What is Luminescence Dating?
Luminescence dating typically refers to a suite of radiometric geologic dating techniques whereby the time elapsed since the last exposure of some silicate minerals to light or heat can be measured. In the luminescence process, when naturally occurring minerals are exposed to low level, ambient, ionizing radiation emissions associated with the decay of U, Th, and K, electrons become stored and collected within defects in crystal lattices referred to as ‘trapping centers’ or ‘traps’. When dosed minerals are then re-exposed to light or heat, they release the stored electrons, emitting a photon of light that is referred to as luminescence. This 'bleaching' process empties the electrons stored in the traps and resets or 'zeroes' the signal.
Above: In silicate minerals, when radiation interacts with the crystal (Irradiation), energy pushes an electron into the conduction band and leaves a ‘hole’ in the valence band. The electron may become trapped at a defect site (T1, T2 etc) for some time (Storage). When the crystal is stimulated by light or heat, the electrons in the traps are evicted into the conduction band (Eviction). From there, they can recombine with holes at the luminescence centers (L), resulting in the emission of a photon of light – the luminescence signal that is observed in the laboratory. (Modified from Aitken, 1990; Duller, 2008)
The burial age is calculated as a ratio of the equivalent dose (total energy accumulated during burial) and the dose rate (energy per year delivered by radioactive decay): Age (yr) = Equivalent dose (Gy)/Dose rate (Gy/year)
Through controlled experiments the emission of luminescence can be controlled and measured and then used to estimate the equivalent dose (De). The dose rate (Dr) is the amount of energy absorbed per year from radiation in the environment surrounding the sample material and is estimated by measuring the amount of radioactivity directly or by chemically analyzing the surrounding material and calculating the concentration of radioisotopes.
Above: The build-up and resetting of luminescence signals is similar to a rechargeable battery. When mineral grains are exposed to light or heat, energy stored in the form of trapped electrons is released, similar to emptying a battery of its charge. During burial, energy builds, recharging the signal. In the lab, mineral grains are stimulated to release the stored energy in the form of light. The brightness of the luminescence signal is related to the amount of energy stored in the mineral. (Adapted from Duller, 2008)
Aitken, M.J., 1985. Thermoluminescence Dating. London: Academic Press. (Out of print.)
Duller GAT. 2008. Luminescence Dating: Guidelines in Using Luminescence Dating in Archaeology. Swindon: English Heritage.