Т-Т method
The definition of background radiation doze absorbed by minerals in a sample location is the base of all dosimetric dating methods. The absorption of this radiation leads to conservation of electrons at point defects of crystalline lattice (electronic traps). Concentration of the trapped electrons (n) grows in a mineral according to the saturation function (a dose curve: n = f (D)). Knowing this function, we have a possibility to determine the natural dose (Dn) absorbed by the mineral (Fig. 1a). The ratio of Dn to the level of background (Е) in sample location determines an exposition of a mineral or the age of sediment:
t = Dn/E.
In luminescent methods the measure of concentration of the trapped electrons is light energy (a lightsum – S), emitted by minerals during their thermal (TL) or optical (OSL) stimulation. The value S is proportional to the concentration of trapped electrons, but the factor of proportionality depends on optical transparency of a mineral. The main problem of luminescent methods is connected with optical properties, which have a wide dispersion even within one group of minerals. As a result the dose curve of the lightsum, registered during measurements (Fig. 1b) is individual for each sample. The identification of this individual function is the primary task of all luminescent methods.
Fig.1. The same dose curve of trapped electrons quantity transforms into a great number of dose curves due to individual transparency of specimens.
In the traditional dating technology the main means of individual dose curve definition is the calibration of each sample by artificial irradiation. We have revealed (Shlukov et al., 1987, 1993), that application of artificial radiation, which dose rate exceeds the natural background on 7-9 orders, causes strong distortions of artificial dose curve in comparison with its natural analogue. According to our estimations the error of dating with such approach can achieve 300% and more.
The refusal from calibration by artificial radiation is the concept of our alternative direction. The methodology of our approach is an analytical definition of natural dose curve instead of its empirical reconstruction by means of calibration curve.
The T-T method is the recent development of our direction. Suggested earlier S-S and S-T methods were used as prototypes. The exception of expensive and labor-consuming procedure of radiating calibration has allowed to carry out a wide range prototype testing in short terms (Shlukov et al., 1999). This check has completely confirmed our concept, however has revealed the great lack of both prototypes. They appeared to be applied in limited territories only. Such a restriction has been coursed by the same dispersion of mineral optical properties, which is the reason of individual calibration of samples in the traditional direction. Successful results have been received in territory of Russian plain, in the valleys of Altai, in the Negev desert. Distinctive feature of these territories is the origin of a material from the same source, which provided identity of optical properties of minerals. However outside such territories the application S-S and S-T prototypes was impossible.
Basic difference of the T-T method consists in replacement of a lightsum as basic criterion of age by the temperature of TL maximum. In comparison with optical properties, which undergo strong changes due to a microimpurity included in a mineral structure, the thermo-physical properties are defined by the basic crystalline structure. For this reason minerals have strict thermo-physical parameters and do not depend neither on theirs origin, nor on subsequent history. In other words they have a global invariance.
The using of temperature for age determination is provided by features of the electron-hole kinetics proceeding in minerals during the absorption of radioactive radiation and at TL registration. It is well known from physics of crystallophosphorus, that this kinetics can follow either to the first, or to the second order. In case of the first order the elementary TL peak has stable position on a temperature scale (Fig. 2а). At the second order of kinetics this maximum has distinct shift to high temperatures with reduction of its lightsum (Fig. 2а).
Fig. 2. The elementary TL peak has a stable position under the first order kinetics (a) and has distinct shift under the second one (b).
There is an erroneous representation about the first order of kinetics in a traditional dating. In the middle of 80s we have shown, that the TL of quartz submits to the second order (Shlukov, Shakhovets, 1987; Shlukov et al., 1993). In particular we have shown that the second order of kinetics is the reason of the great error at modeling a natural dose curve with the help of an artificial irradiation. Recently we have carried out an exact experiment, which gives the direct proof of the second order for the quartz (Shlukov, 2002, 2005 unpublished – see this site). Taking the great relationship between quartz and feldspars into account, we have no doubt, that this conclusion has the direct relation also to this mineral group.
The formula of a trend
establishes biunique connection between the density of the trapped electrons (n) and the temperature of an elementary peak maximum (Tm). So the function expressed by this formula is an independent dose curve, which can successfully be used for definition of age. The major feature of this formula is the fact, that it gives direct interrelation with density of the trapped electrons instead of light energy emitted by mineral. Thus we have excluded the main handicap of all luminescent methods, including our both prototypes, which is wide dispersion of mineral’s optical properties.
Fig. 3. The curve of Tm trend reflects biunique connection with density of trapped electrons (a). The 90-degree turn transforms it in usual outward appearance (b).
Despite the seeming complexity of the received transcendental equation, it is simply solved by a numerical calculation as function of one variable: n = f (Tm). All other values included in the equation are constant parameters, the decoding of which contains in our last works (Shlukov, 2002, 2005).
Within the framework of the experiment carried out we have developed technology of extracting of the basic elementary peak of quartz, have received all necessary parameters, and have offered the measurement procedure for determining the temperature of peak’s maximum. All technology of T-T dating has passed successful tests on two Quaternary sections.
We will plan to present T-T method in detail, and our first results also in the conferences “LED-2005” in Cologne.