Luminescene dating
Description
The first observation of a kind of luminescence of a heated diamond was published by Robert Boyle as early as 1663, but it took until the second half of the twentieth century until the phenomenon of luminescence was investigated scientifically and even used as a dating method, first in archaeology. Today, the two methods of thermoluminescence (TL) and optically stimulated luminescence (OSL) are well established and belong to the standard methods of age determination for archaeological and geoscientific research together with radiocarbon dating.
Application
The two most important application fields of luminescence dating are the determination of the age of ceramic objects or fired inorganic materials on the one hand and the dating of sediments on the other. The method uses the phenomenon that during heating or exposure to light, the signal that has built up in the material of an artefact or a sediment as a result of the ionizing radiation is effectively reset and then re-accumulated. From the intensity of the light emitted it is possible to estimate the radiation dose to which the material was exposed. The annual dose-rate in the sample itself and its surrounding can be determined by various methods and then the ratio between the accumulated dose and the annual dose yields an age in years.
Several types of stimulation for luminescence are used for dating. In archaeology the widest known type is the thermally stimulated luminescence (TL), where the light emission of the sample is measured while it is being heated to 500 °C. An alternative method especially used in sediment dating is optically stimulated luminescence (OSL), where the heating is replaced by stimulation with light of a selected wave range. Other methods used more and more often include radio luminescence, in which stimulation takes place during irradiation with an ionizing source, or spatially resolved luminescence, which allows individual mineral grains to be dated in their original structure. The selection of the optimal method depends on the research object or goal and the requirements and restrictions associated with each method. For example, OSL often takes longer than TL.

The radiation dose to which the material was exposed can be determined from the signal intensity. The annual dose rate from the sample itself and its environment can be measured in various ways so that an age can be calculated from the ratio of the accumulated radiation dose and the annual dose rate. Several types of stimulation for luminescence are used for the measurement (Fig. 2).

The best known method in archaeology is thermally stimulated luminescence (TL), in which the glow of the sample is measured when it is heated to 500 °C. Alternatively, and mainly used for sediment samples, is optically stimulated luminescence (OSL), in which the stimulation is not thermal but is achieved by targeted exposure to light in a selected wavelength range. Other methods that are being used more and more frequently are radioluminescence, in which stimulation takes place during irradiation with an ionising source, or spatially resolved luminescence, which allows individual mineral grains to be dated in their original formation. The specific method is selected according to the object or objective of the investigation and the requirements and restrictions associated with each method. For example, an OSL often takes longer than the TL dating of a ceramic.
Basics
In naturally occurring minerals such as quartz or feldspar, which are abundant mineralogical components of fired ceramics, energy is stored over time in form of radiation damage in the crystal lattice. (Fig. 3).

This damage is caused by the decay of naturally occurring radioactive nuclides (thorium-232, uranium-235, uranium- 238, and potassium-40) and by cosmic radiation, which raise indi-vidual electrons in the minerals to elevated energy levels and thus store the radiation energy. Only by heating or exposure to light, are the electrons released from their higher energy levels and emit the energy difference in the form of light, which can be measured.
The sample preparation includes the removal of the surface or any material that may have been exposed to light. Accordingly, this takes place in dark laboratories where – ideally under red light – the sample is crushed and the material sieved. In general, steps of preparation include the removal of organic materials and carbonates followed by some additional steps before the ac-tual measurement.
This can either be performed on a very fine-grained polymineralic fraction of the sample in the grain size-range of 4-11 μm, where the signal is dominated by potassium feldspar whose luminescence is more intense thanthat of quartz. Alternatively, a coarse grain quartz fraction, usually of extracted, of a grain size of 100-200 μm can be measured. The quartz signal is often more stable over time than the feldspar signal.
In both cases, the samplematerial is subdivided into subsam-ples and measured in groups. One possibility is the additive measurement where one subset of samples is used to measure the natural signal, whereas further subsamples are irradiated with different well-defined laboratory doses on top of the natural signal. In this case, the result is a single growth curve where each subsample is taken into account to calculate the dose. In contrast, the regenerative method allows to determine a dose for each subsample individually by applying cycles of measurement and irradiaion for each subsample. Hence, each subsample yield a dose value.
Both methods have advantages and disadvantages, which may however be overcome through a combination of methods.
Due to the large number of influencing factors (e.g. formation of the analysed mineral, humidity of the sample over time, radioactivity of the sample and the environment), the results of this dating method are inevitably subject to certain sources of error, which cannot be completely avoided even by the regular measures taken (e.g. determination and compensation of any radioactive imbalance in the sample or the aforementioned combination of measuring methods). Uncertainties of up to 10 % of the determined age are generally assumed, whereby the nature of the sample to be analysed and the context of the find should not be underestimated. Under optimal conditions, the method enables dating of up to 200,000 years in the routine range and thus unique insights into early human history and events relevant to geological history.
Limitations
Due to the large number of influencing factors (e.g. formation of the mineral, moisture and radioactivity of the sample and the surrounding material, radioactivity), the results of this dating method are inevitably subject to a certain degree of uncertainty, which cannot be avoided despite careful measurements (e.g. determination and correction of a possible radioactive dis-equilibrium in the sample or the aforementioned combination of measuring methods). Thus, it is generally assumed that an uncertainty of around 10% of the age determined has to be considered, whereby the quality of the sample submitted as well as the detailed information of the find context should not be underestimated. Under optimal conditions, luminescence dating ranges up to 200,000 years in routine work and thus provides unique insights into early human history as well as relevant events in the Earth ́s history.