A scintillator is a volume of material aimed at responding to high-energy photons (i.e., gamma rays and X-rays), typically used in detectors of such photons and/or particles such as products of radioactivity. Materials used for this may be solid, liquid, or gas, selected for their detection abilities.
Incoming photons of sufficient energy interact with the material via the photoelectric effect, Compton scattering, or pair production. This produces lower-energy (longer wavelength) photons and/or free electrons. Further interactions result in photons within or nearer to the visible light range, with a single high-energy photon resulting in hundreds or thousands of them (scintillation). These can be detected by photomultiplier tubes or photodiodes. Following any pair production, lower-energy photons are created by annihilation. Electrons (and incoming particles) interact with atoms or molecules, exciting them, after which they can relax producing photons. The lower-energy photons resulting from these interactions can undergo further Compton scattering, photoelectric effect, or fluorescence. The result of these is that much of the original photon's energy can be converted to the detectable photons, and the measurement of the sum of their energy yields the energy (thus wavelength) of the original photon.