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Photomultiplier tubes use scientific principles to amplify the effect of a single photon. They are manufactured in various configurations and are used in astronomy, medical imaging, and night vision. They are still widely used despite the invention of semiconductors and were initially developed as television cameras. The gain of a photomultiplier tube varies up to 100 million times, making them indispensable for detecting very small numbers of photons.
A photomultiplier tube uses two scientific principles to magnify the effect of a single incident photon. They are manufactured in various configurations of light sensitive materials and angles of incident light to achieve high gain and low noise response in their ultraviolet, visible and near infrared frequency range. Originally developed as a more responsive camera, photomultiplier tubes are now found in many applications.
With the invention of semiconductors, vacuum tubes have been largely eliminated from the electronics industry, with the exception of the photomultiplier tube. In this device, a single photon passes through a window or faceplate and strikes a photocathode, an electrode made of a photoelectric material. This material absorbs light photon energy at specific frequencies and emits electrons in a result called the photoelectric effect.
The effects of these emitted electrons are amplified by the use of the secondary emission principle. The electrons emitted by the photocathode are focused on the first in a series of electron multiplier plates called dynodes. At each dynode, incoming electrons cause more electrons to be emitted. A cascade effect occurs and the incident photon has been amplified or detected. Thus, the basis for the name “photomultiplier”, the very small signal of a single photon is enhanced to the point where it is easily detected by the current flow from the photomultiplier tube.
The spectral responses of the photomultiplier tube are due primarily to two design elements. The type of window determines which photons can pass into the device. The photocathode material determines the response to the photon. Other variations on the design include windows mounted at the end of the tube or side windows where the photon stream is bounced off the photocathode. Since the gain or amplification is limited by the secondary emission process and does not increase with increasing accelerating voltage, multi-stage photomultipliers have been developed.
The response of the photocathode depends on the frequency of the incident photon, not on the number of received photons. As the number of photons increases, the electric current generated increases, but the frequency of the emitted electrons is constant for any window-photocathode combination, a result Albert Einstein used as proof of the particle nature of light.
The gain of a photomultiplier tube varies up to 100 million times. This property, together with the low noise or unwarranted signal, makes these vacuum tubes indispensable for detecting very small numbers of photons. This sensing capability is useful in astronomy, night vision, medical imaging, and other uses. Semiconductor versions are in use, but the vacuum tube photomultiplier is best suited for sensing photons of light that are not collimated, meaning that light rays do not travel parallel to each other.
Photomultipliers were initially developed as television cameras, which allowed broadcast television to move beyond studio shooting with bright lights, to more natural settings or on-location reporting. Although they have been replaced with charge coupled devices (CCDs) in that application, photomultiplier tubes are still widely specified. Much of the photomultiplier tube development work was performed by RCA at facilities in the United States and the former Soviet Union in the second half of the 20th century. In the first decades of the 20th century, most of the world’s photomultiplier tubes were manufactured by a Japanese company, Hamamatsu Photonics.
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