engleski

Study of relaxation rates of stable paramagnetic centers in gamma-irradiated alanine : influence on alanine/ESR dosimety

ESR spectra of irradiated alanine currently employed in the alanine/ESR dosimetry exhibit a complex spectroscopic structure. The complex structure is mainly due to the unresolved hyperfine structure and motional averaging effects of electron proton hyperfine couplings. From single crystal studies the two major radical species can be expected at room temperature, “stable alanine radical” (SAR) with the well established spectroscopic parameters and so called “second stable radical” (SSAR) with spectroscopic parameters which are recently deduced [1]. Electron spin-lattice relaxation times obtained by various pulse and nonlinear techniques on SAR also exhibit more than one single relaxation rates. One expects that ESR intensity and saturation factor of these centers could strongly depend on the spectral diffusion mechanism. Indeed, it was demonstrated that in an CW saturation experiment the dominant relaxation rate is due to the spectral diffusion mechanism and not due to usually expected spin-lattice relaxation mechanism [3]. In order to get more information on spectral diffusion ENDOR, Transfer Saturation ESR and double modulation ESR (DMESR) techniques have been employed to study relaxation rates of SAR and SSAR. ENDOR linewidths and relaxation rates detected by DMESR exhibit a complex extremal dependence in the wide temperature range (100 K – 300 K). It can be expected that detected linewidths and spectral intensities were modulated by the rotational rate of the methyl groups. Different methyl groups with different rotation rates could be used to describe relaxation data of the stable centers. The obtained results are compared with rotation rates of methyl groups obtained earlier for SAR and with the methyl group dynamics measured by NMR in non irradiated samples. Knowledge of a detail temperature dependence of the relaxation rates may have an important implication for the application of alanine/ESR dosimetry [3]. Signal/noise ratio for alanine spectrum can be enhanced around 30 % by detecting the ESR spectrum at the maximum relaxation rate at about 200 K as compared with room temperature. This enhancement might be of great significance in the therapy-level dose range where detected signal is very small and it is comparable to a background signal. [1] E. Sagstuen, E. O. Hole, S. R. Haugedal and W. H. Nelson, J. Phys. Chem. A, 101, 9763 (1997) [2] M. Brustolon and U. Segre, Appl. Magn. Reson. 7, 405 (1994). [3] B. Rakvin, Appl. Radiat. Isot. 47, 525 (1996)

ESR; alanine; dosimetry

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