Optical-model potential for electron and positron elastic scattering by atoms

Abstract

An optical-model potential for systematic calculations of elastic scattering of electrons and positrons by atoms and positive ions is proposed. The electrostatic interaction is determined from the Dirac-Hartree-Fock self-consistent atomic electron density. In the case of electron projectiles, the exchange interaction is described by means of the local-approximation of Furness and McCarthy. The correlation-polarization potential is obtained by combining the correlation potential derived from the local density approximation with a long-range polarization interaction, which is represented by means of a Buckingham potential with an empirical energydependent cutoff parameter. The absorption potential is obtained from the local-density approximation, using the Born-Ochkur approximation and the Lindhard dielectric function to describe the binary collisions with a free-electron gas. The strength of the absorption potential is adjusted by means of an empirical parameter, which has been determined by fitting available absolute elastic differential cross-section data for noble gases and mercury. The Dirac partial-wave analysis with this optical-model potential provides a realistic description of elastic scattering of electrons and positrons with energies in the range from ϳ100 eV up to ϳ5 keV. At higher energies, correlation-polarization and absorption corrections are small and the usual static-exchange approximation is sufficiently accurate for most practical purposes.

The role of absorption in large-angle elastic scattering

“…For small atoms and at low energies the problem can be solved by considering all possible excitations and solving a set of closed coupling equations [3]. At high energies, and for heavy atoms this is computationally too difficult and the problem is usually attacked using a phenomenological optical potential [4,5]. This potential is complex, resulting in a loss of intensity of the elastically scattered electrons, which corresponds to the absorption mentioned above.…”

“…We use ELSEPA by Salvat et al [6], and the absorption part of the program is described extensively in ref. [5]. Other approaches exist, e.g.…”

Radial Secondary Electron Dose Profiles and Biological Effects in Light-Ion Beams Based on Analytical and Monte Carlo Calculations using Distorted Wave Cross Sections

“…The term V ex (r), which applies only for electrons, represents a local approximation to the exchange interaction between the projectile and the atomic electrons (see, e.g., Salvat, 1998;Salvat, 2003). We shall limit our considerations to the case of spin-unpolarised projectiles, i.e., their spin is randomly oriented.…”

“…These effects can be accounted for by introducing local corrections to the static potential (3.5) [see, e.g., Salvat (2003) and references therein]. Calculations with elsepa, including these low-energy effects, yield elastic-scattering DCSs that, at large angles, differ from the static-field DCSs by about 20% at E = 1 keV and by up to 50% at E = 100 eV.…”

Investigation of Interactions Between Low Energy Positrons and DNA Using the Monte-Carlo method

“…For free interaction and and by using the non-relative Rutherford differential influence cross-section for the positron here (Salvat, 2003) the total crosssection for free interaction is…”