Fotomultiplier

A photomultiplier tube, often abbreviated PMT, is a highly sensitive light detector that converts incoming photons into an electrical pulse and provides large signal amplification within a vacuum envelope. It typically consists of a photocathode, a series of dynodes, an anode, and a high-voltage supply. When photons strike the photocathode, photoelectrons are emitted by the photoelectric effect and are directed toward the dynode chain, where each dynode produces multiple secondary electrons, yielding a millionfold or greater amplification.

The resulting current at the anode is proportional to the incident light intensity for low to moderate light levels. Typical gains are in the range of 10^6 to 10^7. PMTs offer very fast response, with rise times on the order of a few nanoseconds and transit-time spread depending on design. The spectral response depends on the photocathode material; common bialkali and multialkali photocathodes cover roughly the visible range, while GaAs or GaAsP variants extend toward the near-infrared. Dark current and afterpulsing are common sources of noise, and devices require careful shielding from magnetic fields and stable high voltage.

Applications span many fields, including particle and nuclear physics, medical imaging (such as PET), scintillation counting, astrophysics, and time-of-flight measurements. Variants include multi-anode PMTs for imaging, and microchannel plate PMTs for superior timing and spatial resolution. Solid-state alternatives such as silicon photomultipliers offer lower operating voltage and magnetic-field insensitivity but typically exhibit higher dark noise and saturation at high light levels.

---