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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.
Thermoluminescence dating (TL) 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.
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.
In the process of recombining with a lattice ion, they lose energy and emit photons (light quanta), detectable in the laboratory.
These imperfections lead to local humps and dips in the crystalline material's electric potential.
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.
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 (Figure 1).
Depending on the depth of the traps (the energy required to free an electron from them) the storage time of trapped electrons will vary as some traps are sufficiently deep to store charge for hundreds of thousands of years.
The sample material is illuminated with a very bright source of green or blue light (for quartz) or infrared light (for potassium feldspars).