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Thermoluminescence

When a radiation is incident on a material, some of its energy may be absorbed and re-emitted as light of longer wavelength. The wavelength of the emitted light is characteristic of the luminescent substance and not of the incident radiation.

Thermoluminescence (TL) is the process in which a mineral emits light while it is being heated: it is a stimulated emission process occurring when the thermally excited emission of light follows the previous absorption of energy from radiation. Energy absorbed from ionising radiation (alpha, beta, gamma, cosmic rays) frees electrons to move through the crystal lattice and some are trapped at imperfections in the lattice. Subsequent heating of the crystal can release some of these trapped electrons with an associated emission of light.
If the heating rate is linear and if we suppose the probability of a second trapping to be negligible with respect to the probability of a recombination, the TL intensity is related to the activation energy of the trap level by a known expression. It is so possible to determine the trap depth.


Thermoluminescence can be used to date materials containing crystalline minerals to a specific heating event. This is useful for ceramics, as it determines the date of firing, as well as for lava, or even sediments that were exposed to substantial sunlight. These crystalline solids are constantly subjected to ionizing radiation from their environment, which causes some energized electrons to become trapped in defects in the molecular crystal structure. An input of energy, such as heat, is required to free these trapped electrons. The accumulation of trapped electrons, and the gaps left behind in the spaces they vacated, occurs at a measurable rate proportional to the radiation received from a specimen’s immediate environment. When a specimen is reheated, the trapped energy is released in the form of light (thermoluminescence) as the electrons escape. The amount of light produced is a specific and measurable phenomenon.

Material and objects of archaeological or historical interest that can be dated by thermoluminescence analysis are ceramics, brick, hearths, fire pits, kiln and smelter walls, heat treated flint or other heat-processed materials, the residues of industrial activity such as slag, incidentally fire-cracked rocks, and even originally unfired materials such adobe and daub if they had been heated in an accidental fire. 


A non-negligible part of materials which ceramic is usually made of (like quartz and feldspars) is thermoluminescent: those materials have trap states that can capture electrons after interaction with alfa, beta and gamma rays existing in nature.
When these materials are heated to several hundreds of Centigrade degrees, electrons are evicted from trap states and energy is emitted in form of light: thermoluminescence (TL). Heating ceramic in a furnace resets TL accumulated by clay and other materials; from this time on, TL begins growing again as time passes; the more concentrated radioactivity where ceramic is, the quicker TL grows.
Thus by measuring TL we can date an object since the last time it was heated above 400°C. Since measured TL depends on time of exposition to natural radiations but also on the intensity of these radiations, to achieve a precise dating we need information about radioactivity of the area where the object was found.

During TL analysis, the sample is reheated by a controlled heating process, so the energy is released in the form of light (thermoluminescence) as the electrons escape. The amount of light produced is measuered by a photomultiplier. The result is a glow curve showing the photon emission in function of the heating temperature:

glow

If the specimen’s sensitivity to ionizing radiation is known, as is the annual influx of radiation experienced by the specimen, the released thermoluminescence can be translated into a specific amount of time since the formation of the crystal structure.

TL formula

Because this accumulation of trapped electrons begins with the formation of the crystal structure, thermoluminescence can date crystalline materials to their date of formation; for ceramics, this is the moment they are fired. The major source of error in establishing dates from thermoluminescence is a consequence of inaccurate measurements of the radiation acting on a specimen.

 

The measure of PALEODOSE

The paleodose is the absorbed dose of natural radiation accumulate by a sample. This paleodose is determined from the TL signal measured by heating sample at a constant rate. The accuracy of the linearity in heating sample is crucial to have a precise measure. The result of this measure is, as described above, a glow curve. Three different types of glow curve can be distinguished:
• The natural thermoluminescence of the sample as it is
• The additive glow curve where a radiation does with a calibrate radioactive source is given in addition to the natural one
• The regenerate signal, when the sample has been zeroed its natural TL by heating and then given an artificial radiation dose

The last two glow curves allow to measure the sensitivity of a sample to natural radiations and are used to determine the paleodose. There are several ways to determine the paleodose comparing the results of the different glow curves measured. The most common methods are:
• The standard method (Aitken, 1985) performs regression analyses for both growth curves and the sum of their absolute values essentially provide the paleodose.
• The normalization method (Valladas & Gillot, 1978; Valladas, 1992; Mercier, 1991), one of the two growth curves is shifted towards the other until they are matched, and the amount of the shift essentially gives paleodose.

 


The Dose Rate

The denominator Dose rate of the age formula consists of two independent parameters, the internal dose rate and the external dose rate. Obviously, the denominator is crucial for the accurate determination of an age.
Internal dose rate all rock material contains radioactive elements that give rise to an internal dose rate. Elements of concern here are only U (Uranium), Th (Thorium), K (Potassium), and to some extent Rb (Rubidium), because other natural radioactive nuclides occur only in very small quantities or do not contribute significantly to the total absorbed dose. Internal dose rate consists of three parameters related to the α, β and γ radiation, where the latter is usually small in most cases.
External dose rate sediment contains not only the flint samples, but radioactive nuclides as well. These give rise to an external dose rate in addition to the one from secondary cosmic rays.


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