Luminescence dating

Luminescence describes the emission of light from insulating solids, independent of black body 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 (predominately 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 irradiation 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 heat source any longer, 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:

$$Age~(ka) = \frac{Equivalent dose~[Gy]}{Dose rate~[Gy/ka]}$$

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 feldspar grains 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:

• green light (GSL) - feldspar and quartz
• infrared (IRSL) - K-feldspar
• blue light (BSL) - quartz
• red light (RSL) - volcanic feldspar and quartz