Physics, Techniques and ProceduresScintillation
emission of light from certain organic and inorganic crystals following an interaction in the crystal. In organic scintillators, the underlying process is
fluorescence arising from transitions in the energy level structure of a single molecule. In inorganic scintillators, a regular crystal lattice structure is required as the basis for the scintillation process. The mechanism depends on the energy states determined by the crystal lattice. In insulators or semiconductors, electrons have available only discrete bands of energy (see
solid). The lower so-called valence band represents electrons bound at lattice sites, whereas, the conduction band represents those electrons that have sufficient energy to migrate through the crystal. There exists an intermediate band, the forbidden band, in which electrons can never be found in the
pure crystal (see
solid (I), Fig. 1). Absorption of energy can result in the elevation of an electron from the valence band into the conduction band, leaving a hole in the valence band. In the
pure crystal, the return of an electron from the conduction band to the valence band across the forbidden band, with the emission of a photon, is an inefficient process. When small amounts of an impurity are added, however,
special sites are created in the lattice at which the normal band structure is modified from that of the
pure crystal. Energy states are created within the forbidden band through which the electron can de-excite back to the valence band. Since the energy is now less than that of the full forbidden gap, de-excitation takes place with increased probability and results in the emission of a light photon. This forms the basis for the scintillation mechanism in inorganic scintillators. An example is the crystal sodium iodide (NaI) which behaves as a scintillator when activated by the addition of thallium as an impurity.
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