Secondary electron spectra: a semiempirical model

A model based on evaluation of coefficients in Bethe's asymptotic expansion of the first Born approximation has been developed to extend the data base of differential ionization cross sections needed in track structure calculations. The coefficient of the lowest-order term in this expansion is related to the photoionization cross section of the target, but optical data alone are generally not sufficient to determine differential cross sections for ionization by charged particles. We have used proton-impact, differential ionization data to evaluate the coefficient of the next higher-order term in the expansion. Since these coefficients are independent of the projectile, the experimentally derived coefficients can be used to predict differential cross sections for ionization of the target by bare ions and electrons that are sufficiently energetic to make the Bethe theory valid. This paper compares model calculations with recent experimental data on ionization of water vapor by high-energy electrons and protons. 20 refs., 2 figs