Intra-atomic double scattering of Binary Encounter electrons in heavy ion collisions with gas targets U. Bechthold a, * ,1 , J. Ullrich a,1 , U. Ramm a,1 , G. Kraft a,1 , S. Hagmann b,2 , C.O. Reinhold c, 3 , D.R. Schultz c,3 , H. Schmidt-B ocking d a Gesellschaft f  ur Schwerionenforschung mbH, Planckstrasse 1, D-64291 Darmstadt, Germany b Kansas State University, Manhattan, KS 66506, USA c Oak Ridge National Laboratory, Oak Ridge, TN 37839, USA d Institut f  ur Kernphysik, August-Euler-Str. 6, 60486 Frankfurt, Germany Abstract Experimental evidence for intra-atomic double scattering of fast Binary Encounter (BE) electrons produced in collisions of 5.9 MeV/u U 29 with various gas targets is presented. A broad distribution of electrons displaying the shape of the (BE) peak at twice of the velocity of the ion beam at angles up to 135° is observed in the double dierential electron emission spectra. This emission is attributed to target electrons ejected in a knock-on collision that are sub- sequently elastically scattered at their own target nuclear potential. Ó 1998 Elsevier Science B.V. All rights reserved. 1. Introduction In swift heavy-ion collisions with solid or gas- eous targets, about two-thirds of the energy loss of the ion is transferred into kinetic energy of the d- electrons [1]. It is well established, that the d- electrons are responsible for radiation eects in biological or other materials in regions far from the primary ionization events [2]. Knowledge about the electron production processes and cross sections are therefore an indispensable ingredient i.e. for the calculation of relative biological e- ciencies on the basis of track structure models [3] used for the treatment planning in hadron cancer therapy at GSI and elsewhere. Apart from the application of the data, it has always been bene- ®cial to compare theoretically predicted and mea- sured double dierential cross sections in order to gain more insight into the ionisation mechanism and to develop new models of ionisation. In this paper we present evidence for an emis- sion mechanism for fast electrons that has not been observed in that clarity before. Electrons of 2v P (where v P is the projectile velocity) at large angles have so far been only observed in heavy-ion solid collisions [4,5] as obvious scattering o other Nuclear Instruments and Methods in Physics Research B 146 (1998) 46±51 * Corresponding author. Fax: +49 6159 71 2106; e-mail: u.bechthold@gsi.de 1 Support by DFG, BMBF, GSI and EC. 2 Support by Division of Chemical Sciences, Oce of Basic Energy Research, Oce of Energy Research and US Depart- ment of Energy. 3 Support by US DOE Oce of Basic Energy Sciences through a grant to Lockheed Martin Energy Research Corpo- ration. 0168-583X/98/$ ± see front matter Ó 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 4 4 0 - 6