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Dating techniques archaeology wiki

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.It will often work well with stones that have been heated by fire.

The heating must have taken the object above 500° C, which covers most ceramics, although very high-fired porcelain creates other difficulties.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.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).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.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.

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