DIRECT-MOLECULAR-BEAM METHOD FOR MASS SELECTIVE OUTGASSING RATE MEASUREMENT

Outgassing rates ranging from 10** minus **1 to 10** minus **6 Pam**3s** minus **1m** minus **2 from a small area of a solid sample can be measured by a quadrupole mass spectrometer completely isolated by a hollow cylindrical cryocollimator and a coaxial liquid nitrogen shroud from a surrounding ultrahigh vacuum chamber, except for a small solid-angle aperture where the molecular beam from the sample passes through it. The 2. 4 K cryocollimator and 80 K shutter (chopper for the molecular beam) assure a clear beam shape and a well-defined signal level from the background, respectively. Outgassing rates of stainless steel 304L and polycrystalline molybdenum having a sampling area of 5 cm**2 are presented for initial pumping and during baking at 500 degree C.