Thermoluminescence (tl) dating
Absolute dating of historical buildings: the contribution of thermoluminescence (TL)
The atoms of crystalline solids, such as pottery and rock, can be altered by this radiation. Specifically, the electrons of quartz, feldspar, diamond or calcite crystals could be displaced from their normal positions in atoms and trapped in defects in crystal lattice of the clay. These electrons progressively accumulate over time. When a sample is heated to high temperatures the trapped electrons are released and return to their normal positions in their atoms.
The rate of energy accumulation depends on the amount of background radiation to which the object has been exposed. Thus, preliminary X-ray or gamma radiography examination of the object can increase the amount of accumulated energy and thus give erroneous dating result.
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TL dating-frequently asked questions, TL authenticity dating, thermoluminescence dating
An input of energy, such as heat, is required to free these trapped electrons. When a specimen is reheated, the trapped energy is released in the form of light thermoluminescence as the electrons escape. The amount of light produced is a specific and measurable phenomenon.
Material and objects of archaeological or historical interest that can be dated by thermoluminescence analysis are ceramics, brick, hearths, fire pits, kiln and smelter walls, heat treated flint or other heat-processed materials, the residues of industrial activity such as slag, incidentally fire-cracked rocks, and even originally unfired materials such adobe and daub if they had been heated in an accidental fire.
A non-negligible part of materials which ceramic is usually made of like quartz and feldspars is thermoluminescent: When these materials are heated to several hundreds of Centigrade degrees, electrons are evicted from trap states and energy is emitted in form of light: Heating ceramic in a furnace resets TL accumulated by clay and other materials; from this time on, TL begins growing again as time passes; the more concentrated radioactivity where ceramic is, the quicker TL grows.
Since measured TL depends on time of exposition to natural radiations but also on the intensity of these radiations, to achieve a precise dating we need information about radioactivity of the area where the object was found. During TL analysis, the sample is reheated by a controlled heating process, so the energy is released in the form of light thermoluminescence as the electrons escape. The amount of light produced is measuered by a photomultiplier.
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The result is a glow curve showing the photon emission in function of the heating temperature:. Because this accumulation of trapped electrons begins with the formation of the crystal structure, thermoluminescence can date crystalline materials to their date of formation; for ceramics, this is the moment they are fired.
The major source of error in establishing dates from thermoluminescence is a consequence of inaccurate measurements of the radiation acting on a specimen. The paleodose is the absorbed dose of natural radiation accumulate by a sample.