An Encyclopaedia of Indian Archaeology v1.c10.s6.1

v1.c10.s6.1

Potsherds, bricks, fired earth, etc., gradually accumulate thermoluminescence (TL) due to internal and external irradiations. Internal irradiations are caused by impurities like uranium, thorium and potassium, and external ones are due to radioactive substances in the surrounding medium and from the cosmic rays. Once the sample is heated, say in the kiln, the accumulated TL then starts and will depend upon the amount of radioactive impurities present and the length of time since the sample was last fired. Through controlled heating the TL accumulated during the archaeological time (i.e. since a pot was fired) can be released and measured by a suitable photomultiplier detection system. A measurement of the radioactive impurity content enables one to calculate the rate of TL accumulation (TL/years). The total accumulated TL and TL/years then enable one to find out the archaeological age. So far only two laboratories have reported TL measurements on Indian archaeological material (table below)—the Research Laboratory for Archaeology at Oxford and the Health Physics Division of the Bhabha Atomic Research Centre, Bombay. Now the Physical Research Laboratory, Ahmedabad, is trying to install a TL-dating system of the same dimensions as its ¹⁴C project [5.3].