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

Abstract

The stopping powers of thin Al foils for H+ and 4He+ ions have been measured over the energy range E=(206.03–2680.05)keV/amu with an overall relative uncertainty better than 1% using the transmission method. The derived 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) data for H+ ions together with those for He2+ ions reported by Andersen et al. (1977) have been analyzed over the energy interval E>1.0MeV using the modified Bethe–Bloch stopping theory. The following sets of values have been inferred for the mean excitation potential, I, and the Barkas–Andersen parameter, b, for H+ and He+ projectiles, respectively: (I=164±3)eV, b=1.40 and (I=163±2.5)eV, b=1.38. As expected, the I 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 I parameter must be determined from precise stopping power measurements performed at high projectile energies where the Bethe stopping theory is fully valid.