Jean Fain

Properties of the red TL peak of quartz relevant to thermoluminescence dating

Abstract A well-defined thermoluminescence (TL) peak was observed in several quartz samples from various origins in the range 380–395°C (heating rate 5°C s -1 ) at a wavelength of around 600–620 nm. This red TL peak (RTL), seemingly common in quartz, could be used for dating in the range 10–150 ka, and probably beyond, so a comprehensive study of its main properties seemed worthwhile. Our study included annealing and heat treatment effects, dose rate effects and fading tests. Preliminary results are also given on crushing effects and light exposure sensitivity.

Attempts at dating ancient volcanoes using the red TL of quartz

Abstract The suitability of the red TL peak of quartz for long-range dating is tested by application to some volcanic events of the French Massif-Central (see map — Fig. 1) having assumed ages in the range 14–150 ka. The samples are sediments heated by lava-flows and pyroclastic products. In most cases the results fit with the geological evidence; no direct comparison can yet be made with other dating methods. On the other hand for two events having ages of 2 and 14 Ma the growth curve using the additive-dose technique had an abnormal shape which could not be reproduced with a sample zeroed in the laboratory. This effect, which is not yet properly understood, may prevent the use of the TL regeneration technique for evaluation of the palaeodose for very old samples.

Intercomparison of red TL and ESR signals from heated quartz grains

Abstract Red thermoluminescence (TL) and electron-spin-resonance (ESR) measurements were made on identically prepared quartz grains, which had been zeroed by a lava flow of the Gravenoire volcano (Clermont-Ferrand, France) during the last glacial period (Wurm glaciation). Samples from two different sites were studied. The additive technique was used in order to evaluate the palaeodoses and the corresponding dose response curves (DRC), approximating saturating exponentials, were fitted on the basis of DRC obtained with laboratory reset samples. Using this technique of regression, results derived from Al and Ti ESR signals and red TL signals converged better than when only the additive DRC were taken into account in a simple exponential fit. Annealing experiments indicated that the traps involved in red TL, Al and Ti ESR centres had different stabilities vs time and temperature; thus the agreement of the palaeodoses derived from the three signals, within error limits, strengthened confidence in the results for each sample. The ages obtained for the two sites agreed with each other, although the palaeodoses and natural radiation dose rates were significantly different: 62.2 ± 9.0 and 61.3 ± 9.0 ka.

Attempts at dating pumice deposits around 580 ka by use of red TL and ESR of xenolithic quartz inclusions

The ESR dating technique and the TL dating technique were applied to the same samples of quartz grains, which were heated during a pumice eruption dated at 580±20 ka by means of 40Ar/39Ar single grain laser fusion. The additive dose technique was used. The age obtained by use of the red TL quartz, 544±42 ka, was in agreement with the expected age. The Al and Ti paramagnetic centres of quartz gave highly scattered age results, depending on the regression method that was used; for this reason, it was not possible to propose any reliable ESR age. This seems to confirm the analysis of the relevant literature, which suggests that the long-range dating limit of the technique is below half a million years.

TEPHROSTRATIGRAPHY OF THE LAST 160 ka IN WESTERN LIMAGNE (FRANCE)

The GEOCLER 1 core extracted from the Clermont crater lake sediments and surface studies made at several localities north of Clermont provided numerous stratigraphic, environmental and chronological data (TL and radiocarbon) which allow us to present a chronostratigraphy of the Limagne for the past 160 ka. 118 ash-falls have been recorded in the crater-lake sediments of the Clermont maar. They demonstrate that a basaltic phreatic activity developed along the western fault-scarp of the Limagne between 160 and 70 ka, after which began the Chaine des Puys activity. Moreover, several acid eruptions occured between 160 and 40 ka. From Older Dryas to Atlantic, between 15 and 7 ka, fourteen ash-falls occured and in some cases the source volcanoes have been identified. These include Puy de La Nugere, Puy de Dome, Puy Chopine, Kilian Crater and Puy de Pariou which contribute mainly to the local tephra succession. Prospects for further prehistoric research in volcanic areas are outlined.

A new slide technique for regressing TL/ESR dose response curves—intercomparisons with other regression techniques

Abstract For TL and ESR dating, the total radiation dose, P , experienced by a sample since the initial resetting event can be evaluated using a slide technique . In the present paper a new computerized slide technique (name LISA) is presented which does not require any mathematical model. The LISA technique can take random experimental uncertainties into account for weighting the data and for deriving the errors in the evaluation of P . Examples of application are given and comparison is made with two other techniques of regression, namely a simple saturating exponential extrapolation of the additive dose response curve (which is widely used in TL/ESR dating laboratories) and an exponential slide technique. The results obtained using LISA were in many cases better than those obtained with the other techniques of regression. An incidental result of the intercomparisons is that the technique of regression (including weighting of the data) strongly influences the paleodose plateau-test , which is a basic notion of TL dating.

The assessment of gamma dose-rate by gamma-ray field spectrometer

Abstract A three-channel γ-ray spectrometer, with a 2.5 in NaI probe, was used for separate determination of γ dose coming from U, Th or K soil contents. A new method is proposed for calibration, based on (a) laboratory absorption measurement with point radioactive sources and (b) measurements in holes bored in homogeneous reference lava flows with well determined U, Th and K contents. These calibrations revealed the importance of inhomogeneous media on the γ dose-rate evaluation. Applications are made to cases of practical interest, especially for the determination of the minimum hole depth for probe location that is required for acceptable accuracy.

A TL model based on deep traps competition

Abstract TL appears as a very complex phenomenon. However, some general features are widely presented by TL materials: low TL yields, nonlinear growth with dose (supralinearity or saturation), lower efficiency for high LET particles etc., which are not explained by the well-known classical one-trap band model. In the past, the competition between TL traps and deep (thermally disconnected) traps was proposed in theoretical models accounting for some specific TL features. Arguments are presented here for a more general role of deep traps, in order to get an overall satisfying (at least qualitatively) description of the TL phenomenon either in the irradiation stage (TL or ESR) or in the heating one. A corresponding theoretical model is proposed. It is outlined that the competition between TL and deep traps leads to spatial correlations between electrons in TL traps and recombination centres (trapped holes). On the other hand, occurrence of recombination is taken into account as well in the irradiation step as during the heating one. As a result, on a very simple basis, the model accounts for supralinear behaviour of TL output with dose, non-exponential shape of TL or ESR growth curves, slight temperature shift of glow peaks with dose and scarceness of observed second-order kinetics.

Gamma and enclosure dosimetry for TL/ESR dating with the new Al2O3:C TL dosemeter

Al2O3: C manufactured by LAIKS-CORUND, Ekaterinenburg, Russia, offers several advantages for the measurement of environmental radiation: stability, low self-dose, high sensitivity and energy deposition features under gamma and beta irradiation very similar to quartz and quartz-like TL minerals. In the present work those properties were tested for in situ gamma dosimetry, by comparison with other dosimetric techniques. It was confirmed that this phosphor is very well adapted to this purpose. On the other hand, the new alumina can be made insensitive to external alpha radiation by means of thermal treatments. This α-insensitized alumina is very interesting for enclosure dosimetry in the perspective of quartz inclusion dating. In this technique, the quartz grains to be dated are faithfully simulated by alumina: the phosphor allows us to measure directly the dose that would be experienced by quartz grains (HF etched, as usual). However, because the attenuation of beta radiation by coarse grains must be taken into account in inclusion dating, the role of this factor was theoretically and experimentally—with Al2O3—explored. The preliminary experiments indicated that enclosure dosimetry with this new alumina was a promising technique.

Zircon dating: Regeneration and evaluation of the external dose

Abstract Zircon dating of a trachytic volcanic event (Puy de Dome, Chaine des Puys, France) is presented using the measurement of TL regenerated in a low-temperature, high intensity peak. Special attention is given to the contribution of the external dose, carried by γ- and β-rays coming from the bulk trachyte. This dose does not appear to be negligible as is usually assumed.