MULTIPLE IONISATION AND COLLECTIVE ELECTRON EMISSION IN MeV/u URANIUM-ION RARE GAS COLLISIONS

Abstract

The multiple ionisation of Ne and Ar gas targets in collisions with high-energy, highly-charged Uranium projectiles (1.4 MeV/u U32+, 5.9 MeV/u U65+) has been investigated. Applying a recently developed experimental technique, the charge state q as well as the transverse momentum (with respect to the beam axis) [MATH] of the recoiling target ion was measured in coincidence with charge-state analysed projectiles. An accuracy in the determination of [MATH] relative to the momentum of the incoming projectile [MATH] in the order of 10-6 was obtained which is comparable to laboratory projectile scattering angles θ in the microradian regime. Thus, the multiple ionisation process and the collision dynamics could be investigated in a very sensitive manner. The statistics of multiple ionisation for definite momentum transfer to the target nucleus is shown to follow a binomial distribution for the case of a Ne target as expected in the Independent Particle Approximation (IPA). The measured differential (with respect to [MATH]) scattering cross sections were compared with the results of n-body classical trajectory Monte Carlo (nCTMC) calculations, which take into account the correct momenta of all involved particles, including the target electrons. From this joint experimental and theoretical study of the collision dynamics of high-energy heavy-ion atom collisions we conclude, that the target electrons are emitted collectively with a mean high energy (? 300 eV) in a cone around 60° with respect to the beam axis as was predicted by theory. The trajectories of the heavy nuclei are influenced considerably by the momenta of these ejected electrons. The strong polarisation of the target atom during the collision is found theoretically to lead to negative deflection angle scattering by the projectile at large impact parameters.