No abstract

A deterministic computational procedure for describing the transport of electrons in condensed media is formulated to simulate the effects and exposures from spectral distributions typical of electrons trapped in planetary magnetic fields. The primary purpose for developing the procedure is to provide a means of rapidly performing numerous repetitive transport calculations essential for electron radiation exposure assessments for complex space structures. The present code utilizes well-established theoretical representations to describe the relevant interactions and transport processes. A combined mean free path and average trajectory approach is used in the transport formalism. For typical space environment spectra, several favorable comparisons with Monte Carlo calculations are made which have indicated that accuracy is not compromised at the expense of the computational speed.

Mean excitation energies for stopping by isolated atoms are accurately predicted by the plasma absorption spectrum associated with the atomic orbitals when the plasma frequency shift due to individual electron motion proposed by Pines [Phys. Rev. 92, 626 (1953)] is incorporated.

Molecular mean excitation energies for ionic bonded molecules calculated according to the local plasma approximation are compared to the Bragg rule. Adjustments of 15% are calculated for LiF in agreement with experiments while 6% adjustments are predicted for HF and 3% for LiH.

The basic model of Lindhard and Scharff, known as the local plasma model, is utilized to study the effects of the chemical and physical state of the medium on its stopping power. Unlike previous work with the local plasma model, in which individual electron shifts in the plasma frequency were estimated empirically, the Pines correction derived for a degenerate Fermi gas is shown herein to provide a reasonable estimate even on the atomic scale. Thus, the model is moved to a completely theoretical base requiring no empirical adjustments, adjustments characteristics of past applications. The principal remaining error is in the overestimation of the low-energy absorption properties characteristic of the plasma model in the region of the atomic discrete spectrum, although higher energy phenomena are accurately represented and even excitation-to-ionization ratios are given with fair accuracy. Mean excitation energies for covalently bonded gases and solids, ionic gases and crystals, and metals are calculated using first-order models of the bonded states for which reasonable agreement with the recently evaluated data of Seltzer and Berger is obtained. Hence the methods described herein allow reasonable estimates of mean excitation energy for any physical-chemical combination of material media for stopping power applications.