Electron spin relaxation of radicals in irradiated tooth enamel and synthetic hydroxyapatite

Spin–lattice relaxation times, T1T1, for EPR signals created by irradiation of tooth enamel or carbonate-doped hydroxyapatite were studied by three-pulse inversion recovery and long-pulse saturation recovery. The recovery curves were modeled as the sum of two log-normal distributions of relaxation times. The dominant component in the EPR signal for tooth samples is assigned to CO2- replacing phosphate (B-sites) in the hydroxyapatite matrix. For this component the center of the T1T1 distribution was 1.4–1.6μs at 294 K, and did not vary significantly with radiation dose or the source of the sample. The smaller, slower relaxing, component of the signals from the tooth samples had a T1T1 distribution centered at 30–60μs and is assigned to an organic radical (g∼2.0045)(g∼2.0045). At 294 K the spin–spin relaxation time T2T2 measured by two-pulse echo decay was dominated by motion of the CO2- and was independent of dose. The dose independence of T1T1 and T2T2 at ambient temperature provides the basis for using dose-independent microwave powers to record dosimetric tooth signals at microwave powers above the linear response regime.