A Monte Carlo study of the spatial aspects of electron energy degradation in water vapor has been carried out in the energy range from 30 eV to 30 keV to obtain the spatial yield spectral distribution. This four-dimensional function is represented by a number of simple analytical forms which display the progression of shapes with increasing incident energy. These forms are used to calculate the spatial distributions of the initial species created by electron energy deposition in water vapor. Results on the gross yields are also presented. The influence of diffusion in the steady state is calculated with the help of a Legendre polynomial expansion of the source function. Having obtained the concentrations associated with this source function in the form of a Legendre expansion, we found a particularly simple closed formula which yields approximately the same results. These analytic source and concentration distributions can serve as convenient inputs for studies of the chemical and biological stages of radiation action.

Spatial (radial and longitudinal) yield spectra for electron energy degradation in neon gas for 20 eV to 1 keV incident electrons are generated using a Monte Carlo technique. The differential elastic and inelastic scattering cross sections needed for these calculations are obtained from atomic independent particle models augmented by semiempirical modifications. Numerical four-dimensional yield spectra are represented analytically within the framework of a microplume model. Some special features of two-dimensional and four-dimensional yield spectra for a gas with a high threshold energy such as neon are discussed. Finally, yields of some prominent emission lines in neon are calculated and their spatial characteristics are represented by contour diagrams.

Recent optical absorption measurements for liquid water in the vacuum uv show both a redistribution of oscillator strengths relative to the gas phase, and the possible existence of a collective oscillation (plasmon). We investigate the effects of such a plasmon on the partition of energy deposited by fast electrons. To accomplish this, we develop a heuristic model of the liquid based primarily upon the extensive body of gas phase generalized oscillator strength data. The model has one major free parameter, the strength of the 21.4 eV plasmon. Using standard resonance shapes to approximate the energy loss spectrum of the plasmon, we estimate that the water plasmon has an oscillator strength of 2 (± about 0.6) and a half-width of about 3.5 eV. We then calculate the slowing down of an electron in liquid water according to continuous-slowing-down theory, and after allowing for plasmon decay, compare the yields for the liquid and gas phases. Our results indicate that when the plasmon decays by ionization it produces a spectrum of only subexcitation electrons. We find a modest change in the yields with the eV per-ion-pair lowered from 30.7 in the gas to 28.5 in the liquid.

A fundamental description of charged particle deposition phenomena must include detailed characterizations of the production and interactions of secondary electrons which arise along the track of a primary particle. Since these secondary electrons predominately have energies below that describable by the Born approximation, one must go beyond the Bethe theory of stopping power and adopt a detailed microscopic approach. In this paper, we review our work involving secondary electron distributions since 1963, beginning with the work of Green and Barth, largely in relation to applications but also in relation to the fitting of experimental data and the testing of theoretical models. In the latter context, we utilize the independent particle model of Green, Sellin, and Zachor in the Born approximation and in the Eikonal approximations. Our past efforts will be reviewed and our current efforts described.

We present a two parameter nonlinear differential equation and solution which quite accurately characterize radiobiological survival curves. The equation is an adaptation of a nonlinear rate equation first used by Verhulst to characterize the saturation of population growth. In the present context it appears capable of accommodating biological recovery, as manifested by the shoulder which appears in many survival curves.

A microscopic approach based upon the assignment of electron impact cross sections has been used by Green et al. (1) to calculate the efficiencies for degradation of low energy electrons in water. By Monte-Carlo techniques Berger (2) has calculated the spatial distribution of the total energy deposited by δ-rays ejected at various energies and at various angles with respect to a heavy ion beam. In the present work we develop a model which combines these two results to calculate the energy per unit volume deposited in various excitations of the H 2 O molecule by δ-rays ejected by a 300 keV proton. Results are presented for vibrational excitations, electronic excitations, and ionization continua as a function of the radial distance from the proton beam.

Proton energy degradation, the fate of the energy lost by protons in slowing down, is analyzed from a microscopic point of view for protons of <tex-math>$0.5-10^{4}$</tex-math> keV in water vapor. The analysis is based on the assignment of cross sections for the excitation of the H 2 O molecule by proton and neutral hydrogen impact. Given these cross sections and the efficiencies for electron degradation in water vapor, we calculate the efficiency for population of ionization continua, dissociative continua, and bound excited states of H 2 O after complete degradation of the proton. Each efficiency includes the contribution from direct proton excitation, excitation by neutral hydrogen produced in charge exchange reactions, and excitation by secondary electrons. We also calculate the average energy lost per ion pair and obtain a value of approximately 25 eV at high energies.