Radiation-induced defects in annealed carbonate-containing hydroxyapatite

The effect of preliminary (before irradiation) annealing of synthetic carbonate-containing hydroxyapatite powders on the formation of paramagnetic centers under $\gamma$-ray and ultraviolet irradiation has been investigated. Annealing of the samples has been performed in the temperature range from 100 to 700$^\circ$C. It has been found that electron paramagnetic resonance spectra of radiation-induced defects depend substantially on the annealing temperature. The paramagnetic centers CO$_2^-$ dominate in the samples annealed to 250$^\circ$C ($\gamma$-ray irradiation) and 500$^\circ$C (ultraviolet irradiation). In the samples annealed above 400$^\circ$C, other defects, in particular, the $O^-$ and CO$_3^{3-}$ centers, play a significant role. Annealing at some temperatures leads to an increase in the radiation sensitivity of the material. The observed effects can be associated with the escape of molecular water from the annealed hydroxyapatite samples and with the corresponding transformation of the defect subsystem of the material.