Optically stimulated luminescence dating accuracy
The corresponding Equivalent dose is calculated by reading it down to the x-axis. where there is still scope for much more charge accumulation) the interpolation is simple as illustrated in Figure 1a.If the sample is relatively old then the interpolation of the natural value is at relatively high positions on the OSL axis because it is near a state of saturation as illustrated in Figure 1b.Samples subjected to a high dose rate will become saturated more quickly, and fully saturated samples will not record the full duration of their burial history.In these cases only a minimum age can be determined.The luminescence of each sample is measured using industry-standard Luminescence Readers (manufactured by Risoe National Laboratories, Denmark) which incorporate 90Sr beta-sources, and 470nm LED optical stimulation.The total absorbed dose (termed De, measured in units of Gy) is measured using standard luminescence dating procedures (Murray and Wintle, 2000).
These slowly decay over time and the ionising radiation they produce is absorbed by other constituents of the sediments such as quartz and feldspar.
The method is routinely applied to sediments as old as 250Ka and, consequently, is ideal for dating material beyond the range of 14C dating (A short length of undisturbed core is sub sampled in a dark room to extract a few grams of sediment.
A residue of pure quartz is extracted by chemical digestion in hydrochloric acid, hydrogen peroxide and fluorosilicic acid, in a process which may take several weeks.
The limiting factor in the age range for luminescence dating is the 'saturation' of the signal at large dose rates (i.e. Accurate age determination therefore becomes increasingly difficult for older samples and there is a loss in dating precision (an increase in statistical uncertainty).
The point at which a sample becomes saturated depends on the dose rate of the sample.