Photomultiplier

A photomultiplier detects weak light by converting a photon into a photoelectron at a photosensitive cathode, then multiplying electrons through successive dynodes. Each impact can release several secondary electrons, and an anode collects the amplified packet as an electrical pulse.

If each of $m$ stages has mean multiplication $\delta$, the mean gain is approximately ${\delta }^m$. This amplification makes small signals measurable but does not eliminate photon shot noise or fluctuations in multiplication. Quantum efficiency controls the probability of producing an initial useful photoelectron, while dark current and background produce additional signals. Optical shielding, stable applied voltages and a blocked-light measurement are therefore essential to interpreting a weak-light count.^[ref]

A dynode is an electrode that emits secondary electrons when struck by an incoming electron. A photomultiplier arranges successive dynodes at increasing potentials so that photoelectrons are accelerated and multiplied through a cascade.

If the mean multiplication factor per stage is $\delta$ and there are $n$ comparable stages, the idealized mean gain is ${\delta }^n$. Fluctuations in secondary emission contribute to output noise. Excessive current can disturb the electrode voltages or create space-charge effects, so gain calibration and linearity need experimental checks.