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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 > 1.0 MeV 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. |
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