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It is recognised that the luminescence generated following laboratory irradiation contains a proportion of electrons that are considered thermally unstable over geological time and are not present when measuring the natural signal in a sample.
To remove this element a preheat is administered, which in turn may increase the sensitivity of the sample to further irradiation.
Figure 4 A dose response curve created by the SAR procedure.
The curve is constructed following the response to a series of known laboratory doses (Lx/ Tx) onto which the response of the natural signal (Ln/ Ln) can be projected, to identify the Equivalent Dose (DMeasurement of De is now generally made using the Single Aliquot Regenerative (SAR) protocol (Murray and Wintle, 2000).
Analysis of fully bleached samples is preferred as this ensures that associated errors are kept to a minimum.
Despite this, procedures exist with which to identify and take account of partially bleached grains, as may be seen in fluvial, or more likely glacial sediments, where light exposure may have been attenuated by turbid or turbulent conditions.
The SAR approach provides multiple Des and also allows for the explicit testing of partial bleaching.
The ‘depth’ of the traps within which the electrons are held effectively dictates the point at which a sample will become saturated with charge and so the limit of the dating range.
Quartz has been used for dating to at least 200 ka, while the deeper traps of feldspar have produced dates as old as 1 ma.
The use of fine-grain dating for samples such as pottery, loess, burnt flint and lacustrine sediments, and coarse-grain dating of aeolian, fluvial and glacial sediments is regularly undertaken.
While thermoluminescence (TL, the generation of a luminescence signal generated by thermal stimulation) is still conducted on pottery and burnt flint samples, the bulk of luminescence dating now uses optical stimulation as this releases a signal that is far more readily zeroed than that re-set by heat.