A study of small impact parameter ion channeling effects in thin crystals

TL;DR

This study investigates how small impact parameter ion trajectories produce distinctive channeling patterns in thin silicon crystals, revealing complex angular structures and focusing effects relevant to ion transport phenomena.

Contribution

It provides new insights into the effects of small impact parameter trajectories on ion channeling patterns in thin crystals, highlighting radial focusing and angular structures beyond the critical angle.

Findings

Large angular deflections occur for ions with small impact parameters.

Radial focusing influences channeling and scattering in thin crystals.

Characteristic angular structures relate to peaks in nuclear encounter probability.

Abstract

We have recorded channeling patterns produced by 1 to 2 MeV protons aligned with <111> axes in 55 nm thick silicon crystals which exhibit characteristic angular structure for deflection angles up to and beyond the axial critical angle, psi(a). Such large angular deflections are produced by ions incident on atomic strings with small impact parameters, resulting in trajectories which pass through several radial rings of atomic strings before exiting the thin crystal. Each ring may focus, steer or scatter the channeled ions in the transverse direction and the resulting characteristic angular structure beyond 0.6psi(a) at different depths can be related to peaks and troughs in the nuclear encounter probability. Such radial focusing underlies other axial channeling phenomena in thin crystals including planar channeling of small impact parameter trajectories, peaks around the azimuthal distribution at small tilts and large shoulders in the nuclear encounter probability at tilts beyond psi(a).