A mass selective electron‐impact ionization detector for use in energy resolved thermal He scattering experiments is presented. The detector combines a short effective ionization region (approximately 3 mm FWHM) with a high detection efficiency of 10−5at 2 mA electron emission. An efficiency of 10−4at 10 mA emission current can be achieved if time resolution below 20 &mgr;s is dispensable. The design is discussed and measurements of detection efficiency and time resolution are presented. The ionizer can likewise be employed for an efficient detection of other types of atoms or molecules.