EXPERIMENTAL-EVIDENCE FOR A DISCRETE TRANSITION TO CHANNELING FOR 1.0-MEV PROTONS IN SI(100)

The present work reports the experimental evidence of anomalies exhibi ted by the energy loss and energy straggling of channeled protons in s ilicon in transmission measurements versus the incident angle. Results are presented for 1.0-MeV protons channeled along the [100] axis for a silicon foil of 3.8 mu m thickness. It is shown that the transition from random to a channeling condition is discrete. The energy spectra of transmitted ions show a random peak (lower energy) and a channeled peak (higher energy). The random peak has a fixed energy, while the en ergy of the channeled peak increases as the target crystal's axis appr oaches alignment with the direction of the incident ion beam. The resu lts support a model suggesting that the channeled ions lose energy onl y to valence electrons and are concentrated in a narrow cone about the direction of incidence when they emerge from the crystal. The energy straggling of channeled particles reaches a minimum in the hyper-chann eled condition. Both the energy loss and the energy straggling of chan neled protons show a dependence on the local electron density.