Abstract: An emitter produces a beam of electrons or ions accelerated at a relatively high accelerating voltage. The beam is sharply focused by a condenser lens at the final stage. An electrostatic field for retarding the beam is produced between the lens at the final stage and a target on which the beam impinges. The retarding field lowers the landing energy of the beam. An auxiliary electrode which is maintained at substantially the same potential as the target is disposed between the lens and the target. A secondary electron detector is mounted between the auxiliary electrode and the target.