Kari Eskola, researcher
Luminescence is a phenomenon occurring in crystal materials, when electrons, trapped in special energy stages (traps) caused by defects in crystal structures, are released and emitting light (luminescence) which wavelength corresponds the change in charge carriers energy stages during the process. Electrons are getting trapped because of the natural radioactive background radiation. The longer the crystals are affected by this radiation the more electrons are trapped. Electrons can be released from traps by stimulating the crystals with external energy for example by heating (thermoluminescence, TL) or by lighting (optically stimulated luminescence, OSL).
When electrons are released from traps the intensity of emitted luminescence follows linearly the amount of released electrons and can be used to find out the total amount of trapped electrons. From the amount of trapped electrons we can find out the total dose of radioactive radiation that has affected the crystals. This total dose can be used to the age determination when we measure the dose rate of radiation that has given that dose. Age (the time from the latest emptying of traps) can be found out by dividing the total dose with dose rate.
In dating the TL-method can be used to determine the age from samples that have been heated during their manufacturing process or when used (for example stones from fireplaces, bricks and ceramics). OSL-method can be used for example when dating sand formations. In both methods the main idea is that the traps are emptied at the specific time we are searching for. The traps in bricks and ceramic items are emptied during their manufacturing while they are heated in high temperatures. This gives possibility to determine the manufacturing moment, when heating has been done. As for sand formations light sensitive traps are emptied by daylight exposure while sand is transported to the formation from where it has been earlier. Later when formation builds up the sand in lower parts covers from sunlight and the emptying of light sensitive traps ends. This gives us the opportunity to determine the time period that sand formations are formed.
Luminescence dating methods are specifically used in geology and archaeology. In more extensive dating models the usability of method grows when separate luminescence dating results can be added for example with radiocarbon results and thereby grow up the knowledge behind the models.