Angular distributions of very low energy recoil ions

We present the first measurements of the angular distribution of recoil ions near 90 ° with respect to the incident projectile direction. Beams of 22.5 and 33 MeV chlorine ions (incident charge states q = 4,5,8) have been used as "hammer" beams incident on Ne atoms. We do not confirm the long standing assumption that these recoil ions are ejected preferentially at angles very near 90 ° with respect to the primary beam direction, although they do have energies typically less than 5 eV. Recoil ions ejected around 90 ° have an energy distribution appreciably wider than those ejected at either larger or smaller angles.