Luminescence

 

Luminescence describes the emission of light from insulating solids, independent of blackbody radiation produced in the course of heating. Luminescence for example can be caused by chemical reactions, mechanical energy (so-called triboluminescence) or by ionising radiation. If the luminescence is caused by irradiation and ceases immediately after the radiation source has vanished, it is known as fluorescence. If the luminescence continues after the irradiation has stopped, it is known as phosphorescence.

Luminescence dating uses the ability of minerals (predominantely quartz and feldspars) to store energy transmitted by ionising radiation. This radiation is caused by the decay of naturally occurring radioisotopes of potassium, thorium and uranium but also consists of cosmic radiation. The result of this irridation are free electrons which are trapped in defects of the crystal lattice. By stimulating these traps with suitable energy such as light or heat, one can free the electrons and measure the amount of released energy (emitted photons), generated by electronic transitions. So light and heat can empty all the traps in the minerals and set the counter to zero. After the minerals are buried again (covered with sediments) and are not exposed to sunlight or a heatsource anylonger, they start to accumulate energy from the surrounding sediments. That means the longer the minerals are buried, the more radiation energy they can accumulate and therefore the more photons can be released. For every sample there is a functional relationship between the luminescence signal and absorbed dose, which allows the determination of the equivalent dose.
The age can be calculated as follows:

         

Datable materials

The main categories of materials, which may be dated by Luminescence, are archaeological pottery and stone tools, fluvial, aeolian, glacial and lacustrine sediments. From the latter ones, quartz- and feldspargrains are extracted for dating.

Dating-techniques

Two main methods have to be distinguished:

1) Thermoluminescence (TL)
    where the sample is heated (ceramics, heated rocks, sediment)
2) Optically Stimulated Luminescence (OSL)
    where the sample is exposed to light (sediment)

 

Different OSL-techniques:


      For more information see Measurement

 

 

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