What is Thermoluminescence Dating?
Thermoluminescence is a property of insulating (generally crystalline) materials that have been exposed to ionizing radiation. Ionizing radiation (alpha and beta particles and gamma rays coming from naturally-occurring uranium and thorium and radioactive potassium in the environment, as well as cosmic rays) knocks electrons loose from atoms it hits. In crystalline materials the electric field holding the atoms together in a regular array (the 'crystal lattice') may be perturbed by impurities or physical damage, leading to areas to which electrons are attracted and may be trapped. These are called 'electron traps'. Some will be deep, holding the electrons tightly, others shallow. The electrons knocked loose by ionizing radiation may be captured by an electron trap and for the deeper traps, may remain for many thousands of years.
Electrons caught in traps will languish there until sufficient energy is put into the system to knock them loose. Typically this will be thermal energy (but high energy photons--blue and UV-- in sunlight remove electrons, too). Heat is perceived in a crystal lattice as vibration; the higher the temperature, the more violent the vibration. As the temperature increases, first the shallow, then the deep traps empty. The freed electrons pass through 'color centers' on their way back to re-combining with an atom, and in the process emit a photon (a particle of light). This light emission resulting from heating irradiated substances is called thermoluminescence, or 'TL' for short.
Because the number of trapped electrons is proportional to the radiation dose absorbed, it is possible to use TL to measure radiation dose.
TL was first observed in 1663 by the English chemist Sir Robert Boyle who reported to the Royal Society a remarkable diamond owned by his friend, a Mr. Clayton, that glowed when '...holding it a good while upon a warm part of my naked body'. It wasn't until 1905 that it was realized that this phenomenon was due to radiation. After WW II, TL started to be exploited for personnel dosimetry, to monitor the radiation exposure to people working with radiation and radioactive materials. Now TL dosimeter badges have largely supplanted film badges at nuclear reactors and hospital radiology departments. In 1952 Farrington Daniels, at the University of Wisconsin, a leader in TL materials research, suggested that TL might be used to determine how long ago pottery was fired. The firing of clay into pottery releases all previously accumulated TL (the 'geological TL'), and the TL builds up from zero. The first attempt made was in 1959 by George Kennedy at UCLA. The idea was taken up in the 60's at a number of institutions, but especially by Martin Aitken and his students at Oxford University. By 1971, after some difficulties were surmounted, all the pieces were in place for successful use of TL for pottery dating.
By comparison of the naturally-occurring TL in pottery with that due to a series of known calibrating doses, the radiation dose accumulated since firing the clay can be determined. When the radiation dose rate is computed by measuring the amounts of uranium, thorium, and potassium in the pottery itself and in the burial environment, the dose divided by the dose per year yields the age of the pottery in years. Unfortunately, the devil was in the details, and it has taken considerable effort to achieve precision and reliability.
Since the 80's TL (and now a related technique, OSL--optical stimulated luminescence) has been extended to other materials and other types of dating. For example George and Helene Valladas in France have dated burnt flints at Neanderthal and Cromagnon habitation sites in Israel and Southern Lebanon. The dates range from 54,000 to 105,000 years before present, and remarkably the two populations lived in close proximity for many thousands of years. TL and OSL have established a very early date for the presence of Australian aborigines on that continent. The major use for luminescence dating now is in geology where sediments are dated from their last exposure to sunlight, which like heat can de-trap electrons. Dating of sediments in some cases as old as 1 million years has been reported. This has proven to be of great use in paleoclimatology. With the current interest in climate due to global warming, accurate climate modeling is extremely important. Understanding, and accurately 'predicting', the past climate will lead to better predictions about the future.