text: The scintillation process in inorganic materials is due to the electronic band structure found in crystals and is not molecular in nature as is the case with organic scintillators. 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; see [http://upload.wikimedia.org/wikipedia/commons/4/46/Bandgap_in_semiconductor.svg picture]). This leaves an associated hole behind, in the valence band. Impurities create electronic levels in the forbidden gap. The excitons are loosely bound electron-hole pairs which wander through the crystal lattice until they are captured as a whole by impurity centers. The latter then rapidly de-excite by emitting scintillation light (fast component). The activator impurities are typically chosen so that the emitted light is in the visible range or near-UV where photomultipliers are effective. The holes associated with electrons in the conduction band are independent from the latter. Those holes and electrons are captured successively by impurity centers exciting certain metastable states not accessible to the excitons. The delayed de-excitation of those metastable impurity states again results in scintillation light (slow component).
  BGO (bismuth germanium oxide)  is a pure inorganic scintillator without any activator impurity. There, the scintillation process is due to an optical transition of the bismuth| ion, a major constituent of the crystal. In tungstate scintillators calcium tungstate| and cadmium tungstate| the emission is due to radiative decay of self-trapped excitons.
  The scintillation process in GaAs doped with silicon and boron impurities is different from conventional scintillators in that the silicon n-type doping provides a built-in population of delocalized electrons at the bottom of the conduction band. Some of the boron impurity atoms reside on arsenic sites and serve as acceptors. A scintillation photon is produced whenever an acceptor atom such as boron captures an ionization hole from the valence band and that hole recombines radiatively with one of the delocalized electrons. Unlike many other semiconductors, the delocalized electrons provided by the silicon are not “frozen out” at cryogenic temperatures. Above the Mott transition concentration of  free carriers per cm, the “metallic” state is maintained at cryogenic temperatures because mutual repulsion drives any additional electrons into the next higher available energy level, which is in the conduction band. The spectrum of photons from this process is centered at 930 nm (1.33 eV) and there are three other emission bands centered at 860, 1070, and 1335 nm from other minor processes. Each of these emission bands has a different luminosity and decay time. The high scintillation luminosity is surprising because (1) with a refractive index of about 3.5, escape is inhibited by total internal reflection and (2) experiments at 90K report narrow-beam infrared absorption coefficients of several per cm. Recent Monte Carlo and Feynman path integral calculations have shown that the high luminosity could be explained if most of the narrow beam absorption is actually a novel optical scattering from the conduction electrons with a cross section of about 5 x 10 cm that allows scintillation photons to escape total internal reflection. This cross section is about 10 times larger than Thomson scattering but comparable to the optical cross section of the conduction electrons in a metal mirror.