thermoluminescence dating

A method of dating ancient pottery by measuring the energy accumulated in the crystal lattice of its inclusions of quartz, through the breakdown over time of naturally occurring uranium. The technique is now increasingly extended to burnt flint, calcite, and sediments.

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 the material is heated during measurements, a weak light signal, the thermoluminescence, proportional to the radiation dose is produced.

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. 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. Depending on the depth of the traps (the energy required to free an electron from them) the storage time of trapped electrons will vary- 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. 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 the 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 sediments), that removes the pre-existing trapped electrons. As time goes on, the ionizing radiation field around the material causes the trapped electrons to accumulate. The radiation dose rate - the dose accumulated per year-must be determined first. Often the gamma radiation field at the position of the sample material is measured, or it may be calculated from the alpha radioactivity and potassium content of the sample environment, and the cosmic ray dose is added in. Once all components of the radiation field are determined, the accumulated dose from the thermoluminescence measurements is divided by the dose accumulating each year, to obtain the years since the zeroing event.