These changes amount to storage of the energy from the radiation. Since the energy is stored, these materials can be used for dosimeters. There are two categories of thermoluminescence. This is not a particularly useful reaction for TLD use. TLDs use phosphorescence as their means of detection of radiation.
Phosphorescence is the emission of light after the irradiation period.
The delay time can be from a few seconds to weeks or months. Electrons in some solids can exist in two energy states, a lower energy state called the valence band and a higher energy state called the conduction band. Electrons in the conduction band or in the band gap have more energy than the valence band electrons. Normally in a solid, no electrons exist in energy states contained in the band gap. In some materials, defects in the material exist or impurities are added that can trap electrons in the band gap and hold them there.