Electron spin resonance study on γ-ray-induced methyl radicals in methane hydrates

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 +/- 0.0005 with the intensity ratio of approximately 1 : 3 : 3 : I and hyperfine coupling constant A = 2.3 +/- 0.1 mT was identified as the methyl radical (CH3..). 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 +/- 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.