What is a Photomultiplier Tube?

A photomultiplier tube uses two scientific principles to amplify the effect of a single incident photon. They are made in many different configurations of light-sensitive materials and incident light angles to achieve a high gain and a low noise response in their working range of ultraviolet, visible, and near-infrared frequencies. Originally developed as a more responsive television camera, photomultiplier tubes now are 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 face plate and impacts a photocathode, an electrode made of a photoelectric material. This material absorbs the energy of the light photon at specific frequencies and emits electrons in a result called the photoelectric effect.

The effects of these emitted electrons are amplified by use of the principle of secondary emission. The electrons emitted from the photocathode are focused onto the first of a series of electron multiplier plates called dynodes. At each dynode, the incoming electrons cause additional electrons to be emitted. A cascade effect occurs, and the incident photon has been amplified or detected. Hence, the basis for the name "photomultiplier," the very small signal of a single photon is strengthened to the point where it is easily detectable by the flow of current from the photomultiplier tube.

Spectral responses of the photomultiplier tube are due primarily to two design elements. The type of window determines what photons can pass into the device. The photocathode material determines the response to the photon. Other variations on the design include tube-end mounted windows or side windows where the photon stream is bounced off the photocathode. As the gain or amplification is limited by the secondary emission process and does not increase with increased acceleration voltage, multiple-stage photomultipliers were developed.

The photocathode response depends on the incident photon frequency, not the number of photons received. If 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 that Albert Einstein used as evidence of the particle nature of light.

The gain of a photomultiplier tube ranges up to 100 million times. This property, along with the low noise or unwarranted signal, makes these vacuum tubes indispensable in detecting very small numbers of photons. This detection capability is useful in astronomy, night vision, medical imaging and other uses. Semiconductor versions are in use, but the vacuum tube photomultiplier is better-suited for the detection of light photons that are not collimated, meaning the light rays are not traveling parallel paths with each other.

Photomultipliers were first developed as television cameras, which enabled television broadcasting to move beyond studio shots with bright lights to more natural settings or on-site reporting. While they have been replaced with charge-coupled devices (CCDs) in that application, photomultiplier tubes are still widely specified. Much of the development work on the photomultiplier tube was performed by RCA in facilities in the United States and the former Soviet Union in the latter half of the 20th century. In the opening decades of the 21st century, most of the world’s photomultiplier tubes are manufactured by a Japanese firm, Hamamatsu Photonics.