Thermoluminescence dating TL is the determination, by means of measuring the accumulated radiation dose, of the time elapsed since material containing crystalline minerals was either heated lavaceramics or exposed to sunlight sediments. As a crystalline material is heated during measurements, the process of thermoluminescence starts. Thermoluminescence dating archaeology leveling emits a weak light signal that is proportional to the radiation dose absorbed by the material.
It is a type of luminescence dating. Sediments are more expensive to date. It will often work well with stones that have been heated by fire. The clay core of bronze sculptures made by lost wax casting can also be tested.
Different materials vary considerably in their suitability for the technique, depending on several factors. Subsequent irradiation, for example if an x-ray is taken, can affect accuracy, as will the "annual dose" of radiation a buried object has received from the surrounding soil.
Ideally this is assessed by measurements made at the precise findspot over a long period. For artworks, it may be sufficient to confirm whether a piece is broadly ancient or modern that is, authentic or fakeand this may be possible even if a precise date cannot be estimated. Natural crystalline materials contain imperfections: 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—both from cosmic radiation and from natural radioactivity —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 "Thermoluminescence dating archaeology leveling," storing part of the energy of the radiation in the form of trapped electric charge Figure 1.
Depending on the depth of "Thermoluminescence dating archaeology leveling" 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 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 quantadetectable 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—light produced when the material is heated to Thermoluminescence dating archaeology leveling radiation dose that caused it, it is necessary to calibrate the material with known doses of radiation since the density of traps is highly variable. Thermoluminescence dating presupposes a "zeroing" event in the history of the material, either heating in the case of pottery or lava or exposure to sunlight in the case of sedimentsthat removes the pre-existing trapped electrons.