FREQUENTLY ASKED QUESTIONS How does thermoluminescence dating work? What is the accuracy of TL dating? What materials can be dated by TL? Warning about fakes using ancient materials Re: Thermoluminescence dating. Thermoluminescence — Wikipédia La thermoluminescence est un phénomène physique lié à la capacité de certains cristaux d'accumuler l'énergie cédée par les rayonnements ionisants issus de la radioactivité et de restituer cette énergie sous forme de lumière lorsqu'ils sont chauffés.
Luminescence dating (including thermoluminescence and optically stimulated luminescence) is a type of dating methodology that measures the amount of light emitted from energy stored in certain rock types and derived soils to obtain an absolute date for a specific event that occurred in the past. The method is a direct , meaning that the amount of energy emitted is a direct result of the event being measured. Better still, unlike , the effect luminescence dating measures increases with time.
Two forms of luminescence dating are used by archaeologists to date events in the past: thermoluminescence (TL) or thermally stimulated luminescence (TSL), which measures energy emitted after an object has been exposed to temperatures between 400 and 500°C; and optically stimulated luminescence (OSL), which measures energy emitted after an object has been exposed to daylight. To put it simply, certain minerals (quartz, feldspar, and calcite), store energy from the sun at a known rate.
This energy is lodged in the imperfect lattices of the mineral's crystals. Heating these crystals (such as when a is fired or when rocks are heated) empties the stored energy, after which time the mineral begins absorbing energy again. TL dating is a matter of comparing the energy stored in a crystal to what "ought" to be there, thereby coming up with a date-of-last-heated.
In the same way, more or less, OSL (optically stimulated luminescence) dating measures the last time an object was exposed to sunlight. Luminescence dating is good for between a few hundred to (at least) several hundred thousand years, making it much more useful than carbon dating. The term luminescence refers to the energy emitted as light from minerals such as quartz and after they've been exposed to an of some sort.
Minerals—and, in fact, everything on our planet—are exposed to : luminescence dating takes advantage of the fact that certain minerals both collect and release energy from that radiation under specific conditions.
Two forms of luminescence dating are used by archaeologists to date events in the past: thermoluminescence (TL) or thermally stimulated luminescence (TSL), which measures energy emitted after an object has been exposed to temperatures between 400 and 500°C; and optically stimulated luminescence (OSL), which measures energy emitted after an object has been exposed to daylight.
Crystalline rock types and soils collect energy from the radioactive decay of cosmic uranium, thorium, and potassium-40. Electrons from these substances get trapped in the mineral's crystalline structure, and continuing exposure of the rocks to these elements over time leads to predictable increases in the number of electrons caught in the matrices. But when the rock is exposed to high enough levels of heat or light, that exposure causes vibrations in the mineral lattices and the trapped electrons are freed.
The way you measure energy stored in an object that you expect has been exposed to heat or light in the past is to stimulate that object again and measure the amount of energy released. The energy released by stimulating the crystals is expressed in light (luminescence).
The intensity of blue, green or infrared light that is created when an object is stimulated is proportional to the number of electrons stored in the mineral's structure and, in turn, those light units are converted to dose units. • Pottery: The most recent heating measured in pottery sherds is assumed to represent the manufacturing event; the signal arises from quartz or feldspar in the clay or other tempering additives.
Although pottery vessels can be exposed to heat during cooking, cooking is never at sufficient levels to reset the luminescence clock. TL dating was used to determine the age of civilization occupations, which had proved resistant to radiocarbon dating, because of the local climate.
Luminescence can also be used to determine the original firing temperature. • Lithics: Raw material such as flints and cherts have been dated by TL; fire-cracked rock from hearths can also be dated by TL as long as they were fired to sufficiently high temperatures.
The resetting mechanism is primarily heated and works on the assumption that the raw stone material was heat-treated during stone tool manufacture. However, heat treatment normally involves temperatures between 300 and 400°C, not always sufficiently high enough.
The best success from TL dates on chipped stone artifacts likely are from events when they were deposited into a hearth and accidentally fired. • Surfaces of buildings and walls: The buried elements of standing walls of archaeological ruins have been dated using optically stimulated luminescence; the derived date provides the age of burial of the surface.
In other words, the OSL date on a foundation wall of a building is the last time that foundation was exposed to light before being used as the initial layers in a building, and hence when the building was first built. • Others: Some success has been found dating objects such as bone tools, bricks, mortar, mounds, and agricultural terraces. Ancient slag left from early metal production have also been dated using TL, as well as absolute dating of kiln fragments or vitrified linings of furnaces and crucibles.
Thermoluminescence was first clearly described in a paper presented to the Royal Society (of Britain) in 1663, by , who described the effect in a diamond which had been warmed to body temperature. The possibility of making use of TL stored in a mineral or pottery sample was first proposed by chemist in the 1950s. During the 1960s and 70s, the Oxford University for Archaeology and History of Art led in the development of TL as a method of dating archaeological materials.
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Thermoluminescence dating ( TL ) is the determination, by means of measuring the accumulated radiation dose, of the time elapsed since material containing crystalline minerals was either heated ( lava, ceramics) or exposed to sunlight ( sediments).
As a crystalline material is heated during measurements, the process of thermoluminescence starts. Thermoluminescence emits a weak light signal that is proportional to the radiation dose absorbed by the material. It is a type of luminescence dating. The technique has wide application, and is relatively cheap at some US$300–700 per object; ideally a number of samples are tested.
Sediments are more expensive to date.  The destruction of a relatively significant amount of sample material is necessary, which can be a limitation in the case of artworks. The heating must have taken the object above 500° C, which covers most ceramics, although very high-fired porcelain creates other difficulties.
It will often work well with stones that have been heated by fire. The clay core of bronze sculptures made by lost wax casting can also be tested.  Different materials vary considerably in their suitability for the technique, depending on several factors.
Subsequent irradiation, for example if an x-ray is taken, can affect accuracy, as will the "annual dose" of radiation a buried object has received from the surrounding soil. Ideally this is assessed by measurements made at the precise findspot over a long period.
For artworks, it may be sufficient to confirm whether a piece is broadly ancient or modern (that is, authentic or a fake), and this may be possible even if a precise date cannot be estimated.  Functionality Natural crystalline materials contain imperfections: impurity ions, stress dislocations, and other phenomena that disturb the regularity of the electric field that holds the atoms in the crystalline lattice together.
These imperfections lead to local humps and dips in the crystalline material's electric potential. Where there is a dip (a so-called " electron trap"), a free electron may be attracted and trapped. The flux of ionizing radiation—both from cosmic radiation and from natural radioactivity —excites electrons from atoms in the crystal lattice into the conduction band where they can move freely. Most excited electrons will soon recombine with lattice ions, but some will be trapped, storing part of the energy of the radiation in the form of trapped electric charge ( Figure 1 ).
Depending on the depth of the traps (the energy required to free an electron from them) the storage time of trapped electrons will vary as some traps are sufficiently deep to store charge for hundreds of thousands of years . In practical use In thermoluminescence dating, these long-term traps are used to determine the age of materials: When irradiated crystalline material is again heated or exposed to strong light, the trapped electrons are given sufficient energy to escape. In the process of recombining with a lattice ion, they lose energy and emit photons (light quanta), detectable in the laboratory.
The amount of light produced is proportional to the number of trapped electrons that have been freed which is in turn proportional to the radiation dose accumulated. In order to relate the signal (the thermoluminescence—light produced when the material is heated) to the radiation dose that caused it, it is necessary to calibrate the material with known doses of radiation since the density of traps is highly variable.
Thermoluminescence dating presupposes a "zeroing" event in the history of the material, either heating (in the case of pottery or lava) or exposure to sunlight (in the case of sediments), that removes the pre-existing trapped electrons. Therefore, at that point the thermoluminescence signal is zero. As time goes on, the ionizing radiation field around the material causes the trapped electrons to accumulate ( Figure 2 ).
In the laboratory, the accumulated radiation dose can be measured, but this by itself is insufficient to determine the time since the zeroing event. The Radiation Dose Rate - the dose accumulated per year-must be determined first. This is commonly done by measurement of the alpha radioactivity (the uranium and thorium content) and the potassium content (K-40 is a beta and gamma emitter) of the sample material.
Often the gamma radiation field at the position of the sample material is measured, or it may be calculated from the alpha radioactivity and potassium content of the sample environment, and the cosmic ray dose is added in. Once all components of the radiation field are determined, the accumulated dose from the thermoluminescence measurements is divided by the dose accumulating each year, to obtain the years since the zeroing event.
Relation to radiocarbon dating Thermoluminescence dating is used for material where radiocarbon dating is not available, like sediments. Its use is now common in the authentication of old ceramic wares, for which it gives the approximate date of the last firing.
An example of this can be seen in . Thermoluminescence dating was modified for use as a passive sand migration analysis tool by ( Figure 3 ), demonstrating the direct consequences resulting from the improper replenishment of starving beaches using fine sands, as well as providing a passive method of policing sand replenishment and observing riverine or other sand inputs along shorelines ( Figure 4 ).
Figure 3 : Thermoluminescence signature lost during migration of two sand grain sizes (Keizars, 2008). Relation to other luminescence dating methods Optically stimulated luminescence dating is a related measurement method which replaces heating with exposure to intense light. The sample material is illuminated with a very bright source of green or blue light (for quartz) or infrared light (for potassium feldspars).
Ultraviolet light emitted by the sample is detected for measurement. See also • Geochronology • Luminescence dating • Rehydroxylation dating • Thermoluminescence • Thermoluminescent dosimeter Notes
it works by The thermoluminescence technique is the only physical means of determining the absolute age of pottery presently available. It is an absolute dating method, and does not depend on comparison with similar objects (as does obsidian hydration dating, for example). Most mineral materials, including the constituents of pottery, have the property of thermoluminescence (TL), where part of the energy from radioactive decay in and around the mineral is stored (in the form of trapped electrons) and later released as light upon strong heating (as the electrons are detrapped and combine with lattice ions).
By comparing this light output with that produced by known doses of radiation, the amount of radiation absorbed by the material may be found. When pottery is fired, it loses all its previously acquired TL, and on cooling the TL begins again to build up.
Thus, when one measures dose in pottery, it is the dose accumulated since it was fired, unless there was a subsequent reheating.
If the radioactivity of the pottery itself, and its surroundings, is measured, the dose rate, or annual increment of dose, may be computed. The age of the pottery, in principle, may then be determined by the relation Age = Accumulated dose / Dose per year Although conceptually straightforward, TL has proven to to be far from simple in practice. In all, close to two dozen physical quantities must be accurately measured to establish the relationship between doses of different kinds of radiation and light output, and to compute dose rate.
A leaflet from Daybreak describing the TL technique in more detail and giving a bibliography will be provided to interested persons. The phenomenon of thermoluminescence was first described by the English chemist Robert Boyle in 1663. It was employed in the 1950's as a method for radiation dose measurement, and soon was proposed for archaeological dating. By the mid-1960's, its validity as an absolute dating technique was established by workers at Oxford and Birmingham in England, Riso in Denmark, and at the University of Pennsylvania in the U.S..
The Research Laboratory for Archaeology at Oxford, in particular, has played a major role in TL research. While not so accurate as radiocarbon dating, which cannot date pottery (except from soot deposits on cooking pots), TL has found considerable usefulness in the authenticity of ceramic art objects where high precision is not necessary.
Since the university laboratories involved with TL are research facilities, they generally will not accept art objects for authentication on a routine basis.
The TL laboratory at Daybreak was established in 1977 to make TL available to the art community in general. Radiocarbon dating, or Carbon 14 dating, relies on processes of radioactive decay. It can be used on any organic matter.
Every living thing contains carbon. When something dies its body stops taking in carbon. From this point onwards the carbon in the body will begin to decay. It takes roughly … 5568 years for the half the carbon in any sample to decay (this is its half life) Decay is at a relatively constant rate. By looking at how much carbon remains in a sample it is possible to calculate how long ago it stopped taking in new carbon (when it died).
There is always an error range associated with results, which is given with a + and - sign. A result of 550 +-50 BC means that a sample is likely to date from 500-600BC. Carbon dating is accurate up to roughly 50,000 years ago. After this the rate of decay is too small to get accurate dates from a sample, and other dating methods (such as Thermoluminescence and Potassium-Argon) have to be used.
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