1. General Seminars

Ion channeling through ultra-thin silicon membranes

by Dr Mark Breese (Singapore Univ.)

Aula Bruno Touschek (LNF INFN)

Aula Bruno Touschek


Via Enrico Fermi, 40 00044 Frascati
At CIBA we have developed at process to fabricate perfectly crystalline 55 nm thick silicon membranes. These have been used to observe transmission ion channeling patterns showing the early evolution of the axially channeled beam angular distribution. The reduced multiple scattering through such thin layers allows fine angular structure produced by the highly non-equilibrium transverse momentum distribution of the channeled beam during its initial propagation in the crystal to be resolved. In this presentation, channeling patterns where clearly resolved effects of the narrow {111} planes are observed in axial and planar alignments for MeV protons passing through such a thin silicon membrane are shown. At certain axes, the offset in atomic rows forming the narrow {111} planes results in shielding from the large potential at the wide {111} planes, producing a region of shallow, asymmetric potential from which axial channeling patterns have no plane of symmetry. At small tilts from such axes, different behavior is observed from the wide and narrow {111} planes. At planar alignment, distinctive channeling effects due to the narrow planes are observed. There are two fields where this insight into the dechanneling behavior from the narrow {111} planes is important. One is in extraction of high energy proton beams using bent crystal channeling, where the silicon {111} rather than the {111} direction is commonly used owing its larger critical angle. The other field is the production of channeling radiation, where a high energy charged particle beam is used to induce electromagnetic radiation of keV energies due to the planar channeling action of particles oscillating back and forth between the channel walls.