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

The energy and angular distribution of delta-electrons produced in collisio ns of swift heavy ions with helium gas targets are presented. Double differ ential electron emission cross sections d(2)sigma/dE(e)d Omega(e) for diffe rent projectiles have been measured as a function of the electron emission energy E-e and the polar electron emission angle theta(e). 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 pro jectile energy was kept constant within 10%, i.e. 5.4 less than or equal to E-P less than or equal to 6.0 MeV/u for the different projectiles. The inv estigations are focussed on the emission cross sections of the Binary Encou nter (BE) electrons. The BE electron emission cross section depends strongl y on the ionic q as well as on the nuclear charge state Z(P). At small emis sion angles theta(e) the electrons are most enhanced but converging towards the q(2)-scaling at larger angles. An appropriate parameter was found to b e the ratio of the ionic charge and the nuclear charge q/Z(P). The observat ion of forward peaked, enhanced electron emission is of great importance fo r the understanding of radiation damages in dense targets caused by heavy i on collisions. (C) 1998 Elsevier Science B.V. All rights reserved.