Enhancement of binary encounter electron emission in collisions of highly charged ions with helium gas targets

The energy and angular distribution of δ-electrons produced in collisions of swift heavy ions with helium gas targets are presented. Double differential electron emission cross sections d2σ/dEedΩefor different projectiles have been measured as a function of the electron emission energy Eeand the polar electron emission angle ϑe. For systematics the cross sections were investigated for different projectiles from neon up to bismuth at ionic charge states between q=7+ and 24+. The specific projectile energy was kept constant within 10%, i.e. 5.4⩽EP⩽6.0 MeV/u for the different projectiles. The investigations are focussed on the emission cross sections of the Binary Encounter (BE) electrons. The BE electron emission cross section depends strongly on the ionic qas well as on the nuclear charge state ZP. At small emission angles ϑethe electrons are most enhanced but converging towards the q2-scaling at larger angles. An appropriate parameter was found to be the ratio of the ionic charge and the nuclear charge q/ZP. The observation of forward peaked, enhanced electron emission is of great importance for the understanding of radiation damages in dense targets caused by heavy ion collisions.