Abstract: A device for directing a primary particle beam at a sample and detecting particles in accordance with the emergence of particles from the sample. The device comprises a source for generating the primary beam and a passage leading from the source to the sample. It has one end towards the source and a second end towards the sample and, alongside the passage, is placed a particle detector having a face for receiving particles to be detected. At the first end of the passage is placed a cover suitable for generating secondary electrodes, which has a central aperture for admitting the primary beam. The particle detector is orientated with the face for receiving particles towards the cover.