Investigations on the development conditions necessary to obtain reliable results of the detection of irradiated herbs, spices, mushrooms and strawberries by thermoluminescence (Tl) method in whole samples and/or in minerals isolated from them were carried out. Tl intensities of whole samples were measured between 40 and 355°C. Thereshold values for non-irradiated samples were obtained by multipying Tl values of control samples by 3 (safety factor). Samples in which Tl intensities were higher than the thereshold values were identified as irradiated. In 4 out of 10 kinds of herbs and spices examined the results of TL measurements led to false identification.
Minerals isolated from foodstuffs were examined for their Tl intensity between 40 and 400°C. The results were normalized to avoid self-absorption and mass effect. After registration of Tl (first glow curve) the mineral sample was irradiated with 1 kGy of gamma radiation of Co-60 and measured again (second glow curve). The ratio of Tl intensity of the first curve to that of the second curve gives the K_Tl coefficient. In our measurements K_Tl values for non-irradiated samples were between 0.002 and 0.13 while those for irradiated ones between 0.134 and 3.425. Basing on these data we propose to set critical K_Tl value as equal to 0.4. It means that a sample with K_Tl value higher than 0.4 is regarded as irradiated one.