ABOUT LUMINESCENCE DATING
Luminescence dating is an absolute radiometric method used to determine the age of ubiquitous mineral grains (such as quartz or K-feldspars) since their burial or firing, i.e. their last exposure to sunlight, or to heat.
After burial or heating, the sediment/artefact is exposed to a low level of radiation coming from both cosmic radiations and the decay of naturally-occurring radionuclides (mainly uranium, thorium and potassium) that are present in the surrounding sediment matrix. This ionizing radiation causes the accumulation of electrons within single mineral grains at a constant rate, the amount of stored energy being related to the time since deposition or exposure to heat (Figure 1). After stimulation in laboratory with light or heat (respectively called Optically-Stimulated Luminescence OSL, or Thermoluminescence TL) the energy stored in the crystals is released and results in the formation of light, also named luminescence. Quantifying the amount of stored energy (palaeodose De, in gray), and the in situ rate of ionizing radiation (Dose rate D, in gray per year) allows estimating the age of burial or firing following the formula:
Age (year) = Equivalent dose (De) (Gray) / Dose rate (D) (Gray/year)
OSL and TL dating can be used on a wide range of environmental and sedimentary contexts summarized in figure 2, and on heated archaeological artefacts (pottery, bricks, ceramics, burnt flint, etc.). The method has been applied successfully to date samples most commonly ranging from 10 to 300,000+ years, the upper limit being dependent on the properties of the luminescent minerals and the composition of the sample and surrounding medium. The uncertainty in the age is calculated as a percentage of the age and is typically in the range of ±4% to ±12% of the sample age (68% level of confidence) for single determinations.
Figure 1: Simplified principle of Luminescence dating: zeroing or reset of the signal when exposed to light / heat, and accumulation of signal with time.
Figure 2: Environmental and sedimentary contexts in which Luminescence dating can be applied (from Rhodes, 2011). The applicability of the method is both sample and site specific.