FLIGHT MASS-SPECTROMETER CALIBRATION IN A HIGH-VELOCITY ATOMIC-OXYGEN BEAM

Calibration and characterization of the quadrupole mass-spectrometer component of the Evaluation of Oxygen Interactions with Materials III space-flight experiment are reported in this paper. A high-velocity atom beam system was used to characterize the response of the flight mass spectrometer to high-velocity oxygen atoms (0.8 to 2.5 eV). The response factor based on oxygen atom flux in the high-velocity beam was found to be logarithmically dependent on the exposure history of the instrument, i.e., the calibration factor was logarithmically dependent on atomic oxygen fluence. This dependence was independent of the background pressure over the range between 10(-6) and 10(-4) Torr. Subsequent contamination of the instrument restored the instrument sensitivity to the original value before exposure to atomic oxygen. Carbon dioxide, carbon monoxide, and water were observed in the mass spectrometer whenever high-velocity oxygen atoms were present. The intensity of reaction products caused by interaction of atomic oxygen with contaminated surfaces within the instrument decreases with increasing atomic oxygen fluence, whereas O-2 resulting from recombination of atomic oxygen on surfaces increases with fluence.