Thermoluminescence (TL) Dating

David M. Price, School of Earth & Environmental Sciences, University of Wollongong, Northfields Avenue, Wollongong, NSW, 2522.  Phone: (02) 4221 3632, Fax: (02) 4221 4250  Email: dprice@uow.edu.au

Basic Principles

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 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. 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 (P), and the rate at which this energy is acquired, the annual radiation dose (ARD).

TL Age = Palaeodose (P) 
                _____________
            (ARD)

Sampling techniques and strategies

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 of the specimen is not important. 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. 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 radiation dose received by the sample. This effect cannot be determined in the laboratory 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 of 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 sample 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 also be taken to ensure that the sample is taken from an undisturbed site and has not been subject to re-exposure by bioturbation.

Laboratory sample processing

Upon arrival in the laboratory TL samples normally consist of two parts: the sample to be dated and a modern analogue sample for the surface residual correction. Both specimens are carefully sieved to separate the 90-125 micrometre grain size fraction, chemically cleansed in dilute HCl,etched in 40% w/w HF and finally subjected to heavy liquid separation. The sample so prepared consists of better than 99% pure quartz grains.

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 the 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 500 degC at a controlled rate and in an oxygen free atmosphere. The light emitted(TL) is recorded and in this way it is possible to establish a TL growth curve 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 by means of a similar procedure and this correction is applied to the palaeodose value. In the absence ofa 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 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 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 immediate contact with a scintillation screen which is sealed in an alpha counting cell which in turn is positioned on a photo multiplier tube assembly. Because 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 chains to be re-established. The amount of potassium present in the sample is determined by means of atomic emission spectroscopy and the rubidium content by X-ray fluorescence. Thus,assuming the cosmic contribution and applying a correction for the modifying effect of the sample moisture content,the radiation dose received upon an annual basis (ARD)may be computed and the depositional age of the sample determined from the equation shown.

Dateable material

TL dating as practised in the Wollongong laboratory may be applied to aeolian, fluvial, coastal and, in some cases, marine sediments. The technique is also successfully applied to volcanic materials and heated fire hearth samples and therefore may be directly applicable in certain archaeological contexts. The age determination of pottery is also undertaken and the method may also be applied to casting core material removed from bronze artefacts. Certain of these analytical procedures may make use of the polymineral 1 – 8 micrometre fine grain sample fraction rather than the 90 – 125 micrometre quartz grains.

Cost and turn around time

The full cost of a sedimentary age determination is currently $770.00 (inc GST) although in certain circumstances a reduced collaborative rate of $440.00 (inc GST) may be applicable. This charge includes the analysis of the modern analogue sample which may be applicable to more than one sediment. A charge of $297.00 (inc GST) is made for the authentication of pottery and bronze artefacts the purpose of which is to establish the originality of the piece rather than to accurately determine the age. This pricing structure will be in place commencing July 1st 2010 until further notice.

The turn around time for authenticity work is generally around two weeks. Often a verbal result is available within a week of submission of the ware. It is preferable to bring the item for test to the laboratory such that it can be sampled and taken away on the same day. Sediments may be sent to the laboratory but full field data must also be submitted. It is essential to work closely with the laboratory to ensure a full understanding of the processes involved in the analysis and also to provide any special information that my have an effect upon the dating of the sample. TL dating is both a costly and time consuming process and, as with most dating methods,good samples give good dates, poor samples seldom provide reliable ages. By the nature of the procedure practised TL dating of sediments cannot be achieved reliably in less than a month and because of the backlog analysis will generally take longer. Small numbers of samples are normally analysed rather more promptly than larger batches which tend to reduce the flexibility of the laboratory. Every effort is made to meet all agreed realistic deadlines.

Australian quarantine regulations

Australia necessarily has quarantine regulations which prohibit the entry of unauthorised soil/samples from other countries. Before submitted such a sample to the laboratory it is necessary to obtain an import permit.  These may be obtained from Australian Quarantine and Inspection Service at cost of AUD$60 and are valid over two years. Additionally the laboratory possesses a permit of approval to work upon imported samples which is renewed upon an annual basis. When sending such samples to the laboratory it is essential that a copy of the import permit is attached to the package and that it is clearly marked with the laboratory approval number N0555. It is also necessary to clearly label sample packages to prevent x-ray or exposure to light. The addition of the laboratory phone number may also assist in the case of problems encountered during quarantine inspection.

Other applications and advantages

TL is an electron trap method of dating as opposed toa radiometric technique such as carbon dating. This has the advantage that the TL signal increases with time rather than decreasing. At a palaeodose level, dependent upon the physical properties of the quartz grains, a saturation point is reached beyond which there is no further increase in the TL with additional irradiation.This saturation level represents the point at which all of the electron traps are full. Once this palaeodose level has been reached a finite depositional age for the sediment cannot be determined and it is only possible to determine a minimum age value. This level frequently occurs at around 250 Grays in the case of quartz but this value may vary from one sample to another. An accumulated palaeodose of this level may be reached as soon as 50kaor, in exceptional cases, almost 1ma dependent upon the operative radiation flux level.

Naturally occurring quartz grains contain chemically in built impurities such as silver and manganese and it is from within these electron trap sites that TL is emitted.Pure quartz in fact does not emit a TL signal. Quartz is then often the preferred phosphor employed in the TL dating process because of the stability of the electrons within the crystalline lattice ie long lifetime c109years. Feldspars may be subject to loss of TL signal over a period of time and must be treated with caution.

 

Because of the finite electron trap energy levels associated with each of the inbuilt impurities within the quartz crystals, TL peaks are exhibited at defined temperatures as the quartz crystals are heated and the trapped electrons released. Thus a TL spectra may be taken as being indicative of the impurities contained within the quartz grains which in turn may represent the formation conditions and history of the quartz grains. This property, in some instances, may be utilised in the detection of changes in provenance of the quartz and the sediment in which it is contained which can indicate a change in palaeo wind direction or an altered water flow regime.

Electrons stored at low energy electron trap levels are more easily released and thus, upon heating, a naturally accumulated TL spectrum will not contain TL peaks in these regions. When a quartz sample is irradiated in the laboratory and fairly quickly heated these low energy TL peaks are immediately evident in the so-called second glow curve. If the amplitude of this spectrum is compared with a natural TL glow curve over a range of temperatures a plateau region will result over the region where the trapped electrons are stored in a stable fashion, usually between 200 and 500 degC.

If a sediment is only partially exposed during its transport phase, or perhaps, has been re-exposed by a process of bioturbation electrons stored at less stable, lower energy electron traps, may be released leaving only those trapped at the higher energy levels. If the TL spectrum displayed by such a sample is compared with that emitted by a recently laboratory induced spectrum then a stepped temperature plateau characteristic may well result. This is sometimes evident in the case of tsunami deposited sediment where there is little time and solar exposure to enable the complete resetting of the previously acquired TL signal.  A similar process may take place if a sediment is partially re-exposed perhaps as a result of bioturbation or other processes.

Recent literature references and handbooks

Extremely readable accounts of TL dating, processes and application may be found in Thermoluminescence Dating by M.J. Aitken, Academic Press, 1985 and Science-based Dating in Archaeology, Longman, 1990 by the same author. There is also a users newsletter available, Ancient TL,in which the recent developments are reported. This appears three or four times a year and is available from the Laboratoire de Physique Corpuschulaire, IN2P3-CNRS Universite Blaise-Pascal, 63177 Aubiere Cedex, France. The results of the application of TL in its many aspects are reported in many international and Australian scientific journals,Australian Geographer, Australian Journal of Earth Sciences,Quaternary Research, Geomorphology, Marine Geology etc.An international meeting of TL and related electron capture practitioners is held every three years and the proceedings of these meetings are reported in Radiation Measurements and Quaternary Science Reviews. The last such meeting was held in Lake Tahoe on the Californian-Nevada border during 2002 and the proceedings of the 1999 meeting,held in Rome, appear in Rad. Meas. vol 32, No. 5-6 and QSR vol. 20, Nos. 5-9. These volumes provide the most up to date information regarding the state of the art.

 

June 2010