Definition of thermoluminescence dating
Thermoluminescence (TL) dating of sediments depends upon the acquisition and long term stable storage of TL energy by crystalline minerals contained within a sedimentary unit.
This energy is stored in the form of trapped electrons and quartz sand is the most commonly used mineral employed in the dating process.
The exposed material at either end of the tube is used in the determination of the annual radiation dose and the internal unexposed portion for the palaeodose determination.
The sample is taken by introducing the tube into a freshly cleaned back surface; if this proves difficult a block may be cut from the unit of interest.
The specimen tube or block should then be wrapped in black plastic to prevent further exposure to light and to preserve the environmental moisture content.
In the case of an older sample this correction may only represent a small proportion of the total age.
The radiation dose received annually by the sample is measured by means of calibrated thick source alpha counting which determines the specific activity of the uranium and thorium decay chains assuming that secular equilibrium exists.
This correction becomes a little more difficult in the case of waterborne sediments but may be determined from a subsurface sample of recently deposited similar material.
In general the older the sediment the less significant the surface residual correction becomes.
Prior to the final depositional episode it is necessary that any previously acquired TL is removed by exposure to sunlight.
After burial the TL begins to build up again at a rate dependent upon the radiation flux delivered by long-lived isotopes of uranium, thorium and potassium.
Aliquots of both the bleached and the unbleached quartz are deposited onto a series of aluminium planchettes and a number of these are incrementally irradiated using a calibrated90Sr plaque source.
Each planchette, complete with its sample aliquot, is heated to 500 deg C at a controlled rate and in an oxygen free atmosphere.
The technique is best suited to the age determination of sediments which have undergone long transport distances and, in the case of water borne sediments, as suspended load under shallow water, low energy conditions.