Charge-state dynamics of 1.4- and 11-MeV/u uranium ions penetrating H2 and He gas targets

Theoretical description and interpretation are presented of the recent experimental data on stripping of fast ²³⁸U ions, penetrating H₂ and He gaseous targets: obtained in GSI, Darmstadt, Germany at 1.4 MeV/u with a H₂ stripper, and in RIKEN, Saitama, Japan at 11 MeV/u with H₂ and He strippers. Electron-loss and capture cross sections for uranium ions, interacting with H₂ and He targets are calculated and used as input data in the BREIT code to obtain dynamic characteristics of uranium-ion beams: non-equilibrium and equilibrium charge-state fractions, mean and equilibrium charges, and equilibrium thicknesses. Special attention is paid for the calculation of the dynamic characteristics of 1.4-MeV/u uranium ions stripped by a H₂ in a windowless gas target at GSI, using a newly designed pulsed gas cell with a pressure changing from $\sim$0.1 to $\sim$100 mbar along the ion beam. To explain the experimental data, a Gaussian pressure model for the H stripper is applied with account for the target-density effect. A possible deviation of the Bragg's additivity rule for electron-capture cross sections for a H₂ target is also considered and we predict that the equilibrium charge of a 1.4-MeV/u uranium beam in atomic hydrogen is larger than in molecular hydrogen, i.e., it is indicated that atomic hydrogen can be a more effective stripper than molecular hydrogen. Charge-changing cross sections and dynamic characteristics of 11-MeV/u uranium ions passing through He and H₂ targets are calculated as well. Calculated equilibrium mean charge states $\overline{q}\left(\mathrm{He}\right)=67.8$ and $\overline{q}\left(H_2\right)=75.7$ are in agreement with RIKEN experimental data within 10%.