 thumb|figure 1: the three stages of thermoluminescence as outlined by aitken (1985, 1998) and applied to a quartz grain (keizars, 2008b) thumb|figure 2: the process of recharging and discharging thermoluminescent signal, as applied to beach sands. (modified from aitken, 1998; keizars, 2008a) thumb|figure 3: thermoluminescence signature lost during migration of two sand grain sizes (keizars, 2008). thumb|figure 4: illustrated method of passively monitoring sand input (keizars, 2003). thermoluminescence (tl) dating is the determination, by means of measuring the accumulated radiation dose, of the time elapsed since material containing crystalline minerals was either heated (lava, ceramics) or exposed to sunlight (sediments). as a crystalline material is heated during measurements the process of thermoluminescence starts. thermoluminescence emits a weak light signal that is proportional to the radiation dose absorbed by the material. == how it works == 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. where there is a dip (a so-called "electron trap"), a free electron may be attracted and trapped. the flux of ionizing radiation&mdash;both from cosmic radiation and from natural radioactivity&mdash;excites electrons from atoms in the crystal lattice into the conduction band where they can move freely. 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. == in practical use == 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. the amount of light produced is proportional to the number of trapped electrons that have been freed which is in turn proportional to the radiation dose accumulated. in order to relate the signal (the thermoluminescence&mdash;light produced when the material is heated) to t
Conservation_and_restoration Dating_methodologies_in_archaeology Geochronology Luminescence