Thermoluminescence dating
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Thermoluminescence (TL) dating is the determination by means of measuring accumulated radiation dose of the time elapsed since crystalline mineral materials were either heated (lava, ceramics) or exposed to sunlight (sediments). As the material is heated during measurements, a weak light signal (the TL) proportional to the radiation dose is produced.
Natural crytalline 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. This leads to local humps and dips in the electric potential. Where there is a dip (an 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 trapped, storing part of the energy of the radiation in the form of trapped charge. Depending on the depth of the trap (the energy required to free an electron from the trap) the storage time of trapped electrons will vary. Some traps are sufficiently deep to store charge for hundreds of thousands of years. When such irradiated crystalline material is heated or exposed to strong light, the trapped electrons are given sufficient energy to escape from the traps. In the process of recombining with a lattice ion, an electron loses energy and a photon (a light quantum) is emitted, 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—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, 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. Therefore, at that point the thermoluminescence signal is zero. As time goes on, the ionizing radiation field around the material causes the trapped electrons to accumulate. In the laboratory, the accumulated radiation dose can be measured, but this by itself is insufficient to determine the time since the zeroing event. 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. 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.
Thermoluminescence dating is used for material where radiocarbon dating is not available, like sediments. Its use is now common in the authentication of old ceramic wares, for which it gives the approximate date of the last firing.
A related measurement method replaces heating with exposure to strong light. The sample material is illuminated with a very bright source of infrared light (for feldspars) or green or blue light (for quartz). Ultraviolet light emitted by the sample is detected for measurement. This is called Optically Stimulated Luminescence (OSL).
References
- Quaternary TL Surveys - Guide to thermoluminescence date measurement (http://www.users.globalnet.co.uk/~qtls/)
- Frequently asked questions about TL dating (http://www.daybreaknuclear.com/bortolot_faq.html)
- Aitken, M.J., Thermoluminescence Dating, Academic Press, London (1985) – Standard text for introduction to the field. Quite complete and rather technical, but well-written and well organized. There is a second edition.
- Aitken, M.J., Introduction to Optical Dating, Oxford University Press (1998) – Good introduction to the field.de:Thermolumineszenz