A limited study has been made of the gamma radiolysis of methyl iodide vapor and mixtures of phosphine and methyl iodide in the gas phase. Analyses of methane and hydrogen yields from irradiation of the pure vapor at 270 Torr show that both are proportional to absorbed dose from$1\times 10^{20}$ to$4\times 10^{20}\ {\rm eV}$, with$G({\rm CH}_{4})=2.3$ and$G({\rm H}_{2})=0.6$. Both yields show a dependence on methyl iodide pressure.$G({\rm CH}_{4})$ increases with pressure above 200 Torr, but$G({\rm H}_{2})$ falls off. The mechanism for methane production is presumed to involve hot methyl radicals. Hydrogen production may be via a molecular elimination or hot atom process. Those mixtures containing a few mole percent phosphine show a greatly enhanced methane yield. A maximum$G({\rm CH}_{4})$ of 12.6 occurs in a mixture containing 3.5 mole % phosphine and in which 99% of the radiation dose is absorbed by methyl iodide. This yield probably approximates the G for methyl radical production in pure methyl iodide vapor. In mixtures with greater phosphine concentrations the methane yield falls off sharply, perhaps due to an energy transfer process.

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