Abstract: A leak detector for sensing a test gas leaking into a vacuum chamber in accordance with this invention comprises a vacuum chamber having an opening, and a momentum transfer pump connected to such opening and serving to compress the gases emanating from the vacuum chamber. An entrainment pump is connected to receive the gases exhausted from the momentum transfer pump, with the entrainment pump having means for entrapping the active exhaust gases coming from the vacuum chamber, but not the noble gases, which remain in a gaseous state and continually increase in pressure. A mass spectrometer is connected to receive the output from the entrainment pump, with the entrapment of the active exhaust gas by the entrainment pump reducing the total pressure at the mass spectrometer gas detector, thus making possible the detection and measurement by the mass detector of extremely small noble gas leaks.