Optically stimulated luminescence dating accuracy

Where De is the laboratory beta dose that induces the same luminescence intensity in the sample emitted by the natural sample, and DT is the annual dose rate comprised of several components of radiation that arise in the decay of natural radioactive elements.

Artifacts which can be dated using these methods include ceramics, burned lithics, burned bricks and soil from hearths (TL), and unburned stone surfaces that were exposed to light and then buried (OSL).

Luminescence dating (including thermoluminescence and optically stimulated luminescence) is a type of dating methodology that measures the amount of light emitted from energy stored in certain rock types and derived soils to obtain an absolute date for a specific event that occurred in the past.

The method is a direct dating technique, meaning that the amount of energy emitted is a direct result of the event being measured.

TL dating is a matter of comparing the energy stored in a crystal to what "ought" to be there, thereby coming up with a date-of-last-heated.

In the same way, more or less, OSL (optically stimulated luminescence) dating measures the last time an object was exposed to sunlight.

Two forms of luminescence dating are used by archaeologists to date events in the past: thermoluminescence (TL) or thermally stimulated luminescence (TSL), which measures energy emitted after an object has been exposed to temperatures between 400 and 500°C; and optically stimulated luminescence (OSL), which measures energy emitted after an object has been exposed to daylight.

To put it simply, certain minerals (quartz, feldspar, and calcite), store energy from the sun at a known rate.

This page was contributed by Dr Georgina King from the Aberystwyth Luminescence Research Laboratory in the Institute for Geography and Earth Sciences. OSL is used on glacial landforms that contain sand, such as sandur or sediments in glacial streams. The energy released by stimulating the crystals is expressed in light (luminescence).The intensity of blue, green or infrared light that is created when an object is stimulated is proportional to the number of electrons stored in the mineral's structure and, in turn, those light units are converted to dose units.Better still, unlike radiocarbon dating, the effect luminescence dating measures increases with time.As a result, there is no upper date limit set by the sensitivity of the method itself, although other factors may limit the method's feasibility.

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Hollie Wynne (Aberystwyth University) stirs OSL samples being treated with acid in the preparation lab of the Aberystwyth Luminescence Research Laboratory. We make an approximation of the number of trapped electrons by measuring the light that they emit following stimulation by light (hence the name of the technique, “Optically stimulated luminescence”).

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