In relativistic atomic collisions the x-ray emission due to charge changing and excitation processes is accompanied by continuum radiation originating from electron bremsstrahlung. For low-Z systems the bremsstrahlung cross-sections are generally predicted with success by the Coulomb modified relativistic Born approximation (for a review see eg. [1]). However, for highly charged high-Z systems this process occurs in the presence of strong Coulomb fields, thus requiring theoretical description which goes beyond first order Born approximation [2]. Up to now very few experimental investigations have been reported for bremsstrahlung production in relativistic ion-atom collisions involving very-heavy highly-charged ions [3]. These studies demonstrated that the experimental data for doubly differential bremsstrahlung cross-sections for heavy ion impact are significantly underestimated by the Born approximation calculations even if Coulomb distortion effects are taken into account in an approximate way (Born-Elwert formula).
We report here on the recent experimental results for electron bremsstrahlung in collisions of He-like uranium with light gaseous targets at relativistic energies. Such systems provide a unique possibility to study bremsstrahlung in the domain of strong Coulomb fields while simultaneously screening effects play a negligible role. Furthermore, the gaseous targets used in the present experiments reduce the background associated with secondary electron bremsstrahlung as compared to solid-target measurements. The experimental results for the doubly differential bremsstrahlung cross-sections are compared to "exact" independent particle approximation (IPA) calculations of Pratt et al. [2] and to calculations based on the Born approximation (Bethe-Heitler formula with Elwert correction factor).