Abstract: A signal detector for detecting electrically charged particles in a gaseous medium has an electrode having a tip region for gathering charged particles. The electrode is connected to a voltage source for applying a voltage that generates electro-static field for attracting the particles to the electrode, and at least part of the tip region has a radius of curvature which is sufficiently small or (in the case of a point tip or sharp edge) infinitesimally small so as to create a localised high intensity electro-static field which defines a detection zone in which, in use, charge particles are accelerated by the electric field so as to ionise gas molecules in the zone and thus amplify the signal to be received by the detector. The received signal is supplied through an output of the detector. The detector may be provided in a sample chamber of a scanning electron microscope.

Abstract: A scanning electron microscope has means for generating a beam of electrons which is scanned over a specimen held within a holder in a chamber which contains a gaseous medium. A negative potential is applied to the holder so as to generate an electric field which accelerates secondary electrons, formed by the interaction or the primary beam with the specimen, in a direction away from the specimen surface and into a collision zone in the chamber. In that zone, the accelerated secondary electrons collide with gas molecules of the gaseous medium, thereby initiating a cascade of collisions which, in effect, amplifies the secondary electron signal. That signal (which may take the form of photons generated as a result of the collisions) is detected by detecting means, such as a photo-multiplier.