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Electron paramagnetic resonance spectra of human tooth enamel with preferentially oriented nanocrystals

Electron paramagnetic resonance spectra of a selected -irradiated human tooth enamel fragment expressed anisotropy that increased considerably after thermal annealing. By repeated irradiation, the spectrum of the fragment approached the disoriented (powder type) spectrum, but repeated thermal treating restored the anisotropy. The anisotropy is characteristic of an ensemble of hydroxyapatite nanocrystals, predominantly oriented with the hexagonal axis perpendicular to the outer surface of the tooth. A Monte Carlo spectral simulation, corres ponding to the bulk arrangement of paramagnetic CO2 - radicals possessing the v 2 C symmetry and normal distribution of y-axes (parallel to the O -O direction) in a space angle of 27 ° (± 5°) around the surface normal, gives a satisfactory agreement with the experimental spectra, whereas the agreement for alternative models with specific surface arrangement of the radicals is worse.