EPR Properties of Irradiated Hydroxyapatite and Fluorapatite Cements: Carbonated and Sodium Effects

In a recent study, electron paramagnetic resonance (EPR) spectra of γ-radiation-induced paramagnetic defects in carbonated hydroxyapatite (CHA), prepared via calcium phosphate cement method, revealed a pure and stable EPR signal from orthorhombic CO₂^●– (g_x = 2.0029, g_y = 1.9967, g_z = 2.0017) radicals. The signal is proportional to the dose received from 5 Gy to 20 kGy¹. The findings suggested that CHA could be a promising substrate for EPR-based dosimetry. Understanding the effects of anion and cation incorporation on the EPR properties of apatite cements is critical to improving their dynamic range for dosimetry. Here, we studied the impact of carbonate (CO₃^2–) and sodium (Na) concentrations on the EPR properties of irradiated hydroxyapatite (HA) and Fluorapatite (FAP) cements. We synthesized nine HA and nine FAP cement samples, each containing three levels of CO₃^2– and three levels of Na. After γ-irradiation at 10 kGy, EPR spectra revealed a major EPR signal from orthorhombic CO₂^●– and a smaller signal from CO₃^●– (g_x = 2.0091, g_y = 2.01718, g_z = 2.00584) radicals. For a given pair of CO₃^2– and Na contents, FAP generally produced significantly stronger EPR signals than did HA. Without added Na, the FAP/HA EPR signal intensity ratio was 2.2 ± 0.88; this ratio decreased to 1.09 ± 1.31 as Na concentration was increased. These findings show that changing the elemental content improves the EPR signal intensity in apatite cements.