The predose effect in thermoluminescent dosimetry

We present a model of the predose effect of thermoluminescence in crystalline quartz in terms of known impurities and defect processes. It involves the recombination-induced dissociation of an aluminium-alkali complex [Al M⁺] and the reaction of an alkali [M⁺] with an activator [X], possibly Ge, to make the former an efficient electron trap [X M⁺]. This excitation-induced ionic process is a new feature, in addition to the simple carrier redistribution processes usually assumed, and is consistent with a number of experiments. The test dose detects [X M⁺], which is present in a concentration within the dynamic range of thermoluminescence measurements. Our model enables us to understand the enhanced sensitivity of the predose method, which has been widely used in archaeological and accident dosimetry, and to link it to established impurity and defect centres in α-quartz.