Fotomultiplicator

Fotomultiplicator, or photomultiplier tube (PMT), is a vacuum device that detects light and converts it into a large electrical signal. It operates by the photoelectric effect: photons strike a photocathode and eject electrons, which are then amplified by a cascade of dynodes inside the tube, producing a measurable current at the anode.

The standard design includes a sealed glass envelope, photocathode, dynode chain, and anode, all powered by a high-voltage supply. Typical gain is 10^6 to 10^7, allowing single-photon sensitivity. Timing is very fast, with pulses from hundreds of picoseconds to a few nanoseconds, and PMTs can be used in analog or single-photon counting modes.

Spectral response depends on the photocathode material, covering ultraviolet to visible and sometimes near-infrared. Quantum efficiency varies by wavelength and device, sometimes over 30%. Performance is affected by dark current, afterpulses, and gain drift; magnetic fields and temperature influence timing and gain.

Applications include physics experiments, medical imaging (PET), scintillation and fluorescence spectroscopy, astronomy, and environmental monitoring. Variants such as microchannel plate PMTs (MCP-PMTs) offer faster timing, while hybrids combine PMT with semiconductor stages. Solid-state photomultipliers (SiPMs) are a different technology.

Compared with solid-state detectors, PMTs deliver very high gain and low noise for faint light but require high voltage, are fragile, and can be sensitive to magnetic fields, requiring shielding. They remain in use when fast timing and high sensitivity to low light are essential.