Thermoluminescence

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The property of some minerals or other substances to glow when heated. The presence of casual impurities and activator atoms in the crystal leads to the appearance of localized energy levels in the band gap. Some of them are deep, that is they are located at a considerable distance from the top of the valance band or from the bottom of the conduction band. Such levels are often metastable and play the role of traps for charge carriers. For the electron at the metastable level to be able to recombine with a hole it should first be transferred to the conduction band. This process can be stimulated by an electric field, by infrared light, or by simply warming crystal. The luminescence of the pre-excited crystal phosphor stimulated by warming is called Thermoluminiscence.