Abstract: An electron multiplier having an access for allowing a beam to pass is presented. The electron multiplier collects particles traveling back along the beam and is capable of collecting the particles arbitrarily close to the beam. The electron multiplier includes at least two plates having secondary electron emitting surfaces, the at least two plates being separated by a small distance. The electron multiplier has a beam access through the at least two plates. Particles enter the electron multiplier in a direction opposite that of propagation of the beam and impact a secondary electron emitting surface, thereby being captured between the top plate and the bottom plate. In some embodiments of the invention, the electron multiplier is segmented so that azimuthal distributions of the particles can be determined. In some embodiments, the electron multiplier includes a stack of electron emitting surfaces arranged so that an angular distribution of the particles can be determined.