Application of thermoluminescence in Iran: Dating of pottery samples of Iran National Museum

While a crystalline material is heated from room temperature to around 500°C, a weak but measurable light will be emitted. This light is known as thermoluminescence (TL)‎ and is based on storage of energy from ionizing radiation in many naturally occurring TL minerals, including quartz and feldspar. The source of radiation is the radioactive materials such as uranium, thorium, potassium and cosmic radiation. The physical bases for this phenomenon in the simplest model can be expressed as follow: In nature all minerals are exposed to radiation, which leads to ionization of the atoms as electrons and holes. Freed electrons may become trapped in structural defects. The number of trapped electrons will be accumulated up to the time that the material is heated sufficiently. Heating stimulate the atoms and electrons can be released and recombine with the ‘holes’. Some of these holes are called 'the luminescence center's in the mineral crystal lattice which recombination of electron with them, results to an emission of light which is called thermoluminescence. When mineral is heated to high temperature, it loses all its previously acquired TL signal, and sets luminescence clock to zero. After cooling, the natural radioactivity causes thermoluminescence to build up again and thus the amount of TL induced is proportional to the time that has elapsed since the minerals were fired. For dating in laboratory, it is necessary to measure equivalent dose and dose rate. The intensity of the radiation damage in crystal lattices is a measure of the Equivalent Dose (DE)‎ which the mineral has received since last “resetting” by exposure to heat. DE is obtained by means of a TL measurement. The dose rate (DR)‎ is a measure of radiation dose per unit of time absorbed by mineral. The equation for obtaining an age is: Age (ka)‎ = DE (Gy)‎/DR (Gy/ka)‎ Two approaches are employed to determine the DE: the additive-dose method and the regeneration method. In this study, we used the first method. In additive-dose method, a number of nearly equal portions of the sample (aliquots)‎ are divided into groups; one is reserved for measurement of the natural TL signal only, while the others are given various doses of laboratory radiation. Then, all the aliquots are measured together and the luminescence intensity is plotted against laboratory radiation dose; this forms the sample’s dose response. The DE is determined from the intercept of the fitted line with the dose. The dose rate is calculated from an analysis of the radioactive elements in both the sample and its surroundings. These are determined using the measured concentrations of radioactive elements (uranium, thorium, potassium-40) within the sample and its surroundings, which are, in turn, converted into dose rates using standard conversion factors and formulae. Contribution of cosmic rays is also determinated. There are different methods for obtaining concentrations of radioactive elements such as γ-Spectrometer, ICP Mass Spectrometer, Notrun Activation, α counting and flame photometry. Thermoluminescence dating began in 1960- 1970, with age determination of pottery and other fired material in archeology and then it was employed in the other science such as paleoseismology, paleoclimatology, geology, archeology and geography. Iran is an ancient country which is located on the belt of earthquake, so most of ancient monuments and old civilizations have been destroyed by earthquake or other natural disasters. Dating can be a device to relate paleoseismology, paleoclimatology and archeology in Iran. Therefore, having knowledge of dating methods, especially luminescence, is so important for experts in geography, archeology, geology, seismology students and etc. In addition to an introduction to TL dating, this study has dated five potteries from Iran national museum in a range of 1000 to 4000 years.