Electron energy loss in fluids: Thermalization distances in liquid and gaseous sulfur hexafluoride

Ionizing radiation passing through a fluid produces an ion–electron pair by knocking an electron off a molecule. The electron possesses excess energy, which it loses in collisions with molecules as it moves away from the ion. These are stochastic processes. The distance travelled during thermalization determines the probability that the electron ultimately escapes the Coulombic field of the ion to form freely diffusing ions. Free-ion yields were measured in X-irradiated sulfur hexafluoride at 5.7 ≤ d(kg m−3) ≤ 1860, corresponding to the vapor and liquid at 202.8 ≤ T(K) ≤ 324.1. (The critical fluid has dc = 730 kg m−3 and Tc = 318.7 K). The electric field dependence of the yield was best fitted using an electron thermalization distance distribution function F(y) that was Gaussian with a power tail. The most probable thermalization distance bGP was estimated at each density. The density-normalized electron-thermalizing ability of the fluid decreased with increasing gas density and was independent of density in the liquid phase. The dependence is different from those observed in hydrocarbons and might reflect a density effect on the energy dependence of the electron-attachment reaction.