Elsevier

Radiation Measurements

Volume 42, Issues 6–7, July–August 2007, Pages 997-1004
Radiation Measurements

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

https://doi.org/10.1016/j.radmeas.2007.05.048Get rights and content

Abstract

Spin–lattice relaxation times, T1, 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 T1 distribution was 1.41.6μs 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 T1 distribution centered at 3060μs and is assigned to an organic radical (g2.0045). At 294 K the spin–spin relaxation time T2 measured by two-pulse echo decay was dominated by motion of the CO2- and was independent of dose. The dose independence of T1 and T2 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.

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 CO3-, CO33-, CO2- and organic radicals (Ikeya, 1993, Amira et al., 2001, Callens et al., 2002). The dominant radical species is CO2-, 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 g-value 2.0045 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 T1 and T2. The product T1T2 can be estimated by CW progressive saturation, but separation of the product into component contributions (T1 and T2) is difficult. It has been reported (Ignatiev et al., 1996) that saturation of the CO2- signal in tooth enamel requires higher power than that for the organic radical (g=2.0045), which implies that relaxation times are longer for the organic radical. The signal for CO33- in hydroxyapatite saturates at microwave power below 1 mW (Amira et al., 2001). In calcium carbonate, CO33- is already saturated at 10-3mW, orthorhombic CO2- starts to saturate around 0.3 mW, and freely rotating isotropic CO2- starts to saturate around 4 mW (Ikeya, 1993), indicating that the relaxation times decrease in the order CO33-> orthorhombic CO2-> freely rotating CO2-.

The purpose of this study is to determine the extent to which spin-relaxation times T1 and T2 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 Co60 radiation doses were applied. Comparison data were obtained for irradiated synthetic hydroxy-apatites samples with CO2- 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 Co60. 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 CO2- in the A-sites of hydroxyapatite the g 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 CO2- 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 CO2- radical in B

Conclusions

Spin–lattice relaxation times for the dominant CO2- signals in irradiated tooth enamel and CO2- 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 CO2- radical, is a reasonable approximation for T2, 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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