  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).
