Electron paramagnetic resonance of human tooth enamel at high gamma ray doses

Powdered human tooth enamel was exposed to ⁶⁰Co γ rays up to a dose of 100 kGy. The electron paramagnetic resonance (EPR) signal intensity (I) of the radiation-generated carbon dioxide radicals was measured for dependence on absorbed dose (D). The EPR dose response can be fitted with an exponential saturation function I = I_M[1 - exp(-D/D₃₇)] with the saturated signal intensity (I_M) and the dose saturation value (D₃₇). The obtained value D₃₇ = 9.64 (± 0.96) kGy (measured at least one month after irradiation) exceeds those given in the literature. The saturated concentration of orthorhombic CO₂- radicals was estimated at 6.5 × 10¹⁷ per gram of enamel by comparing the integrated EPR spectra of enamel and a standard MgO:Cr probe. For enamel samples, which were heated before irradiation for one hour at +405°C, the value of D_37T = 3.89 (± 0.44) kGy and the saturated value of CO₂^- radicals 3.4 × 10¹⁷ per gram of enamel were lower than for unheated samples. The initial rise of the signal with the dose was slightly higher (8.8 × 10¹³ radicals/g × Gy) for heated compared with unheated samples (6.8 × 10¹³ radicals/g × Gy).