Optically stimulated luminescence

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In physics Optically stimulated luminescence or (OSL) is a method for measuring doses from ionizing radiation (commonly known as radioactive radiation).

The method makes use of electrons trapped between the valence and electron band in the crystaline structure of certain types of matter (such as quartz, feldspars, aluminum oxide). The trapping sites are imperfections of the lattice - impurities or defects. The ionizing radiation produces electron-hole pairs - electrons are in the conduction band and holes in the valence band. The electrons which have been excited to the conduction band may become entrapped in the electron or hole traps. Under stimulation of light the electrons may free themselves from the trap and get into the conduction band. From the conduction band they may recombine with holes trapped in hole traps. If the centre with the hole is a luminescence centre (radiative recombination centre) emission of light will occur. The photons are detected using a photomultiplier tube. The signal from the tube is then used to calculate the dose that the material had absorbed.

OSL is used in at least two different applications: optical dating of ancient materials (mainly geological sediments, but also sometimes fired pottery, bricks etc., although in the latter case thermoluminescence is used more often), and as a method of radiation dosimetry which is the measurement of accumulated radiation dose in the tissues of health care, nuclear, research and other workers, as well as in building materials in regions of nuclear disaster.

The difference between radiocarbon dating and luminescence dating is that the former is used to date organic materials and luminescence dating to date minerals, like quartz and feldspars. Events that can be dated using OSL dating are for example last exposure to sunlight - for example the deposition of geological sediments after being transported by air (eolian sediments) or rivers (fluvial sediments). In archaeology, OSL dating finds application to dating of ceramics - the dated event is the time of last heating to high temperature (in excess of 400 °C)

The most popular OSL method is called Single-aliquot regeneration (or SAR) - see optical dating. In order to carry out dating, from the sample mineral grains have to be extracted. Most commonly these are so-called coarse grains -100-200 μm, or fine grains - 4-11 μm. Occasionally other grain sizes are used.