scintillator

A scintillator is a volume of material aimed at responding to particles such as cosmic rays, the products of radioactivity, and high-energy photons such as gamma rays and X-rays. The material may be solid, liquid, or gas, selected for their detection abilities. Such scintillators are used in lab equipment, particle experiments, and within astronomy, some detectors of cosmic rays, neutrinos, muons, gamma rays, and X-rays.

Incoming photons or other particles 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.

Earth atmosphere itself can serve as such a scintillator, producing air showers, which can be spotted and recorded by optical sensors. The term Compton telescope has been used for some of these. Sea water is also used.