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Articolo pubblicato il 30-07-2005
P.R. González Martínez¹, D. Mendoza Anaya¹, A. Ramírez Luna², Peter Schaaf²
Numero 18/19 - Anno 2
30 Luglio 2005
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 Mexican prehispanic ceramic dating by thermoluminescence method
Abstract
One of the most frequently recurring questions in Archaeometry concerns the age of the studied objects. The first dating methods were based on historical narrations and style of buildings manufacture techniques. However it has been observed that as a consequence of the continuous irradiation from naturally occurring radioisotopes and from cosmic rays some materials, such as archaeological ceramic, accumulate a certain quantity of energy. These types of material can be dated, in principle, through the analysis of this accumulated energy. In such a case, the objects have been subjected to high temperature heating in the manufacture process; after that the object accumulates energy from ionizing radiation and then the thermoluminescence (TL) dating technique can be applied. In this work, results obtained by our research group about TL dating of ceramic belonging to several archaeological zones like Edzná (Campeche), Calixtlahuaca and Teotenango (Mexico State) and Water legs (Durango) are presented. The analysis was realized using the fine grained mode in a Daybreak model 1100 reader TL system. The radisotopes that contribute in the accumulate annual dose in ceramic samples (40K, 238U, 232Th) were determined by means of techniques such as Energy Dispersive X-ray Spectroscopy (EDS) and Neutron Activation Analysis (AAN), while the artificial irradiation of the samples was carried out with a 90Sr source beta radiation. Our results are in agreement with results obtained through other methods.
Introduction
Among the different archaeological materials, pottery is extremely valuable because it is long-lived and characteristic in shape and decoration. The use of ceramics in the construction of chronologies has a long history in archaeological studies, extending back to the works of the late nineteenth century. Using stratigraphy, archaeologists can examine changes in ceramic form over time using only the vessels themselves, largely independent of their context of recovery. Seriation has been defined as the procedure of working out a chronology by arranging local remains of the same cultural tradition in the order to produce the most consistent patterning of their cultural traits. However this dating method could be conduced to serious errors if pottery analysis is not adequate. The thermoluminescence technique is the only physical way for determining the absolute age of pottery presently available. It is an absolute dating method, and does not depend on comparison with similar objects.
Thermoluminescence (TL) is the emission of light when a substance is heated at temperatures lower than its incandescence temperature. The TL coming out from the minerals present in a pottery sample is produced by the prolonged exposure to the ionizing radiation emitted by the radioactive elements like U, Th and 40K embedded in the sample as impurities and present in the surroundings. A little contribution to TL is due to the cosmic rays too.
The accumulated dose which is responsible of the TL is named the paleodose (P). Due that all the radioisotopes responsible for TL have a very long half-life, their emission of radiation remains constant and the amount of TL induced is proportional to the time that has elapsed since the pottery was fired(1). This fired removed all the TL acquired by the sample during geological times and sets the clock to zero. The basic form of the age equation is:
The sites
Edzná is an archaeological site of the Mayan Culture at 19°25´ of Latitude North of the Yucatan Peninsula in Campeche, Mexico. This site is known by its cultural development from Early Classic since 600 to 1200 years A.D. however, the age determined by means of 14C inside some buildings disagree with the historical data(1). Due this chronological discrepancy it is necessary to confirm this age by means of other dating methods.
The archaeological site "El Olote" is situated in the region of Hervidero in Durango, Mexico, located at 25° 14' North and 105° 27' West. According to the running studies this site has presented two occupations of different intensity. The occupation, which left the most important and abundant vestiges, was the Chalchihuites Culture. However, until now it is no possible to establish exactly the nature of this occupation. The type of materials found at the surface and into the excavations seems to indicate that the terrace where the well probe was done corresponds to the Chalchihuites Culture.
Calixtlahuaca is an archaeological site of the Mexica Culture located at 19°20' North and 99°41' West, about 50 km from Mexico City. The site was occupied by the Matlazinca before the Spanish arrival. However, it seems that there were probably inhabitants in this place for a long while. Ceramic fragments which could belong to the period from the Early Classic (approx. 200 - 500 A.D.) up to the Early Post-classic (approx. 900 - 1100 A.D.) have been found at this site.
Teotenango is an archaeological site located at 19°06' North and 99°44' West, about 65 km from Mexico City. The site shows three stages of human occupation; the Teotenanca, (900-1200 A.D.), the Matlatzinca (1200-1476 A.D.) and the Aztec (1476-1521 A.D.). The earliest and less known stage is related to this site with a transitional type of ceramic identified as Coyotlatelco; during the second one the Matlatzinca ceramic types are widely spread into the site. Sample results on both ceramics are reported here.
Experimental procedures
Various fragments of ceramics have been recovered for this study. The samples were obtained from archaeological pits made in Edzná, El Olote, Calixtlahuaca and Teotenango. Samples collection occurred during the morning, the shreds were not washed and immediately saved on black plastic containers, protected from heat and sun light. The samples were delivered the next days to laboratory for TL analysis; only eight samples, labeled CH3, OLOT, CAL1, CALIX6, T2, T4, T5 and T6 have been completely investigated.
The dating technique used was that of the fine grain method(2). A 2 mm layer was first removed from the surface. The remaining sample was crushed and powdered in an agate pestle and mortar. The grains sized from 4 to 11 µm (fine grains) were treated with H2O2 to remove the organic material and with HCl to neutralize carbonate.
More than 60 discs of each sample, each containing 2 mg of the sample, were prepared to check the reproducibility of the procedure . Discs were made of aluminum 0.5 mm thick and 9 mm in diameter. All the sample preparation and analysis was performed under red light illumination.
The TL reader used was a Daybreak 1100 Automated TL system, Instituto de Geofísica, UNAM. It is equipped with a sample-holder which can carry 20 discs and is coupled to a PC loaded with the software to control the whole reading process. This software permits the analysis of the plateau (region of interest) and uses least squares fitting of the TL curves to give an estimate of the dose equivalent (Q) and the supralinearity correction factor (I)(3).
Artificial irradiation of the samples was performed with a 90Sr beta source (Amersham International), with an activity of 3.7 MBq (100 mCi) at a dose rate of 190 Gy·h-1.
Sixteen readings of unirradiated samples were taken to obtain the natural TL signal (NTL); the remaining samples were irradiated in vacuum at four different artificial doses, which were labeled as 1ß, 2ß, 3ß, and 4ß respectively in every cases. The samples were stored one week in the dark prior to measurement. No variation in TL intensity was observed during this period of storage. Data took into account were those obtained from the reading of the discs presenting an uncertainty less than 4%. This suggests that the sample does not undergo anomalous fading.
The dose equivalent (Q) was determined by the additive dose method; while the determination of the supralinearity factor (I) was carried out by means of the regeneration method (1,2), using those samples from which NTL had been previously erased.
To estimate the annual dose rate the 40K concentration was determined by means of energy dispersive spectroscopy (EDS) meanwhile, the U and Th contents were obtained by means of neutron activation analysis (NAA) technique, using the Triga Mark III reactor operating at 1 MW.
The gamma radiation dose rate was measured using locally made CaSO4:Dy TL dosimeters(4).
Results
Table 1 shows the U, Th and K concentrations, the average gamma radiation by soil and cosmic rays measured with TLD´s, the moisture contribution as well as the annual dose rates in Gy/ka. Taking an efficiency factor of 0.10 and a Th/U ratio of 3:1(5), annual dose rates in dry conditions were obtained.
In figure 1 we can observe the plateau which appears in a wide temperature range where it is possible to evaluate the parameters within a high confidence level(1).
In figure 2(a) we can observe the glow curves of unirradiated samples (NTL) as well as those irradiated with 90Sr beta radiation at four different artificial doses (NTL+ATL). The dose response curves obtained by the additive dose method(3), for the T6 sample, are shown in figure 2(b). Interpolation of these curves to zero TL intensity (TL=0) gives the dose equivalent (Q); this was determined at each 10°C temperature intervals in the range from 250 up to 400°C for T6 sample. The correction factor due to supralinearity was determined by means of the regeneration method at the same 10°C temperature intervals. These results are shown in figure 3(a) and 3(b) respectively.
Fig. 2. (a) Glow curves of unirradiated samples (NTL) as well as those irradiated with 90Sr beta radiation at four different doses (NTL+ATL). Sample T6 irradiated at; 1ß, 2ß, 3ß and 4ß. (b) Dose response curves of the samples obtained by the additive method to sample T6
Fig. 3. (a) Glow curve of T6 sample beta irradiated at; 1ß, 2ß, 3ß and 4ß. (b) Regenerated growth curve of T6 sample obtained from the glow curves
Table 2 shows the beta radiation for the equivalent dose (Q) and supralinearity factor (I) for each sample studied as well as the paleodose (P) and age corresponding to the type of samples are presented.
Conclusions
From the results obtained we can conclude that archaeological dating by means of TL is highly confident. This encourages us to continue this research due that our country is rich in archaeological sites which need to be dated. Finally the method should be applied to date all archaeological zones of Mexico.
References
- Aitken M.J., (1985). Thermoluminescence Dating (London: Academic Press).
- Zimmerman, D.W., (1971). Thermoluminescent Dating Using Fine Grains from Pottery. Archaeometry 13, 29-52.
- González P., Chung H., Azorín J., Schaaf P. and Ramírez A., (1998). Thermoluminescence Dating of a Pottery Sample From Edzná-Campeche in Mexico. Nucl. Sci. J. 35(4) 280-284.
- Azorín, J., González, G., Gutiérrez, A. and Salvi, R., (1984). Preparation and Properties of a Highly Sensitive CaSO4:Dy Thermoluminescent Dosimeter. Health Phys. 46(2) 269.
- Adamiec G., Aitken M., (1998). Ancient TL 16(2) 37-50.
- González P., Azorín J., Schaaf P., Ramírez A., (1999). Assessing the Potential of Thermoluminescence Dating of Pre-Conquest Ceramics From Calixtlahuaca, Mexico. Rad. Prot. Dosim. 84(1-4) 483-487.
- Piña Chan Román, (1975). Teotenango, el antiguo lugar de la muralla. Memoria de las exploraciones. Gobierno del Estado de México.
- Fernando Berrojalbiz, Peter Schaaf, Angel Ramírez, Pedro González and Juan Azorín (1999). Thermoluminescence Dating of an Archaeological Sample from Northern Mexico. 9th International Conference on Luminescence and Electron Spin Resonance Dating (LED99), Rome.
1 Instituto Nacional de Investigaciones Nucleares (ININ), Apartado Postal 18-1027, México, D.F., MÉXICO
2 Instituto de Geofísica, UNAM (IGFUNAM), Circuito Interior, C.U, 04510, México, D.F., MÉXICO
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Autore: P.R. González Martínez¹, D. Mendoza Anaya¹, A. Ramírez Luna², Peter Schaaf²
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