On transverse cooling of channeled ions by electron capture and loss

• Published in: Nuclear instruments & methods in physics research. Section B, Beam interactions with materials and atoms Beam interactions with materials and atoms, 2002-06, Vol.193 (1), p.118-127
• Main Authors: Andersen, J.U., Grüner, F., Ryabov, V.A., Uguzzoni, A.
• Format: Article
• Language: English
• ISSN: 0168-583X; 1872-9584
• DOI: 10.1016/S0168-583X(02)00737-1

To study the charge exchange cooling of channeled ions, we have made simulations of a single collision with an atomic string for 1.5 MeV/u Al ions in (1 0 0) Si. The relative magnitude of the probabilities for capture and loss varies with the distance from the string, and the irreversibility leading to cooling is associated with an asymmetry in time of the ion charge state during the collision. Combining the simulations with simple analytical calculations, we investigate the transition from the perturbation limit with very few charge exchanges to the adiabatic limit with a local charge state equilibrium. In this limit, the symmetry in time is restored and the cooling is strongly suppressed. The simulations illustrate the interplay of charge exchange and multiple scattering due to thermal displacements of atoms, and cooling by charge exchange can be included in the diffusion equation for dechanneling. We use the results of the simulations to obtain an estimate of the flux enhancement in an axial direction from an initially isotropic angular distribution. The conclusion is that the observed magnitude of this enhancement can be explained by charge exchange cooling, with plausible assumption about the impact parameter dependencies of electron capture and loss.