When a radiation is incident on a material, some of its energy may be absorbed and re-emitted as light of longer wavelength. The wavelength of the emitted light is characteristic of the luminescent substance and not of the incident radiation. Thermoluminescence TL is the process in which a mineral emits light while it is being heated: it is a stimulated emission process occurring when the thermally excited emission of light follows the previous absorption of energy from radiation. Energy absorbed from ionising radiation alpha, beta, gamma, cosmic rays frees electrons to move through the crystal lattice and some are trapped at imperfections in the lattice.
Luminescence dating refers to a group of methods of determining how long ago mineral grains were last exposed to sunlight or sufficient heating. It is useful to geologists and archaeologists who want to know when such an event occurred. It uses various methods to stimulate and measure luminescence. All sediments and soils contain trace amounts of radioactive isotopes of elements such as potassium , uranium , thorium , and rubidium. These slowly decay over time and the ionizing radiation they produce is absorbed by mineral grains in the sediments such as quartz and potassium feldspar.
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 a crystalline material is heated during measurements the process of thermoluminescence starts. Thermoluminescence emits a weak light signal that is proportional to the radiation dose absorbed by the material. Sediments are more expensive to date.
The use of thermoluminescence TL dating of sediments at the sites of Jinmium Fullagar et al. At Jinmium, the results suggest that the peopling of Australia occurred about , years ago, more than twice as old as previously believed Roberts et al. Similarly, the site of Diring Yuriakh in Siberia, a site regarded by many to not be any older than about 30, years has been dated to greater than about , years. In contrast to dating of sediments where optical exposure resets the clock, the more conventional applications of thermoluminescence dating of burned flint and pottery operate on the basis of resetting by heating.