An electron multiplier is positioned relative to at least one dynode to direct a beam of secondary particles from the at least one dynode to a collector area of the electron multiplier and not to a channel area of the electron multiplier for a range of electron multiplier voltages applied by one or more voltage sources to the electron multiplier and for a dynode voltage applied by the one or more voltage sources to the at least one dynode. The electron multiplier includes an aperture with an entrance cone and walls of the entrance cone comprise the collector area and an apex of the entrance cone comprises the channel area. An electron multiplier voltage of the range of electron multiplier voltages is applied to the electron multiplier and the dynode voltage is applied to the at least one dynode using the one or more voltage sources.