Abstract: An ion detector for use with a mass spectrometer or other instrument and a high-voltage power supply are provided. The detector comprises two dc power sources connected in series at a junction grounded. Each dc power source delivers an output voltage which can be switched between 0 V and a given voltage. The junction between the resistors, or voltage-dividing terminal, is connected with a conversion dynode. The polarity of an ion-accelerating voltage applied to the conversion dynode is switched, depending on whether detected ions are positive or negative. Ions are accelerated and caused to strike the conversion dynode, thus releasing secondary electrons. The secondary electrons are accelerated and detected by a scintillator.