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Thermoluminescence dating is in its developmental stages. Except for doing simple authenticity tests of art objects, thermoluminescence dating is not generally accurate enough for archaeological standards. There are many factors which have to be taken into account and each of these factors has its own random error. This, combined with poorly understood measurement errors, make the accuracy of thermoluminescence dating only about 15% accurate for a single sample and 7 to 10% accurate for a suite of samples in a single context. Thermoluminescence dating is used for rocks, minerals and pottery. It dates items between the years 300-10,000B.P. It is based on the fact that almost all natural minerals are thermoluminescent. Energy absorbed from ionizing radiation frees electrons to move through the crystal lattice and some are trapped at imperfections. Later heating releases the trapped electrons, producing light. Measurement of the intensity of the luminescence can be used to determine how much time has passed since the last time the object was heated. The light is proportional to the amount of radiation absorbed since the material was last heated. Natural radioactivity causes latent thermoluminescence to build up so the older an object is the more light is produced. Therefore, thermoluminescence dating is actually determining the last time a crystal was heated and electrons were released. The minerals that are used for thermoluminescence dating are quartz, feldspar, diamond and calcite. The last time a crystal was reheated and its electrons were released is known as a clock resetting event. This usually occurs when the items are heated to 350 degrees Celsius. Therefore, in archaeology, thermoluminescence dating works best for ceramics, cooking hearths, incidentally fire-cracked rocks and deliberately fire treated rocks such as flint or chert. When collecting samples for thermoluminescence dating, several samples from different vessels not smaller than 1 gram should be taken. Samples should not be exposed to heat and powdery examples should not be exposed to bright light. A sample of the earth also needs to be collected so environmental radiation can be tested. The wetness of the soil and sample should also be recorded. Samples should be placed in a polyethylene bag and sealed with electrical tape. To test the date, three steps are taken:
The formula used in this equation is:
Dose Rate=dose accumulated each year Author: Sara Wenner References Joukowsky, Martha. A Complete Manual of Field Archaeology, Tools and Techniques of Fieldwork for Archaeologists. Prentice Hall Inc. Englewood Cliffs, New Jersey 07632. 1980. The Basic Principle of Luminescence Dating. Former link- http://www.ph.ed.ac.uk/courseinfo/postgrad/html/node70.html |