EPR of carbonate derived radicals: Applications in dosimetry, dating and detection of irradiated food

After exposure of biological (tooth enamel, bone, …) and synthetic apatites to ionizing radiation, the so-called “asymmetric EPR signal nearg = 2” is formed. Although this signal is being used in EPR dosimetry, dating and detection of irradiated food for many years already, its composite character and the precise nature of the radicals contributing to the spectrum are still insufficiently known and/or recognized. For some fifteen years already, the EPR group in Ghent is gaining extensive experience on the radicals present in calcified tissues and model systems like synthetic apatites, calcites and single crystals doped with carbonate. It will be shown that the majority of radicals in calcified tissues are carbonate derived, e.g., CO ₂ ^? , CO ₃ ^? , CO ₃ ^3? while also phosphate derived radicals like PO ₄ ^2? and oxygen species (O^?, O ₃ ^? ) have been identified with EPR and/or ENDOR. For the EPR applications mentioned above, the most important type of radicals is CO ₂ ^t- (g values ranging from 2.0035 to 1.9970). A second type of radicals which is very intriguing but still badly known, exhibits a spectrum atg values around 2.0045. It is very apparent in tooth enamel below doses of 1 Gy, it has been observed in certain fossil teeth in a very prominent way and also in irradiated food containing bone (e.g., frog legs). It will be shown that the organic origin of this signal can be questioned. The importance of other radicals like CO ₃ ^3t- and CO ₃ ^t- for EPR applications will also be discussed.