Pierre Guibert

Changes in OSL properties of quartz by preheating: an interpretation

Abstract A study of OSL variation with preheat temperature showed, in a majority of cases, that OSL recorded at room temperature, increases above 200°C before the normal drainage at higher temperature. To explain this behaviour, an alternative interpretation to the common ‘electronic thermal transfer’ mechanism is suggested, supported by a study of hydrothermally grown quartz crystals. This interpretation involves impurities in substitution of Si 4+ , specially Al 3+ , which are associated with species like, in the case of quartz, hydrogen (H + , in fact, OH − ) and alkali ions (Li + , Na + , K + ). These monovalent ions usually act as charge compensators and are mobile during heating. As a consequence of the mobility and a possible irreversible exchange between compensators, the number of radiative recombination centres associated with the OSL trap(s), observable within the detection spectral window used (250–400 nm), increases during preheating. This phenomenon could lead to a wrong ED determination.

Changes in luminescence properties induced by thermal treatments; a case study at Sipan and Trujillo Moche sites (Peru)

Abstract TL-dating studies were carried out on prehispanic pottery attributed to the Moche culture of Peru (from BC 200 to AD 800 according to the previously known chronology). A greater than expected scatter in ages was found for samples known, from archaeological considerations, to have been contemporaneous. This was related to applying a fixed, but inappropriate, annealing temperature to the grains prior to their irradiation to regenerate the TL growth curve. The dependence of growth of TL and equivalent dose evaluation on this annealing temperature was tested in the range 400–1000°C. A combined cathodoluminescence/TL study was used to select the appropriate thermal treatment for evaluation of the final equivalent dose. This procedure, which we suggest can be generally applied in pottery dating, resulted in a reduction of age dispersion when applied to groups of contemporaneous ceramics.

Thermoluminescence — dating of calcite: study of heated limestone fragments from Upper Paleolithic layers at Combe Sauniere, Dordogne, France

Abstract This paper presents a feasibility study of dating heated limestones by thermoluminescence (TL). Fragments of stone extracted from Upper Paleolithic fire places at Combe Sauniere, Dordogne (France), have been investigated. The problem of spurious TL, usually interpreted as the result of a decarbonation process, was avoided by heating the calcite crystals in carbon dioxide. TL dates obtained using an appropriate grain size were found in good agreement with radiocarbon dates. These encouraging results validate such an experimental approach and confirm the suitability of such archaeological materials as a potential source of chronological information.

IXth millenium B.C. ceramics from Niger: detection of a U-series disequilibrium and TL dating

Abstract A set of pottery sherds collected from two ancient neolithic sites in Niger (Tagalagal and Adrar Bous 10) has been dated by thermoluminescence. The natural radioactivity of these ceramics and of their surrounding sediments was measured using low background gamma spectrometry and atomic emission plasma spectrometry. With gamma spectrometry, the comparison between the activity of 238U (deduced from the 235U and 234Th gamma emissions) and that of 226Ra (deduced from 214Pb and 214Bi γ emissions in equilibrium with 222Rn) shows a significant disequilibrium of the U-series. The activity ratio 38 U 226 Ra , which is greater than unity in this case, is interpreted as a result of either uranium enrichment (234U, 235U, 238U) or radium impoverishment that has been occurring since the burying of the archaeological artefacts. The effects of the changes in radiochemical composition on the annual dose are discussed and various determinations of the annual dose are analysed according to different hypotheses of disequilibrium (either permanent state or recent occurrence). The TL results: Tagalagal — 9820±780−10, 180±780 years/1993, Adrar Bous 10 — 9530±750−10,500±730 years/1993, are consistent with the radiocarbon dates obtained from charcoals collected at the same locations (the uncalibrated 14C dates belong to the 9100–9370 BP range). These results are of fundamental interest for the chronology of early neolithic cultures.

Methodological study on single grain OSL dating of mortars: Comparison of five reference archaeological sites

Abstract The paper focuses on single grain OSL dating of quartz extracted from known age archaeological mortars, potentially representing a new tool for dating the construction of historical buildings. Apart from SG-OSL measurements and annual dose rate determination, the samples are systematically studied by means of optical microscopy, EDX-SEM cartography and beta autoradiography in order to evaluate the possible microdosimetric heterogeneity of each sample, arising principally from local variations of potassium content. Besides the practical aspects concerning sampling, preparation and appropriate choice of measurement conditions, the paper aims at the differences in microstructure and in elementary composition between different mortars and attempts to evaluate the impact of these aspects on the dispersion of equivalent dose distributions. Finally, archaeological doses (paleodoses) are calculated by using central age model (CAM), minimum age model (MAM) and internal-external consistency criterion (IEU). The appropriateness of these models for the exploitation of the measured SG-OSL data as well as for a hypothesis on the estimation of the input parameter needed to run these models are discussed. Three categories of mortars were identified: samples without any exploitable SG-OSL signal, samples that could have been reliably dated and poorly bleached samples affected by microdosimetric variations whose dating still remains complicated. Finally, the hypothesis on distinguishing between reliable and questionable dating results is raised and the potentials of the method for dating mortars are pointed out.

Combined dating methods applied to building archaeology: The contribution of thermoluminescence to the case of the bell tower of St Martin’s church, Angers (France)

St Martin’s church, Angers, is emblematic of the problems raised in pre-12th century history of architecture.In view of the importance of this building, it was necessary to attempt to define its dating and this study particularly focuses on its bell-tower. In addition to the conclusion resulting from the interpretation of written sources and typological criteria positioning the construction of the site at the beginning of the 11th century, not only a significant number of 14C dates were carried out on charcoals from the masonry structures, but also independent dating by archaeomagnetism and thermoluminescence were performed on bricks from the bell-tower. The whole results from these three different methods agree and indicate the lower level of the bell-tower was likely built in the 9th century, disputing evidence to the theory of construction in the 11th century of the church.Presented here are the detailed results obtained from the thermoluminescence (TL) dating analysis.

Ultra stable oscillators dedicated for space applications: oscillator and quartz material behaviors vs radiation

This paper presents the first results obtained in the R&D study initiated by the CNES at the end of 2004. Numerous French experts have been gathered to determine and to tentatively understand the mechanisms responsible for the radiation sensitivity of quartz resonators and to correlate the results of various analyses in order to reduce or to anneal their susceptibilities

Characterization of unswept and swept quartz crystals for space applications

Unswept and vacuum-swept synthetic quartz crystals were investigated in order to determine the mechanisms responsible for the radiation sensitivity of this material. Results were obtained by means of infrared (IR) spectroscopy, dielectric relaxation spectroscopy (DRS), and thermoluminescence (TL). First, the effect of vacuum sweeping was clearly demonstrated in IR absorption by a significant decrease in the amount of hydroxyl ions and in DRS by the disappearance of the dielectric loss peak arising from the relaxation of alkali ions. Second, it was shown that swept quartz is less sensitive to irradiation than the unswept crystal. A sharp decrease in the TL sensitivity of the electrolyzed material was observed in the energy range corresponding to the recombination of alkaline-electronic defects. DRS results indicated that the dielectric signal is shifted toward a lower energy range for both types of crystals, suggesting that the irradiation greatly facilitates the relaxation of alkaline species by creating ...

TL and radiocarbon dating of neolithic sepultures from Sudan: Intercomparison of results

Abstract Thermoluminescence dating of a set of 29 pottery sherds excavated at the neolithic sites of El Kadada and El Ghaba (near Shendi, Central Sudan) was carried out at Bordeaux University. The archaeological dose was measured using the fine grain technique. The annual dose was determined by analytical techniques (neutron activation analysis, ICP spectrometry, XRF, low background gamma spectrometry) and by “on-site” measurements of the environmental radioactivity (gammametry). The crystalline inclusions of the samples were characterized by optical microscopy and cathodoluminescence: the TL minerals mainly consist of quartz and K-feldspar crystals. In some cases, radioactive inclusions of zircons and monazites are observed. The TL and the radiocarbon dates show a good agreement, verifying the validity of the radiocarbon ages which were suspected to be too old because of the nature of the dated material (shells). Taking into account all the chronological data, it is shown that El Ghaba and El Ghaba necropolis were used respectively within the 4800-3300 B.C. and 4200-3000 B.C. date-ranges for neolithic cultures, the occupation of El Kadada starting five or six centuries later than El Ghaba.

Luminescence dating of burnt materials: effects of preheat treatment on OSL and consequences for dating procedures

For dating purposes, OSL experiments were carried out on a variety of silicate materials extracted from ceramics and kiln fragments which were already dated by TL. In order to check the thermal stability of signals, a study of the variation of OSL with preheat temperature showed, in a majority of cases, that OSL, recorded at room temperature, increases above 200°C before the normal thermal drainage at higher temperature, at more than 250°C. According to our data, preheat treatments at a temperature which induces an increase of OSL, lead to underestimated measures of equivalent doses (ED). So, dating protocols avoiding sensitivity changes by preheating have been carried out, using the more appropriate conditions of preheat and stimulation that get rid of unstable components, determined by preliminary studies. A good agreement between OSL and TL results is then obtained when no change in sensitivity occurs during the irradiation/optical bleaching cycles performed in ED measurements.