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The dose rate includes an internal component (short- ranged alpha and beta irradiation) from the sample itself and an external component (long-ranged gammas and cosmic radiation) from the environment.The age equation is expressed as a simple ratio: Age (ka) = De (Gy) / DR (Gy/ka) where De is the equivalent dose in grays (unit of absorbed dose), and DR is the average dose rate over time. Equivalent dose is the amount of radiation dose that is necessary to account for the measured luminescence signal, in other words, how much radiation is needed to get from zero luminescence to the current, natural luminescence. De is measured by calibrating the natural signal against laboratory-administered radiation.Upon release the electrons and holes recombine and return to the ground state, in the process emitting light, or luminescence.If the stimulus is heat, the emitted light is called thermoluminescence (TL).Subsequent to zeroing, traps become refilled because of continued ionization by radioactivity and a latent luminescence signal steadily accumulates.
The time lag can be understood by reference to solid state energy band theory.
Because traps have different energy depths, some traps are not very stable at ambient temperatures and lose their electrons over the time period relevant to archaeology.
Other traps are sufficiently deep that they are not easily emptied during the exposure to heat or light.
Ionizing radiation excites electrons from the valence band, or ground state, across an energy gap to the conduction band where they are free to move about.
Energy levels within the gap cannot normally be occupied, but crystalline defects resulting in localized charge deficiencies allow occupation of metastable energy levels within the gap.