PMTs

Photomultiplier tubes (PMTs) are highly sensitive light detectors that convert faint optical signals into electrical pulses. They operate by the photoelectric effect at a photocathode, followed by amplification of electrons across a series of dynodes inside a vacuum tube. PMTs are widely used where single-photon sensitivity and fast timing are required.

A typical PMT consists of a sealed glass envelope containing a photocathode, multiple dynodes, and an anode. When a photon strikes the photocathode, a photoelectron is emitted and accelerated toward the first dynode. At each dynode, electrons are multiplied, producing a cascade that yields gains commonly in the range of 10^6 to 10^8.

Quantum efficiency depends on photocathode material and wavelength, often around 20–40% in the visible region. The spectral response can extend into the ultraviolet or near-infrared depending on construction. Timing performance is characterized by transit-time spread of a few hundred picoseconds to several nanoseconds. Noise sources include dark current and afterpulses from residual gas.

PMTs require high voltage supplies and are sensitive to magnetic fields, which can deflect electrons and alter gain. They are commonly used with scintillators or Cherenkov radiators in particle physics, astrophysics, and medical imaging (for example PET). They have been superseded in some applications by solid-state detectors such as silicon photomultipliers but remain favored where large active areas and fast timing are needed.

Development dates to the 1930s, with the dynode-based design becoming standard in subsequent decades.