Transfer of Radiation Energy to Solute Molecules in Irradiated Frozen Aqueous Solutions

The effect of ionizing radiation on aqueous systems is in general considerably less when the systems are irradiated in the frozen state than when they are irradiated at temperatures above the freezing point (1). It has consequently been assumed that the contribution of the indirect effect is of little or no importance in the frozen state. However, several lines of evidence are available showing that an indirect effect operates also in the frozen state (2-10). Electron spin resonance (ESR) studies indicate that the same type of radicals is formed when ice and liquid water are irradiated (6, 7, 11-16). The decrease in the magnitude of the indirect effect at low temperatures has in part been explained by assuming that the yield of water radicals is considerably reduced at low temperatures (2, 3, 17, 18). When pure ice is irradiated at 77?K, water radicals are stabilized with a G value of about 1 (6, 7, 16). The purpose of the present investigation was to study the extent and the mechanism of the indirect effect in irradiated frozen aqueous solutions. ESR spectroscopy has been used, since it is possible with this method to study the formation and fate of free radicals without having to melt the system. The data indicate that the indirect effect occurring after irradiation at 77?K involves largely water radicals formed within a small sphere of water surrounding the solute molecules.