Charge fraction of 6.0 MeV/n heavy ions with a carbon foil: Dependence on the foil thickness and projectile atomic number

Abstract

We measured the charge fraction of 6.0 MeV/n heavy ions (C, Ne, Si, Ar, Fe and Cu) with a carbon foil at the NIRS-HIMAC injector. At this energy they are stripped with a carbon foil before being injected into two synchrotron rings with a maximum energy of 800 MeV/n. In order to find the foil thickness ( D E) at which an equilibrium charge state distribution occurs, and to study the dependence of the D E-values on the projectile atomic number, we measured the exit charge fractions for foil thicknesses of between 10 and 350 μg/cm 2. The results showed that the D E-values are 21.5, 62.0, 162, 346, 121, 143 μg/cm 2 for C, Ne, Si, Ar, Fe, Cu, respectively. The fraction of Ar 18+ ions was actually improved to 33% at 320 μg/cm 2 from ∼15% at 100 μg/cm 2. For Fe and Cu ions, the D E-values were found to be only 121 and 143 μg/cm 2; there is a large gap between Ar and Fe, which is related to the differences in the ratio of the binding energy of the K-shell electrons to an electron energy (3.26 keV) corresponding to the velocity of a 6.0 MeV/n projectile. The well-known “two disjoint Gaussians (shell effect)” was also observed in the measured charge fractions of both Fe and Cu. The results of the charge fractions were also compared with other data and to calculations of Rozet et al. [2], in which there was a good agreement for light ions (C); however, a significant difference was observed for ion species heavier than Ne.