Ions produced in the cold-cathode discharge of an inverted magnetron and passing through an aperture in the cathode into an ion collector are separated by the magnetic field according to their mass. Under appropriate conditions of applied potential, magnetic field, electrode geometry, and gas density, a mass spectrometer of this type exhibits high sensitivity and resolution for gases of low atomic mass. It is, therefore, exceptionally well suited for helium leak detection. Simultaneously, it serves as an ionization gauge indicating total pressure. Compared with conventional leak detector mass spectrometers, it has the advantage of operating without a hot filament; it is inexpensive, small, rugged, and bakeable; and the associated electronic circuitry is equally simple. In order to enhance its sensitivity, it can be combined with a selective zeolite pump which does not pump helium but provides a low-background pressure of high stability. A simple leak detection system of this type is described with optimum...