Photomultiplier

A photomultiplier tube (PMT) is a vacuum-tube device that detects light with high sensitivity and converts faint optical signals into electrical pulses with amplification. Incident photons strike a photocathode, ejecting electrons via the photoelectric effect. These photoelectrons are accelerated toward a series of dynodes, each of which emits multiple secondary electrons, creating a cascade that yields a large pulse collected at the anode. Overall gain typically reaches 10^6 to 10^7.

Construction and operation involve an evacuated glass envelope containing a photocathode, multiple dynode stages, and an anode, all powered by a high-voltage supply. Common photocathodes respond to visible light, often alkali-based materials, while some variants extend into ultraviolet or near-infrared. Microchannel plate PMTs use dense arrays of tiny channels to provide compact, fast multiplication, and hybrid PMTs combine a photocathode with a solid-state multiplier.

Performance characteristics include quantum efficiency, which depends on wavelength and photocathode material, and is often in the tens of percent range in the visible. PMTs offer fast timing (nanosecond-scale) and low dark current but require high voltage, magnetic-field shielding, and careful handling to avoid saturation and aging of the photocathode.

Applications span scientific and medical fields: particle and nuclear physics detectors, scintillation counters, Cherenkov detectors, medical imaging (PET and gamma cameras), astronomy, LIDAR, and environmental monitoring.

Variants and alternatives include multi-anode PMTs, microchannel plate PMTs, and hybrid PMTs; solid-state photomultipliers (SiPMs) provide different trade-offs in gain, ruggedness, and magnetic-field tolerance. PMTs remain a foundational detector where single-photon sensitivity and fast response are essential.