Thermoluminescence

Thermoluminescence as the property of a solid body is designated, in the crystal lattice deliver stored energy in the form of light when heated previously. The energy needed for this was previously stored in the excited metastable states. The that process, the thermally stimulated light emission is similar to other luminescence and is described there in more detail.

Discovery

The discovery of the thermoluminescence 1663 attributed to Robert Boyle. He reported this year on 28 October at the Royal Society, that he brought a diamond in the dark to dim lights by keeping him in bed during the warmest part of his naked body. First modern applications have been described in the 1950s, archeology on ceramics by Elizabeth K. Ralph and Mark C. Han and Martin J. Aitken.

Basis

In naturally occurring minerals such as quartz or feldspar is energy in the form of radiation damage caused by the decay of naturally occurring unstable nuclides and cosmic radiation stored in the crystal lattice. Electrons are set in " electron traps " between the valence and conduction band. Quartz or feldspar are in turn mineralogical constituents eg fired ceramic.

When heated, emit body temperature rises first heat radiation, later also visible light. If no thermoluminescence occurs, the radiated power can be predicted as a function of temperature with the aid of the Stefan- Boltzmann law. On further heating to temperatures around 300 ° C - 500 ° C is thermal stimulated light emission, a ( thermoluminescence ), that is, excited electrons leave their metastable state and fall to lower energy levels back ( it is also called recombination ). The energy difference is emitted while the photon of a characteristic frequency (for example in the visible spectrum). Since a relatively short time all excited electrons have fallen to a lower energy level, this occurs thermoluminescence ( TL) mentioned effect on only the first heating, provided that the crystal does not then ( artificial) is irradiated.

From the difference of the two curves can be inferred to the stored energy. This depends on the intensity and the duration of the preceding accumulated energy.

For the radiation measurement technique is used crystals of lithium fluoride, CaSO4, CaF2, and lithium, with various impurities (activators ) such as Mn, Mg, Ti, Cu or P specifically contaminated ( doped) are. These allocations are used for generation of voids in which the liberated electrons in the crystal can be captured and stored.

Archaeological application

Thermoluminescence is, inter alia, in archeology under the name Thermoluminescence dating ( TL dating) or general Luminescence used as a method to determine the age of ceramic objects, or otherwise burned artifacts. It serves as a supplement to radiocarbon dating (also: C14 - dating), particularly where datings are needed beyond the limited range of the C14 - dating or where no organic material is available.

Already proposed in 1953 in a paper by Daniels, Boyd & Saunders, first dating applications were presented 1957/1958 by researchers at the University of Bern ( team led by Friedrich Georg Houtermans and Norbert Grögler ). In the period that followed the dating method was developed in the 1960s championed by Martin J. Aitken in Oxford on. Further methodological improvements led in 1985 to present the Optical Stimulated Luminescene ( OSL) dating by David Huntley. Is dated the date of last exposure, even with the TL dating is possible in principle, but considerably longer exposure times required. The process of the OSL dating, although closely related to the TL dating, therefore, of this delineate (see section related procedures).

The structure of the latent luminescence signal is done by supplying energy from the decay of naturally occurring radioactive nuclides ( 238U, 232Th, 40K, 87Rb ) and by cosmic rays.

During the firing process for the preparation of the artefact, the TL- clock has been reset to "0". Then sets the sketched " charging " again. The older the sample, the stronger the observable at a reheating luminescence. By measuring the TL clock is reset again, however.

• Measurements of the dose rate in the vicinity of the locality occurring radioactive nuclides
• Knowledge of the (regional / local differences ) spectrum of the radioactive isotopes involved and their respective decay time

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

So far it has not counterfeiters demonstrated how to bypass this method of age determination, because it is obviously impossible, freshly baked ceramic by artificial irradiation as " charge " that the time course of TL- radiation is imitated during heating.

Other applications of thermoluminescence

• Another use of thermoluminescence in dosimetry, see main article: thermoluminescence.

In this case, the luminescent, thermally stimulated recombination generated by ionizing radiation, which is stable at room temperature, broken into very sensitive materials, such as, determined, for example lithium fluoride as a measure of the dose received.

• TL measurements can provide important information in photosynthesis research. Here arise after excitation with light, metatstabile radical pairs that recombine by supplying heat. Peak temperature and degree of the emitted light allow us to draw conclusions about the state of the photosynthetic apparatus.
• Detection of food irradiation

Related Procedures

According to the same operating principle as the thermoluminescence other methods that differ primarily by the frequency of the externally supplied, stimulating radiation work:

• Optically stimulated luminescence (OSL; s: optically stimulated luminescence ) with the help of light from the visible region of the spectrum. Applicable in quartz and feldspar, that is, at once the sunlight or a heating exposed rocks ( sandstone, granite ), and in particular quartz-bearing sediments, suitable for dating of samples that are up to 200,000 years old.
• Infrared stimulated luminescence ( IRSL; s: infrared stimulated luminescence ) by means of infrared light.
• Radio luminescence (RL; s: Radioluminescence ) with the help of ionizing radiation.
• Green stimulated luminescence ( GLSL, en: green -light stimulated luminescence ) with the help of green light.