fotomultiplikator

Fotomultiplikator, or photomultiplier tube (PMT), is a highly sensitive vacuum device that converts light into an amplified electrical signal. It detects individual photons through the photoelectric effect at a photocathode, which emits electrons when illuminated. These electrons are guided toward a chain of dynodes, each stage providing secondary emission and thereby multiplying the number of electrons. The result is a large electrical pulse from a single or a few photons.

A PMT typically consists of a sealed glass or metal envelope with a photocathode, a series of dynodes, and an anode. The whole tube is operated at high voltage, creating strong electric fields that accelerate electrons between stages. The overall gain can reach roughly 10^6 to 10^7, and modern designs can approach higher gains under suitable conditions. The spectral response depends on the photocathode material, with common choices covering visible to near-UV light. Timing performance is fast, often with sub-nanosecond to a few nanoseconds transit times, making PMTs suitable for time-resolved measurements.

Performance characteristics include quantum efficiency, dark current, afterpulsing, and noise. PMTs require careful magnetic shielding and a light-tight, well-regulated high-voltage supply. They have relatively large form factors and can be fragile, with gain varying over temperature and aging.

Applications span scientific and medical fields, including scintillation counters for radiation detection, gamma- and X-ray spectroscopy, particle physics experiments, fluorescence spectroscopy, PET imaging, and LIDAR. While solid-state detectors are increasingly used, PMTs remain valued for high gain, fast timing, and wide dynamic range in many setups.