A new type of mass spectrometer leak detector is described which has a sensitivity for helium of 10−13standard cc/sec with a signal‐to‐noise ratio of unity. Operating pressure is in the 10−5to 10−6mm Hg range, time for ⅔ of final response is 2 sec or less, and operating characteristics are similar to present helium‐sensitive mass spectrometer leak detectors.