fotomultipliers

Photomultipliers, or photomultiplier tubes (PMTs), are highly sensitive light detectors that convert photons into electrical signals with high gain. When photons strike a photocathode inside a vacuum tube, electrons are emitted via the photoelectric effect. These photoelectrons are directed toward a series of dynodes held at progressively higher voltages; at each dynode, multiple secondary electrons are emitted, producing exponential amplification. The final anode collects the amplified electron cloud, generating a measurable current or pulse. Typical gains are in the range of 10^6 to 10^8, enabling single-photon sensitivity. PMTs require high voltage supplies, often in the kilovolt range, and are enclosed in a light-tight, evacuated envelope.

PMTs consist of a photosensitive cathode, a vacuum tube envelope, and a dynode chain that provides amplification. The spectral response depends on the photocathode material (for example, bialkali or multialkali) and ranges from the near-UV to the visible. Quantum efficiency is typically a few tens of percent at peak for standard designs, with specialized photocathodes achieving higher values in certain wavelengths. Performance characteristics include fast timing (transit time spreads from hundreds of picoseconds to a few nanoseconds), good linearity over a wide dynamic range, and low intrinsic noise, though dark counts and afterpulses can occur. PMTs are sensitive to magnetic fields and temperature and require careful high-voltage and mechanical handling.

Applications span scientific research and industry, including high-energy and nuclear physics detectors, astrophysical instruments, medical imaging (such as PET), radiation monitoring, and environmental sensing. Variants include microchannel plate PMTs and hybrid photomultipliers, while solid-state alternatives like silicon photomultipliers offer compactness and magnetic-field insensitivity at different noise and dynamic-range trade-offs.