Luminescence and ESR Dating | Institute of Nuclear Sciences
Professor Michel Lamothe and his team from the Lux Luminescence Dating of the Luminescence and Electron Spin Resonance Dating conference (LED). Research presented at Luminescence and Electron Spin Resonance Dating LED . Explore 3 events, speakers and authors. Electron spin resonance (ESR) dating was introduced into archaeology about 20 years Electron Spin Resonance Signal Optically Stimulate Luminescence.
EPR spectroscopy is now widely recognised as a reference technique for routine dosimetry by many international institutions [e. Over the last decades, many applications have been developed, including those for post-accident dose reconstruction in the environment, biophysical dosimetry using human tissues, to identify irradiated foods, and some of them, such as the alanine dosimetry, have reached a high-level of standardisation.
Hydroxyapatite, the main component of bones and teeth, is especially sensitive to ionising radiation: It is now internationally accepted as a valuable natural EPR dosimeter, and is commonly used in the field of retrospective dosimetry for persons accidentally exposed to ionising radiation. An extensive review on this aspect may be found in Reference 4.
From a mineralogical point of view, tooth enamel is mainly made of carbonated hydroxyapatite [Ca10 PO4 6 OH 2] like dentine or bones.
These characteristics make tooth enamel especially stable over time, i. The EPR signal associated with fossil hydroxyapatite is an asymmetric composite signal. The main radiation-induced signal is defined by three peaks T1, B1 and B2, see Figure 1. Many contributions to this signal have been identified, mainly carbonate-derived radicals and some oxygen radicals, 4 but the major contribution comes from three kinds of CO2— radicals, whose precursors are very likely the carbonate groups CO32— present in the hydroxyapatite.
This natural radioactivity is due to the radioelements, mainly U-series, Th-series and 40K elementsthat are not only naturally present in the sediment, but are also progressively incorporated into the dental tissues. Ionising radiations emitted by these radioelements are alpha and beta particles as well as gamma rays Figure 1. Together with cosmic rays, they contribute to build up a dose in the enamel over time, the magnitude of which will mainly depend on two main parameters: This relationship may be converted into an EPR age equation as follows: This work is carried out in two different ways: To obtain an accurate evaluation of the total dose rate, it is important to divide it into several components.
The specificity of teeth dating relies on the complex system that has to be considered, because a tooth is usually made by several tissues enamel, dentine and, sometimes, cement; Figure 2having various thicknesses and composition. The geometry of the enamel and its surrounding thus has to be considered in the dose rate reconstruction.
In the case of a tooth with an enamel layer surrounded by cement and dentine, the dose rate equation may be expressed as follows: Consequently, with this specific configuration, the internal dose rate within the enamel comes from alpha and beta particles, while the surrounding tissues only provide an external beta contribution. The gamma rays contribution only comes from the sediment, since the absorption by the enamel of the gamma rays coming from the enamel itself and the other tissues can be neglected.
In the case of a tooth with an enamel layer in direct contact with the sediment on the outer side i.
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Dental tissues are usually assumed to be free of Th and 40K, since their incorporation into the crystalline network is very complicated, owing to their mobility and atomic radius, respectively.
Consequently, the dose rate components associated to dental tissues are directly, and only, dependent on the uranium concentration. However, dental tissues behave as open systems for U, i. It is therefore crucial not only to measure the actual U-content but also to know its evolution in the past. Indeed, one may intuitively understand that the total dose absorbed by the enamel will be somewhat different if the uranium was accumulated in the dental tissues shortly after the death of the animal or if it happened only very recently.
The US model defined by these authors is based on the following equation: Examples of dating applications may be found in Reference 9. Standard analytical procedure An EPR age estimate is the result of a long analytical process, made by five main steps associating fieldwork and laboratory procedures: Fossil teeth are usually collected either on site or chosen from collections. Large mammal teeth, and especially from herbivores, are usually preferred, since they offer a thicker enamel layer.
Then, in situ measurements of the natural radioactivity at the exact place where the sample was collected during excavations, or at least the closest possible, is carried out to evaluate the gamma dose rate. Classically, various techniques may be employed: In the laboratory, the fossil tooth is prepared by separating mechanically each dental tissue.
The enamel layer is then cleaned, usually with a dentist drill, and gently powdered, in order to avoid significant angular dependence of the EPR signal within the resonator and to improve sample homogeneity. This is why EPR must be considered as a destructive dating method. Each aliquot is then measured at room temperature by EPR spectrometry in order to study the behaviour of the EPR signal with the increasing dose values see Figure 1. Routine quantitative measurements are usually performed by X-band EPR spectrometry, since it offers a good compromise between sensitivity and measurement repeatability in comparison with higher frequency bands.
The experimental setup for quantitative EPR measurements is specifically designed to ensure the stability of the system, including air conditioning and chiller to control the temperature of the water circulating in the magnet. Measurements are thus performed under controlled experimental conditions and following a standardised analytical protocol, in order to minimise any sources of uncertainty that could affect the repeatability of the measurements see further details in Duval et al.
EPR intensities are then extracted from each spectrum, usually by peak-to-peak measurements between T1 and B2 Figure 1 and plotted vs the irradiation doses in order to obtain a growth curve or dose response curve. A given function, usually a single saturating exponential or a double saturating exponential function, is fitted through the EPR experimental data points. By definition, this function is supposed to describe the behaviour of the radiation-induced EPR signal of tooth enamel since the death of the animal i.
If the gamma dose rate is assessed in situ, the beta dose rate from the sediment if it applies should preferably be assessed in the laboratory from the sediment sample that was collected around the tooth.
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Moreover, within the water transportation mode, the clearest transport environments should be preferred. Therefore, sands transported by clear water reveal being the most appropriate for ESR analyses. It means that it is better sampling pure sands than silty sands.
However, even if bleaching is not complete at the deposition time for fluvial, marine or aeolian sands, the low values of the residual dose observed does not prevent dating these sediments by ESR see further details in Voinchet et al. Grain size and bleaching levels may also be closely correlated. Finest and coarser grain size fractions can also be used for dating by ESR but taking into account that the residual dose is higher than for the intermediate particle size.
Loess is a special case in which an overestimation of ages is systematically observed due to insufficient bleaching turbulent transport clouds preventing sunlight contact. In the field, sediment should be examined for evidence of disturbance such as bioturbation from roots or animalspedogenic processes clay illuviation or segregation or post-depositional reworking because it can mix grains of different ages in a sedimentary profile or alter dose rate conditions over time, respectively Bateman et al.
A metallic cap may be used so that the tube can be easily hammered horizontally into the outcrop. Following the sample collection, both ends of the tube should be sealed with tape to prevent light exposure and loss of sediment.
Clearly label the sample using preferably a permanent dark-colored pen by indicating for example the acronym obtained from the name of the site, year of collection and sample number e. Finally, the tube sealed and labelled should be introduced into an opaque ziplock bag labelled with the same code. If the sediment is too hard or compact to insert a tube, then other sampling techniques can be used.
For example, a block of sediment can be carved and securely wrapped with aluminium foil and tape. Then it must be placed in opaque plastic bags or bigger containers for transport to the laboratory where it will be prepared under controlled light conditions. In the case of coarse-grained alluvial deposits where sand lenses are too thin to sample with a tube or a block, the sandy matrix within the gravel or loose sediment can also be collected in a light-proof container under an opaque plastic cover.
In this context, bulk sediment samples for D analyses and moisture content should be collected from the ESR sampling site tube hole or from a 30 cm radius sphere around the sample fig. Samples should be bagged and clearly labelled using the same acronym to that ESR sample.
In addition, the evaluation of the in situ gamma dose rate can be done using either a field portable gamma spectrometer fig. Consequently, the site should be accessible and secured to avoid any perturbation or loss during that period. Another advantage is that the gamma dose rate registered over such a long period is also by definition indirectly taking into consideration any fluctuations of the water content of the sediment with time.