13C hyperfine interactions of CO2- in irradiated tooth enamel as studied by EPR

The EPR spectrum of tooth enamel caused by 13C hyperfine interactions of the CO2- radical were studied on gamma-irradiated powdered samples annealed for 40 min at different temperatures up to 250 degrees C. The lineshape and hyperfine splitting of the spectra were found to depend on the annealing temperature. Experimental spectra were compared with calculated ones assuming that EPR spectra are formed by two CO2- species--axial (rotating) and orthorhombic (braked) radicals. We assumed that the axial CO2- radicals are centers located in perfect areas of the hydroxyapatite crystals of tooth enamel whereas the orthorhombic CO2- radicals are rotating centers which are braked by defects. The thermal treatment of enamel samples leads to defective annealing and transformation of the orthorhombic centers into axial ones. This results in an increasing axial CO2- radical contribution to the EPR spectrum with increase of annealing temperature.