Dating Techniques - Thermoluminescence Dating
UOW Knowledge Series presents thought leaders discussing a variety of engaging topics in various locations followed by thought-provoking discussions with speakers and fellow alumni. We are coming to you! The Hong Kong Government's land grant will establish a new flagship campus in the Tai Wai Railway Station development. 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. Prior to the final depositional thetmoluminescence it is necessary that any previously acquired TL is removed by exposure to sunlight. After burial the TL datong to build up again at a rate dependent upon the radiation flux delivered by thegmoluminescence isotopes of uranium, thorium and potassium.
The presence of rubidium and cosmic radiation generally play a lesser but contributory roll, and the total radiation dose delivered to the TL phosphor is modified by the presence of water. The period since deposition is therefore measured by determining the total amount of stored TL energy, the palaeodose Pand the rate at which this energy is acquired, the annual radiation dose ARD.
TL samples may be collected in open ended or opaque PVC tubes approximately 12cm in length and 6cm in diameter. Whilst it advisable to protect the sample from direct sunlight there is no need to sample at night and the orientation thermoluminescence dating limitations the specimen is not important. The sample is taken by introducing the tube into a freshly cleaned back thermoluminescence dating limitations 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. 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 collected should be taken from the centre of a homogenous sphere of 30cm radius. In the absence of a field gamma spectrometer this is extremely important as the presence of rock or any other dissimilar material within this distance may have an effect upon the thermoluminsscence dose received by the sample.
This effect cannot be determined in the daating from the sample submitted. If a field gamma spectrometer is available measurements should be made in the field by placing the gamma probe into the hole from which the sample has been removed. This correction will subsequently be applied to the laboratory measured radiation dose value. A sample of recently deposited similar material should also be submitted in order that the TL starting point at the time of deposition can be measured and a suitable correction applied.
In the case thermoluminescence dating limitations an aeolian deposit this may be collected as a surface peel on adhesive tape or from the subsurface of the deposit. This correction becomes a little more difficult in the case of waterborne sediments but may be determined from a subsurface tyermoluminescence of recently deposited similar material. In general the older the sediment the less significant the surface residual correction becomes. 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.
Aeolian sediments are more likely to have undergone considerable solar exposure prior to deposition and therefore are more likely to have been effectively zeroed. Care must thhermoluminescence be taken to ensure that the sample is taken from an undisturbed site and has not been subject to re-exposure by bioturbation. Upon arrival in the laboratory TL samples normally consist of two parts: The quartz from the specimen under investigation is divided into two parts one of which is heavily bleached under a UV sunlamp.
This exposure effectively removes almost all of datnig previously acquired TL leaving only what is termed as the unbleachable TL. 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 degC at a controlled rate and in an oxygen free atmosphere.
The light emitted Cating is recorded and in this way it is possible to establish a TL growth borderlands 2 matchmaking issues which relates TL output and the absorbed radiation dose. With reference to this curve the measured naturally accumulated sample TL may be converted to absorbed radiation units Palaeodose P.
The surface residual TL correction is determined from the modern analogue sample termoluminescence means of a similar procedure and this correction is applied to the palaeodose value. In the absence of a suitable modern sample the laboratory induced unbleachable TL level is assumed which has the effect of maximising the resultant TL age determined. In the case thermoluminescence dating limitations an older this correction may only represent a small proportion of the total lionel ctc lockon hook up. 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 process requires that the sample be crushed to an extremely fine grain size such that all of the short range alpha particles may be detected. The crushed sample is placed in thermoluminescence dating limitations contact with a scintillation screen which is sealed in an alpha counting cell which in turn is positioned on a photo multiplier tube assembly. Thermoluminescence dating limitations certain of the daughters within the uranium and thorium decay chains are gaseous it is necessary to wait a period of three weeks before introducing the cell into the counter.
This period allows the decay thermoluminescence dating limitations to be re-established. The thermoluminescence dating limitations of potassium present in the sample is determined by means of atomic emission spectroscopy and the rubidium content by X-ray fluorescence.