliers are uniquely well-suited for applications requiring low-noise, high-sensitivity detection of light that is imperfectly collimated. Photomultipliers are extraordinarily sensitive and moderately efficient.

Photomultiplier Tubes Vs Semiconductor Avalanche Photodetector Photodiodes

  • In a comparison of the relative advantages of photomultipliers and solid-state detectors, The most important consideration is the light or radiation level to be detected. The use of photomultipliers is recommended when the level of light flux is very low. If levels are relatively high, it may be simpler to use a solid-state detector. Furthermore, a high level of light flux could overload and damage the photomultiplier. A photomultiplier may be used if the light flux is 100 microlumens or less or of the order of 0.1 microwatt or less at the peak of the spectral response.
  • When a fairly large detector area is required, a photomultiplier is also recommended, as is the case in scintillation counting where a crystal scintillator may be several centimeters in diameter. Silicon avalanche photodiodes have good low-light-level capability but their area is generally limited to a few square millimeters.
  • Spectral response of the photomultiplier, of course, must be a reasonable match to that of the source of radiation. Photomultipliers are useful in the range 120 nanometers to 1100 nanometers, depending upon the type of photocathode and window material. Responses further in the infrared than 1100 nanometers require an infrared photoconductive detector or some other infraredsensitive device.
  • When a fast response time is an important requirement, the photomultiplier is usually the most suitable detector. Photomultiplier tubes have response-time capability down to the nanosecond range and even better in the case of specially designed tubes.
  • Proper operation of a photomultiplier depends critically upon the applied voltage and the voltage distribution to the dynode stages. A small variation in voltage can result in a much larger percentage change in the tube gain.