Accurate stopping power measurements for (0.21–2.68) MeV/u 1H+ and 4He+ ions crossing thin Al foils; extraction of the (I, b) parameters

The stopping powers of thin Al foils for H+ and 4He+ ions have been measured over the energy range E=(206.03–2680.05)E=(206.03–2680.05) keV/amu with an overall relative uncertainty better than 1% using the transmission method. The derived S(E)S(E) experimental data are compared to previous ones from the literature, to values derived by the SRIM-2008 code or compiled in the ICRU-49 report, and to the predictions of Sigmund–Schinner binary collision stopping theory. Besides, the S(E)S(E) data for H+ ions together with those for He2+ ions reported by Andersen et al. (1977) have been analyzed over the energy interval E>1.0E>1.0 MeV using the modified Bethe–Bloch stopping theory. The following sets of values have been inferred for the mean excitation potential, II, and the Barkas–Andersen parameter, bb, for H+ and He+ projectiles, respectively: {(I=164±3)(I=164±3) eV, b=1.40}b=1.40 and {(I=163±2.5)(I=163±2.5) eV, b=1.38}b=1.38. As expected, the II parameter is found to be independent of the projectile electronic structure presumably indicating that the contribution of charge exchange effects becomes negligible as the projectile velocity increases. Therefore, the II parameter must be determined from precise stopping power measurements performed at high projectile energies where the Bethe stopping theory is fully valid.