A Computer Simulation of Slow-Positron Implantation Depths in Aluminum

Abstract

The mean penetration depths and stopping profiles of positrons with incident energies up to 10 keV impinging on semi-infinite aluminum with normal and oblique angles of incidence are stochastically modelled within a Monte-Carlo framework. The elastic scattering cross sections have been obtained from a partial wave expansion. To model inelastic core and valence electron excitation, we have used the Gryzinski's expression. Our simulated results agree very well with the available experimental data and clearly demonstrate that the mean penetration depths are not adequately described by the simple power law commonly used in defect profiling. This suggests that both mean penetration depths and stopping profiles cannot generally be modelled by materialindependent parameters as was proposed previously. The mean penetration depth decreases non-monotonically with increasing the angle of incidence from 0 • to 80 • . This can be seen from the reduction of the width of the implantation profiles.

Simulation of positron backscattering on Al, Cu, Ag and Au targets using GEANT4 code

“…It could be noticed that the critical thickness decreases with the increase of atomic number Z , which indicates that the impact of target surface on the backscattering coefficient is also related to the atomic number Z . Actually, the mean positron implantation depths for semi‐infinite materials are customarily expressed as: , where E is the positron energy, ρ is the material density, and A and n are energy‐independent constants for a particular material . It could be speculated that the positron implantation depth is deeper for lighter atoms and hence a larger transmission probability considering the same target thickness.…”

“…Actually, the mean positron implantation depths for semi-infinite materials are customarily expressed as: z ¼ A ρ E n , where E is the positron energy, ρ is the material density, and A and n are energy-independent constants for a particular material. [39,40] It could be speculated that the positron implantation depth is deeper for lighter atoms and hence a larger transmission probability considering the same target thickness. As a result, the possibility of emitting out of the target surface for incident positrons becomes smaller.…”

Positron backscattering from solid targets: Modeling of scattering processes via various approaches

“…The ultimate fate of positrons in condensed matter is obviously annihilation in the nanosecond timescale. However, during that time a rich variety of physical phenomena occur whose knowledge is of prime importance for determining the particle ranges as well as the backscattering coefficients (Bouarissa et al, 1998;Bouarissa and Walker, 2000). With respect to the slow-positron-beam experiments, the essential point is that a large fraction of positrons can diffuse back to the surface and then be emitted into the vacuum either as free positron or as Positronium (Ps) (Boev et al, 1987).…”

Study on electron scattering in solid targets using accurate transport cross-sections

“…The knowledge of the collision processes can be encapsulated in scattering cross-sections generally used either to simulate electron trajectories via Monte Carlo (MC) simulations [6][7][8][9][10] or to provide stopping powers and transport cross-sections needed for analytic transport theory [11,12]. In fact, the transport cross-section is a quantity of prime importance.…”