On the application of the Onsager theory to the description of the free-ion yield observed in ''warm liquids'' irradiated by γ-rays

We have analysed existing data on the free ion yield produced by γ-radiolysis of ''warm liquids'', namely neopentane, 2,2,4,4-tetramethyl pentane, tetramethyl silane, tetramethyl germanium and tetramethyl tin, within the framework of the Onsager theory. Four distribution functions (describing the separation distance r between electron-ion pairs at thermalization) were considered: a delta function, an exponential function, an exponential times r, and an exponential times r2. With a suitable choice of the two adjustable parameters, G(∞) and ρ - where G(∞) is the number of electron-ion pairs initially produced per every 100 eV released in the sample, and ρ is the mean thermalization distance - all four distributions describe the data approximately. However, an independent estimate of G(∞) based on the justifiable assumption that the ratio between the average energy W, which is necessary to create an electron-ion pair, and the ionization energy, is the same in the liquid as it is in the gas, and is the same in all warm liquids, allows a choice to be made between these functions. On the other hand, a least-squares polynomial fit representation of the data (in powers of the applied electric field E) indicates that the usual linear expansion of the free ion yield, G(E) = G(0)(1 + E / E0), does not provide a good representation of the data