Electron spin resonance dating range

The present study was aimed at using the ESR method for dating tooth enamel from the Aguazuque archaeological site in Soacha, Colombia. The human tooth used in this work was extracted from a skeleton labelled AG-I taken from a collective burial site where 23 people were found arranged in a foetal position; men, women and children were identified. The burial site was circular shaped 4. Archaeological research concerning the early and middle Holocene period has led tore constructing the way of life for human groups in the region, thereby showing a transition from hunters and gatherers to agricultural societies Groot Electron spin resonance ESR consists of the resonant absorption of electromagnetic energy during electron-spin transitions.

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A static magnetic field should be applied to resolve different electron-spin levels. Unpaired electrons from free radicals have spin equal to V. An important conclusion derived from this formula is the linear dependence between the applied magnetic field and resonance frequency. The most frequently used microwave energies drop in what is called the X band. ESR dating and dosimetry are based on the fact that ionising radiation produces paramagnetic centres having long lifetimes in a number of materials.

Such centres' concentration in a given sample is a measure of the total radiation dose to which a particular sample has been exposed. This effect can be used to determine the length of time of such exposure and hence, in many cases, a sample's burial time Jonas The aim is to determine the accumulated dose D A to which a sample has been exposed whilst being buried; the exposure time is then determined by comparing D A to the average dose rate over such period:. However, no significant difference in the accuracy of dose reconstruction has yet been found using different calibration methods Wieser et al.

Electron spin resonance dating

Each sample's radiation sensitivity is individually calibrated for dose assessment by the additive dose method; each sample is incrementally irradiated with at least 14 additional doses up to Gy or more avoiding the sample becoming saturated. A linear regression analysis is applied to the ESR measurements at each dose increment and the original measurement laboratory-un-irradiated sample.

The original D A in the sample is obtained from the negative intercept of the regression line with the dose axis; is estimated as being the total radiation provided by all radioactive elements present in a sample and soil and also cosmic radiation Ikeya ; Jonas ; Walker ; Grun The dentine was initially removed from teeth using a dental drill and water cooling.

The molar enamel powder was divided into nine aliquots. The spectrometer parameters used were: Soil samples were also collected from the Aguazuque site and sent for U, Th and K content analysis by gamma spectrometry. Radiometric analysis is widely used for determining natural radioisotopes in geological samples by means of spectroscopic methods and, especially, for quantifying the uranium, thorium and potassium present in samples.

A linear model was used for fitting ESR signal intensity to dose, as has been done by other authors Baffa et al.

Electron spin resonance dating in paleoanthropology

ROSY software Brennan et al. Soil water content was The software's default values were used for the remaining parameters. Finding the dose rate to convert D A into an age is a crucial step after D A has been found. ROSY software allows age to be calculated from an enamel sample by taking D A due to radioisotopes from adjacent layers into account. The software computes age based on three uranium absorption models linear, early uptake and a combination of both. Table 1 shows the soil analysis results.

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The present work did not determine U, Th and K concentration in enamel and dentine as their percentages have been negligible for calculating age in previous work, especially in dates for the Holocene period Kinoshita et al. Typical soil density, enamel and dentine values were used for calculating age 2. Using these values and the radioisotope concentrations listed in Table 1 , ROSY software was used for obtaining the alpha, beta and gamma radiation dose rate contribution for each radioisotope in sediment the results are listed in Table 2.

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These results were the same for any uranium absorption model. It should be noted that ROSY software calculates these ages using three uranium accumulation models: The way that uranium accumulates in a tooth is established by these models; accumulation increases at the same rate as time in the linear model and accumulation occurs in a short period of time in the early model, relative to a tooth's age and then remains constant after this short period of time.

The tooth's age was the same when calculated by these three methods; this result was consistent if taken into account that the accumulation of radioactive elements inside the tooth was negligible. Age determined at many countries' archaeological sites is the result of radiocarbon 14 C analysis of particular items found around the burials and not direct dating of individuals found there.

The results regarding either physical or geological phenomena may not be chronologically equal, hence the importance of having techniques like ESR which can directly and absolutely determine the age of individuals of interest. This result agreed with stratigraphic analysis at sites providing soil ages ranging from 2, to 3,y BP for 50 and 80 cm depths Correal, 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 2 , having 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. Dental tissues are usually assumed to be free of Th and 40 K, 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. 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.

Various laboratory analytical techniques may be used to calculate the radioelement contents of the sediment, e. Other techniques, like beta counting for example, may directly provide a total beta or gamma dose rate value.

To do so, mass spectrometry techniques are now usually employed [e. This is done via tables, and the value depends on the depth of the sample, as well as the density of the sedimentary matrix, latitude and altitude of the site. EPR age calculation of fossil teeth is not so straightforward since it may involve up to 25 parameters. In addition to this, the dose rate in dental tissues is not constant over time but has to be modelled from the U-series data collected. An EPR age may be obtained by iteratively solving the integral Equation 1 , since there is only one solution for which the total dose rate built up over time will match the D E value.

Not every chronometric dating method can be used on a given Prehistoric site, since by definition it depends on the presence or absence of suitable materials for this purpose, which is closely related to the geological context and the presumed age of the site. Figure 3 shows the time range applicability for some of the most used dating techniques in Quaternary studies.

EPR is one of the very few dating methods that may be applied to fossil remains. By definition, the dating of a tooth remains provides a direct dating of hominid or animal occupations, whereas other numerical methods can only date the sedimentary matrix that is enclosing the archaeo—palaeontological materials.

Consequently, this application may be also potentially used in any sedimentary context, while other methods like Argon—Argon or cosmogenic nuclides dating can only be used on volcanic minerals and quartz grains, respectively. In addition, the EPR technique is one of the very few possibilities to date fossil remains beyond the C and U-series dating time range.

C, U—Th, Ar—Ar; an overview may be found in Reference 12 , this is nevertheless one of the few methods that can be used for the Early Pleistocene period 2. This is a key period in European Prehistory, marked by the arrival of the first hominids in the continent and who very likely spread from Georgia, about 1. EPR has definitely an important role to play for the improvement of the chronological framework of the oldest hominids settlements in Europe.

This is very likely due to the long and complex analytical process that requires a large diversity of equipment e. U-series analyses facilities, EPR spectrometer, gamma irradiation source, high resolution gamma spectrometer, portable gamma spectrometer , which make it especially complicated to setup a complete and autonomous laboratory.