Eur. Phys. J. B (2018) 91: 49
https://doi.org/10.1140/epjb/e2018-80580-4

Regular Article

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

¹ Center for Ion Beam Applications, Physics Department, National University of Singapore, Singapore 117542, Singapore
² NUSNNI-NanoCore, National University of Singapore, Singapore 117576, Singapore
³ Singapore Synchrotron Light Source (SSLS), National University of Singapore, Singapore 117603, Singapore

^a e-mail: phymbhb@nus.edu.sg

Received: 16 October 2017
Received in final form: 19 December 2017
Published online: 7 March 2018

Abstract

We have recorded channeling patterns produced by 1–2 MeV protons aligned with ⟨1 1 1⟩ axes in 55 nm thick silicon crystals which exhibit characteristic angular structure for deflection angles up to and beyond the axial critical angle, ψ_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 0.6ψ_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 ψ_a.