MEASURED DEVIATIONS FROM RUTHERFORD BACKSCATTERING CROSS-SECTIONS USING LI-ION BEAMS

Bozoian et al. have developed an analytical theory which predicts the incident ion energy at which elastic-scattering cross sections begin to deviate from their Rutherford values as the result of nuclear perturbations. The predictions are in good agreement with experimental data for proton and helium-ion beams, and should be applicable to heavier projectiles. The theory has also been extended to account for isotopic variation of the incident beam. As a further test of the theory, we have performed a series of backscattering measurements using both Li-6 and Li-7 ion beams. Lithium-ion backscattering is of importance to thin-film analysis because it offers improved depth sensitivity and kinematic mass resolution as compared to helium. Elastic-backscattering cross sections for O, F, Al, and Ti were measured relative to those of high-Z reference elements over an energy range of 2.5-9.25 MeV, and the energies at which they began to deviate from their Rutherford values were determined. The experimental results are compared to predictions from the analytical theory, and from optical-model calculations.