Electron spin relaxation of radicals in irradiated tooth enamel and synthetic hydroxyapatite
Introduction
The paramagnetic centers in tooth enamel have been studied extensively because of interest in using the signals from carbonate radicals in the hydroxyapatite matrix for retrospective dosimetry (Ikeya, 1993). The EPR spectra of irradiated tooth enamel and synthetic hydroxyapatites are superpositions of signals from several species including , , and organic radicals (Ikeya, 1993, Amira et al., 2001, Callens et al., 2002). The dominant radical species is , but there is uncertainty whether it is located at A sites (substituted for a hydroxyl group) or B sites (substituted for phosphate) in the hydroxyapatite or at a surface location (Ishchenko et al., 1999, Koshta et al., 2000, Amira et al., 2001, Callens et al., 2002, Tieliewuhan et al., 2006). An organic radical with -value also is found in EPR spectra of native and irradiated tooth samples (Ikeya, 1993).
Accurate dosimetry depends on selection of microwave powers that are appropriate for diverse samples, which requires an understanding of the electron spin relaxation times and . The product can be estimated by CW progressive saturation, but separation of the product into component contributions ( and ) is difficult. It has been reported (Ignatiev et al., 1996) that saturation of the signal in tooth enamel requires higher power than that for the organic radical , which implies that relaxation times are longer for the organic radical. The signal for in hydroxyapatite saturates at microwave power below 1 mW (Amira et al., 2001). In calcium carbonate, is already saturated at , orthorhombic starts to saturate around 0.3 mW, and freely rotating isotropic starts to saturate around 4 mW (Ikeya, 1993), indicating that the relaxation times decrease in the order orthorhombic freely rotating .
The purpose of this study is to determine the extent to which spin-relaxation times and of the signals in irradiated tooth enamel are affected by the radiation dose or sample source when prepared by the same literature protocol. Irradiated tooth enamel samples were prepared in three laboratories following the method of Romanyukha et al., 1994, Romanyukha et al., 1999. For one preparation three different radiation doses were applied. Comparison data were obtained for irradiated synthetic hydroxy-apatites samples with in A or B sites (Oliveria et al., 2000, Schramm and Rossi, 2000).
Section snippets
Samples
Irradiated tooth enamel samples were prepared by the method of Romanyukha et al., 1994, Romanyukha et al., 1999 in the laboratories of Dr. Romanyukha (USUHS, tooth1–10, tooth1–100, tooth1–500), Dr. Robert Hayes (while he was at the University of Utah, tooth2–1740) and Dr. Alexandre Rossi (Universidade Federal do Rio de Janeiro, tooth3–100 000). Samples were irradiated with . A sample of tooth dentine with a particularly strong native signal was included in the study. The tooth samples were
Results
Representative CW spectra of the irradiated hydroxyapatite and tooth samples are shown in Fig. 2. For in the A-sites of hydroxyapatite the values are 2.0035, 2.0022, 1.9972 which agrees with the literature values of 2.0035, 2.0024, 1.998 (Geoffroy and Tochon-Danguy, 1982). For in the B-sites of hydroxyapatite the g values are 2.0032, 20020, 1.9973, which agrees with the literature values of 2.0030, 2.0015, 1.9972 (Callens et al., 1989). In the spectrum of syn-B there is a small
Discussion
The CW spectra of most of the irradiated samples showed overlapping signals (Fig. 2). The SR and IR curves were analyzed in terms of two distributions of relaxation times. The dependence of the relative weightings of the two distributions on position in the spectra was consistent with the assignment of the distributions to the two species observed in the CW spectra. For the irradiated tooth samples the dominant component at most positions in the spectrum is assigned to the radical in B
Conclusions
Spin–lattice relaxation times for the dominant signals in irradiated tooth enamel and in hydroxyapatite measured by long-pulse SR and by IR show negligible dose dependence. At room temperature the spin-echo dephasing time constant is dominated by motion of the radical, is a reasonable approximation for , and is not dose-dependent. These direct measurements of relaxation times support the use of microwave powers that are independent of dose for quantitative tooth dosimetry.
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