Editing Phosphor (section)

==Light-emission process==
[[File:Jablonski Diagram of Fluorescence Only-en.svg|thumb|[[Jablonski diagram]] shows the energy levels in a fluorescing atom in a phosphor. An electron in the phosphor absorbs a high-energy [[photon]] from the applied radiation, exciting it to a higher energy level.  After losing some energy in non-radiative transitions, it eventually transitions back to its ground state energy level by fluorescence, emitting a photon of lower energy in the visible light region.]]

The scintillation process in inorganic materials is due to the [[electronic band structure]] found in the [[crystal]]s. An incoming particle can excite an electron from the [[valence band]] to either the [[conduction band]] or the [[exciton]] band (located just below the conduction band and separated from the valence band by an [[energy gap]]). This leaves an associated [[electron hole|hole]] behind, in the valence band. Impurities create electronic levels in the [[forbidden gap]].

The excitons are loosely bound [[electron–hole pair]]s that wander through the [[crystal lattice]] until they are captured as a whole by impurity centers. They then rapidly de-excite by emitting scintillation light (fast component).

In the conduction band, electrons are independent of their associated holes. Those electrons and holes are captured successively by impurity centers exciting certain [[metastable state]]s not accessible to the excitons. The delayed de-excitation of those metastable impurity states, slowed by reliance on the low-probability [[forbidden mechanism]], again results in light emission (slow component).
In the case of inorganic [[scintillator]]s, the activator impurities are typically chosen so that the emitted light is in the visible range or [[near ultraviolet|near-UV]], where [[photomultiplier]]s are effective.

Phosphors are often [[transition-metal]] compounds or [[rare-earth]] compounds of various types.   In inorganic phosphors, these inhomogeneities in the crystal structure are created usually by addition of a trace amount of [[dopant]]s, impurities called ''[[activator (phosphor)|activators]]''. (In rare cases [[dislocation]]s or other [[crystal defect]]s can play the role of the impurity.) The wavelength emitted by the emission center is dependent on the atom itself and on the surrounding crystal structure.