Strong projectile-dependent forward-backward asymmetry of electron ejection by swift heavy ions in solids

We studied forward and backward electron ejection from carbon foils penetrated by swift ions (energy range, 20 keV/nucleon-74 MeV/nucleon; projectiles, H-U) over four orders of magnitude of electronic energy loss (0.01-25 keV/nm). Ejection of fast electrons from primary ionization and subsequent energy dissipation by secondary cascades are found to be asymmetric. Electronic energy deposition by swift ions is thus different at the entrance surface, in the solid's bulk, and at the exit surface. The effect strongly increases with projectile atomic number due to an increasing contribution of close collisions and enhanced fast-electron ejection.