An apparatus for sterilization of an inner wall of containers has a charge carrier source for generating charge carriers, with an acceleration device by which charge carriers are capable of being accelerated in the direction of a charge carrier emission window. The charge carrier emission window is arranged on a treatment device for introduction through an opening into the container along an insertion direction, in order to act upon an inner wall of the container with the charge carriers issuing from the charge carrier emission window. A medium is flowed into the region of the charge carriers issuing out of the charge carrier emission window for changing the dimension of a charge carrier cloud formed by the discharged charge carriers.