J. C. Eckardt

Stopping power of fluorides for low-velocity protons

A combined experimental and theoretical study of the energy loss of protons in fluorides is presented. The measurements were performed in fresh AlF{sub 3} evaporated in situ on self-supported C foils, extending the low-energy range from 25 down to 0.7 keV. The transmission method is used in combination with time-of-flight spectrometry. The experimental stopping power increases almost linearly with the mean projectile velocity, showing a velocity threshold at about 0.1 a.u. These features are well reproduced by a model based on quantum scattering theory that takes into account the velocity distribution and the excitation of the active 2p electrons in the F{sup -} anions, and the properties of the electronic bands of the insulators. The model also reproduces the threshold behavior measured previously on LiF.

Threshold effect in the energy-loss straggling of protons channeled in Au at very low velocities

The energy-loss straggling of protons channeled in gold crystals in the direction has been experimentally and theoretically studied at the low-velocity limit. The experimental results show a threshold effect in the straggling values, which is similar to that observed earlier in the energy loss. This effect is described by a theoretical model which explains the velocity dependence by considering the individual features of the contributions of free electrons in the conduction band and the nearly free (5d{sup 10}) electrons which generate a threshold effect at low energies due to their binding energy. The model predicts a deviation of proportionality with ion velocity of the energy-loss spread and yields a good description of the obtained experimental data.