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Natural crystalline materials contain imperfections: impurity ions, stress dislocations, and other phenomena that disturb the regularity of the electric field that holds the atoms in the crystalline lattice together.These imperfections lead to local humps and dips in the crystalline material's electric potential.Thermoluminescence dating is used for material where radiocarbon dating is not available, like sediments.
In thermoluminescence dating, these long-term traps are used to determine the age of materials: When irradiated crystalline material is again heated or exposed to strong light, the trapped electrons are given sufficient energy to escape.
In the process of recombining with a lattice ion, they lose energy and emit photons (light quanta), detectable in the laboratory.
Optically stimulated luminescence dating is a related measurement method which replaces heating with exposure to intense light.
The sample material is illuminated with a very bright source of green or blue light (for quartz) or infrared light (for potassium feldspars).
The Radiation Dose Rate - the dose accumulated per year-must be determined first.
This is commonly done by measurement of the alpha radioactivity (the uranium and thorium content) and the potassium content (K-40 is a beta and gamma emitter) of the sample material.
Most excited electrons will soon recombine with lattice ions, but some will be trapped, storing part of the energy of the radiation in the form of trapped electric charge (Figure 1).
Depending on the depth of the traps (the energy required to free an electron from them) the storage time of trapped electrons will vary as some traps are sufficiently deep to store charge for hundreds of thousands of years.
Ideally this is assessed by measurements made at the precise findspot over a long period.