Abstract
The authors investigate the fivefold differential double-ionization cross section of helium by the impact of fast electrons. They consider the case where little momentum is transferred to the target so that a Born-type approximation in the projectile-target potential can be employed. Collision geometries are provided at which maxima and minima arise in the cross section and the associated ionization mechanisms are identified. Necessary kinematical conditions for the Bethe ridge in double ionization are worked out which provide a possibility for the measurement of the momentum profile of the initially bound state.