Scintillation is the physical process where a material, called a scintillator, emits UV or visible light under excitation from high energy photons (X-rays or γ-rays) or energetic particles (such as electrons, alpha particles, neutrons or ions). See scintillator and scintillation counter for practical applications. Overview Scintillation is an example of luminescence, whereby light of a characteristic spectrum is emitted following the absorption of radiation. The scintillation process can be summarized in three main stages: (A) conversion, (B) transport and energy transfer to the luminescence center, and (C) luminescence. The emitted radiation is usually less energetic than the absorbed radiation, hence scintillation is generally a down-conversion process. Conversion processes The first stage of scintillation, conversion, is the process where the energy from the incident radiation is absorbed by the scintillator and highly energetic electrons and holes are created in the material. The energy absorption mechanism by the scintillator depends on the type and energy of radiation involved. For highly energetic photons such as X-rays ( 0.1 keV < < 100 keV) and γ-rays ( > 100 keV), three types of interactions are responsible for the energy conversion process in scintillation: photoelectric absorption, Compton scattering, and pair production, which only occurs when > 1022 keV, i.e. the photon has enough energy to create an electron-positron pair. These processes have different attenuation coefficients, which depend mainly on the energy of the incident radiation, the average atomic number of the material and the density of the material. Generally the absorption of high energy radiation is described by: where is the intensity of the incident radiation, is the thickness of the material, and is the linear attenuation coefficient, which is the sum of the attenuation coefficients of the various contributions: , which will be explained below. At lower X-ray energies ( 60 keV), the most dominant process is the photoelectric effect, where the photons are fully absorbed by bound electrons in the material, usually core electrons in the K- or L-shell of the atom, and then ejected, leading to the ionization of the host atom. The linear attenuation coefficient contribution for the photoelectric effect is given by: where is the density of the scintillator, is the average atomic number, is a constant that varies between 3 and 4, and is the energy of the photon. At low X-ray energies, scintillator materials with atoms with high atomic numbers and densities are favored for more efficient absorption