scintillator and photomultiplier
/ PMT = pee-em-tee /
Some materials light up faintly when a particle passes through them — a brief, dim sparkle far too weak for the eye. The challenge is to catch and amplify that whisper of light into a usable electrical signal. The scintillator-and-photomultiplier pairing solves both halves of this: the scintillator is the material that turns particle energy into a tiny flash of light, and the photomultiplier is the device that detects that flash and multiplies it into a measurable pulse.
Here is the chain in detail. When a charged particle (or the products of a neutral particle's interaction) deposits energy in a scintillator — a special plastic or crystal — the material's atoms absorb that energy and promptly re-emit it as a quick pulse of visible light, with more light for more deposited energy. That light is funneled to a photomultiplier tube. Inside the tube, the light first strikes a surface that ejects a few electrons (the photoelectric effect); those electrons are then accelerated onto a series of charged plates called dynodes, each of which knocks loose several more electrons, so a handful becomes thousands and then millions in a cascade. The result is an electrical pulse big enough to read, whose size reflects the original energy and whose timing can be measured to a fraction of a billionth of a second.
This combination is one of the oldest and most versatile tools in the field, prized for speed and sensitivity. It underlies fast trigger systems, time-of-flight measurements, and many calorimeters, which often use scintillator to convert shower energy into light. The vast walls of photomultipliers lining neutrino detectors like Super-Kamiokande catch the faint Cherenkov flashes from rare interactions. Modern variants increasingly replace the bulky vacuum-tube photomultiplier with compact silicon photomultipliers, but the core idea is unchanged: convert energy to light, then amplify that light enough to count.
A single photon entering a photomultiplier can dislodge one electron, which the chain of dynodes turns into a pulse of millions of electrons — enough that a detector can register the arrival of essentially one quantum of light, and time it to better than a nanosecond.
Energy becomes a flash; the flash becomes a countable pulse.
The scintillator makes the light and the photomultiplier amplifies it; neither alone is the whole detector. Modern silicon photomultipliers now often replace the classic vacuum tube.