Thermoluminescent dating

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When Thermolumineszenzdatierung the property is used some solids when heated beforehand in the crystal lattice energy stored in the form of light delivered. The energy was stored in metastable states , it mainly comes from the decay processes of naturally occurring radioactive nuclides or from cosmic rays . Related processes of stimulated light emission are described in more detail in the article Luminescence .

Thermoluminescence dating is u. a. Used in archeology (abbreviation TL dating or more generally luminescence dating ) as a method for determining the age of ceramic objects or other fired artifacts . Thermoluminescence can also be used to date sediments. Then the event of the last exposure to sunlight is dated. Thermoluminescence serves as a supplement to radiocarbon dating (also: C14 dating), especially where dating beyond the limited range of C14 dating is required or where no organic material is available.

Thermoluminescence of fluorspar

basis

Comparison of normal thermal radiation with the glow curve

In naturally occurring minerals such as B. Quartz or feldspar , energy is stored in the crystal lattice in the form of radiation damage caused by the decay of naturally occurring unstable nuclides and cosmic radiation. Electrons are trapped in “electron traps” between the valence and conduction bands. Quartz or feldspars are mineralogical components such. B. fired ceramics.

When heated, bodies initially emit thermal radiation as the temperature rises , which then also includes the visible spectral range. If there is no thermoluminescence, the radiated power can be predicted as a function of temperature with the help of Stefan-Boltzmann's law . With further heating to temperatures around 300-500 ° C, thermally stimulated light emission (thermoluminescence) begins, i. That is, excited electrons leave their metastable state and fall back to lower energy levels (this is also called recombination). The energy difference is emitted as a light quantum with a characteristic frequency (e.g. in the visible spectrum). Since all excited electrons have fallen to a lower energy level after a relatively short time, this effect, called thermoluminescence (TL), only occurs when the crystal is heated for the first time, unless the crystal is subsequently irradiated again.

The stored energy can be deduced from the difference between two curves determined in this way. This depends on the intensity and duration of the previous accumulated energy.

For radiation measurement technology, crystals made of lithium fluoride , CaSO 4 , CaF 2 or lithium boride are used , which are specifically contaminated (doped) with various foreign atoms (activators) such as Mn, Mg, Ti, Cu or P. These dopings are used to create defects in which the electrons released in the crystal can be captured and stored.

Archaeological application

First modern uses were described in the 1950s, in archeology on ceramics by Elizabeth K. Ralph and Mark C. Han and by Martin J. Aitken . As early as 1953 in an article by Daniels, Boyd & Saunders, the first dating applications were presented in 1957/1958 by researchers at the University of Bern (team led by Friedrich Georg Houtermans and Norbert Grögler ). In the following period, the dating method was further developed in the early 1960s by Martin J. Aitken in Oxford. Further methodological improvements led to the presentation of Optically Stimulated Luminescence (OSL) dating by David Huntley in 1985 . The time of the last exposure is dated, which in principle is also possible with TL dating, but requires considerably longer exposure times. The method of OSL dating, although closely related to TL dating, must therefore be distinguished from it (see section related methods).

Typical quartz TL curve, measured as part of a TL dating

The build-up of the latent luminescence signal takes place through the supply of energy from the decay of naturally occurring radioactive nuclides ( 238 U, 232 Th, 40 K, 87 Rb) as well as through cosmic radiation.

During the burning process to produce the artifact, the TL clock was reset to "0". Then the outlined “charging” starts again. The older the sample, the stronger the luminescence signal that can be observed when it is heated again. However, the measurement will reset the TL clock again.

The following must be included in the evaluation:

  • Measurements of the dose rate in the vicinity of the location of the radioactive nuclides occurring
  • Knowledge of the (regionally / locally different) spectrum of the radioactive isotopes concerned and their decay time

The accuracy of the method is limited. It is around 10% of the age of the sample. Their range is more than 50,000 years, depending on the dosimeter used and the dose rate . Under good conditions, 500,000 years have also been reached.

So far, counterfeiters have not succeeded in circumventing this method of age determination because it is obviously impossible to “charge” freshly fired ceramics by means of artificial irradiation in such a way that the temporal course of the TL radiation is imitated during heating.

Related procedures

Other methods work according to the same operating principle as thermoluminescence dating, in which the de-excitation from the metastable states did not take place by heating the material, but by supplying energy in the form of photons. These photo- or radioluminescence methods can be differentiated according to the frequency of the stimulating radiation supplied from the outside:

  • Optically stimulated luminescence ( OSL; en: optically stimulated luminescence ) using light from the visible range of the spectrum. Applicable to quartz and feldspar, i. H. For rocks that were previously exposed to sunlight or heating (sandstone, granite) and especially quartz-containing sediments, suitable for dating samples that are up to 200,000 years old.
  • Infrared stimulated luminescence ( IRSL; s: infrared stimulated luminescence ) with the help of infrared light.
  • Radioluminescence ( RL; en: Radioluminescence ) with the help of ionizing radiation.
  • Green stimulated luminescence ( GLSL; en: green-light stimulated luminescence ) with the help of green light.

literature

  • Martin Jim Aitken : Science-based dating in archeology. Longman, London et al. 1990, ISBN 0-582-49309-9 , pp. 141-175 (Longman archeology series).
  • Martin Jim Aitken : Thermoluminescence dating. Academic Press, 1985, ISBN 0-12-046380-6
  • Reuven Chen & Stephen W. McKeever: Theory of thermoluminescence and related phenomena. World Scientific, Singapore et al. 1997, ISBN 981-02-2295-5 .
  • Reuven Chen & Vasilis Pagonis: Thermally and Optically Stimulated Luminescence: A Simulation Approach. John Wiley & Sons, 2011, ISBN 978-0-470-74927-2 .
  • Stuart Fleming: Thermoluminescence techniques in archeology. Clarendon Press, Oxford 1979, ISBN 0-19-859929-3 .
  • Claudio Furetta: Handbook of Thermoluminescence. World Scientific, 2010, ISBN 981-238-240-2 .
  • Barthel Hrouda (ed.): Methods of archeology. An introduction to their scientific techniques. Beck, Munich 1978, ISBN 3-406-06699-2 , pp. 151-161 (Beck's elementary books).
  • K. Mahesh, PS Weng & C. Furetta: Thermoluminescence in solids and its application. Nuclear Technology, Publishing, 1989, ISBN 1-870965-00-0 .
  • Stephen WS McKeever: Thermoluminescence of solids. Cambridge University Press, 1988, ISBN 0-521-36811-1 .
  • Stephen Stokes: Luminescence dating applications in geomorphological research. In: Geomorphology. 29, 1999, ISSN  0169-555X , pp. 153-171.

Web links

Footnotes

  1. ↑ Meyer's Big Pocket Lexicon in 24 volumes: Age determination. Vol. 1. A-Ang. 1987, p. 270
  2. ^ Dating of Pottery by Thermoluminescence. In: Nature . 210, 1966, pp. 245-247
  3. ^ F. Daniels, CA Boyd & DF Saunders: Thermoluminescence as a Research Tool. In: Science . 117, 1953, pp. 343-349.
  4. ↑ FG Houtermans & H. Stauffer: Thermoluminescence as a means of studying the temperature and radiation history of minerals and rocks. In: Helvetica Physica Acta. 30, 1957, pp. 274-277.
  5. ↑ N. Grögler, FG Houtermans & H. Stauffer: Radiation damage as a research tool for geology and prehistory. In: 5 ° Rassegna Internazionale Elettronica E Nucleare, Supplemento Agli Atti Del Congresso Scientifico. 1, 1958, pp. 5-15.
  6. ^ DJ Huntley, DI Godfrey-Smith & MLW Thewalt: Optical dating of sediments. In: Nature . 313, 1985, pp. 105-107.
  7. ↑ Michael Balter: New light on ancient samples. In: Science. Volume 332, 2011, p. 658, doi : 10.1126 / science.332.6030.658-b