A method and apparatus for increasing the operation lifetime of a beam tube are provided. The method includes converting an electrical current output by the beam tube into an intermediate voltage to provide a voltage signal used for changing an electron multiplier polarization voltage applied to an electron multiplier by a power supply unit controlled by an electron multiplier gain control signal generated by a controller. The controller regulates the electrical current output by the beam tube to keep it below a characteristic current threshold by adjusting the electron multiplier gain control signal until it reaches a predefined maximum value. The voltage signal applied to the controller is regulated by increasing the variable gain of a voltage amplifier to compensate for a drop in electrical current output by the beam tube caused by the aging of the electron multiplier until the applied voltage signal reaches a predefined minimum value.