Electron Spin Resonance Study on $\gamma$-Ray-Induced Methyl Radicals in Methane Hydrates

概要

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Synthetic methane hydrate irradiated by $\gamma$-rays at 77 K has been measured by electron spin resonance (ESR). The quartet signal at $g=2.0029\pm 0.0005$ with the intensity ratio of approximately $1:3:3:1$ and hyperfine coupling constant $A=2.3\pm 0.1$ mT was identified as the methyl radical (CH3$\cdot$). The dose response of its signal intensities showed that the radicals were formed linearly up to 10 kGy with the $G$-value [the formation efficiency of radical pairs per 100 eV (16 aJ)] of 0.05. According to annealing experiments, the radical became unstable over 200 K, at which thermal dissociation of the hydrate had already started, but a very weak signal was detectable up to 270 K. The methyl radical thermally decayed following second-order kinetics with an activation energy of $0.26\pm 0.02$ eV (42 zJ). The existence of the signal at 270 K would be related to the extremely slow dissociation referred to as the self-preservation effect of the gas hydrate.
Published by the Japan Society of Applied Physics through the Institute of Pure and Applied Physicsの論文
2004-01-15