## Abstract

The radiation-induced decomposition of water by X-rays (200 kV) and $\gamma $-rays ($^{60}$Co) has been investigated by studying the reduction of ceric salts in dilute aqueous solutions under the influence of these radiations. For the elucidation of the mechanism, the ceric reduction yields have been studied under various conditions, i.e. at different acid concentrations, in de-aerated and in oxygenated (1 atm) solutions, in the presence of dissolved hydrogen and with formic acid as an added solute. The initial reduction yields were found to be greater than the steady-state reduction yields, the following values were obtained (G in molecules/100 eV): $ \matrix\format\l \\ \text{for X-rays}\quad G(\text{Ce}^{\text{III}})=3\cdot 15\pm 0\cdot 10, \\ \text{for}\ \gamma \text{-rays}\quad \,G(\text{Ce}^{\text{III}})=2\cdot 45\pm 0\cdot 08. \endmatrix $ These yields were the same both in the presence and in the absence of oxygen during irradiation. In the presence of formic acid the reduction yields are increased: $ \matrix\format\l \\ \text{for X-rays}\quad G(\text{Ce}^{\text{III}})=8\cdot 50\pm 0\cdot 10, \\ \text{for}\ \gamma \text{-rays}\quad \,G(\text{Ce}^{\text{III}})=8\cdot 15\pm 0\cdot 10. \endmatrix $ These yields were found to be independent of the formic acid concentration in the range 0$\cdot $001 to 0$\cdot $05 M. The yield of the gaseous products (hydrogen and oxygen) from dilute ceric salt solutions, irradiated in vacuo, were found to be: $\aligned \text{for X-rays}\quad & G(\text{H}_{2})=0\cdot 57\pm 0\cdot 03,\\ \quad & G(\text{O}_{2})=1\cdot 07\pm 0\cdot 05,\\ \text{for}\ \,\gamma \text{-rays}\quad & G(\text{H}_{2})=0\cdot 40\pm 0\cdot 02,\\ \quad & G(\text{O}_{2})=0\cdot 84\pm 0\cdot 04. \endaligned $ A mechanism is proposed on the basis of which the experimental results can be accounted for. It has been possible to draw some definite conclusions about the primary chemical processes in the radiation-induced decomposition of liquid water; a small but significant difference in the yields of the primary chemical products for X-rays and for $\gamma $-rays has been clearly established.