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 exhibited by the energy loss and energy straggling of channeled protons in silicon 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 μm 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 energy of the channeled peak increases as the target crystal’s axis approaches alignment with the direction of the incident ion beam. The results support a model suggesting that the channeled ions lose energy only 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 hyperchanneled condition. Both the energy loss and the energy straggling of channeled protons show a dependence on the local electron density.