FLIGHT MASS-SPECTROMETER CALIBRATION IN A HIGH-VELOCITY ATOMIC-OXYGENBEAM

Calibration and characterization of the quadrupole mass-spectrometer c omponent of the Evaluation of Oxygen Interactions with Materials III s pace-flight experiment are reported in this paper. A high-velocity ato m 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 respo nse factor based on oxygen atom flux in the high-velocity beam was fou nd to be logarithmically dependent on the exposure history of the inst rument, i.e., the calibration factor was logarithmically dependent on atomic oxygen fluence. This dependence was independent of the backgrou nd 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 when ever high-velocity oxygen atoms were present. The intensity of reactio n products caused by interaction of atomic oxygen with contaminated su rfaces within the instrument decreases with increasing atomic oxygen f luence, whereas O-2 resulting from recombination of atomic oxygen on s urfaces increases with fluence.