III—The Oxidation of Ferrous Sulphate Solution

This symposium is primarily concerned with the determination of the R.B.E. of different radiations for a variety of biological materials. A simultaneous measurement of the chemical efficiency of the radiations for the oxidation of ferrous sulphate was also carried out. An experiment of this kind is of value because of the relative simplicity of this chemical reaction which is less subject to vagaries in response than are biological systems. In order to make a distinction between the radio-chemical and radio-biological measurements it would seem to be useful to call the result of a chemical measurement R.C.E. (Relative Chemical Efficiency). This paper reports a value for R.C.E. for the oxidation of ferrous iron by 20 and 4 MV X rays. A survey of the literature on yields for ferrous oxidation in the Fricke dosimeter suggests that the chemical yield increases slightly over the range 300 kV to 4 MV to 20 MV (see, for example, Shalek and Sinclair, 1959). Such a result may also be expected on theoretical grounds since at lower ionization densities the chance of recombination of radicals within tracks is less (Burch, 1959). The experiments described here constitute a measurement of R.C.E. for 20 MV versus 300 kV X rays. The value so obtained is used together with a value already published (Dale, Davies and Keene, 1957) for 4 MV versus 300 kV to calculate a value for 20 MV versus 4 MV X rays.