Projectile electron loss and capture in MeV/u collisions of U²⁸⁺ with H₂, N₂ and Ar

Electron capture and loss cross sections for U28+ colliding with H2, N2 and Ar were measured at 3.5 and 6.5 MeV/u. These data were used to benchmark n-body calculations using the classical trajectory Monte Carlo method. The n-body calculations include electrons on both nuclear centres and all electron–electron and electron–nuclear interactions between each centre. For the U28+ ion, 36 electrons were incorporated in the calculations (4s24p64d104f145s25p2), while for the H, N and Ar targets all electrons were used except those for the K-shell of Ar, leading to 39-, 45- and 54-body calculations, respectively. Projectile electron loss was predicted for U28+ at energies from 2 to 150 MeV/u. Only for the H-target did the projectile electron loss cross section decrease approximately as E−1. The heavier targets exhibited slower energy dependences, contrary to the E−1 prediction of one-electron theories. Moreover, the collisional interactions are quite strong with an average of 1.64 and 2.88 electrons removed from the U28+ ion at 10 MeV/u in each collision with N and Ar, respectively. These data and calculations were used to assess the vacuum requirements for the SIS-100 synchrotron ring under construction at GSI-Darmstadt. For the residual gases expected to be in the ring, the U28+ lifetime was found to be essentially constant as a function of projectile energy, leading to very stringent vacuum requirements.

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