Performance of a Mass Spectrometric Modulated Beam Detector for Gas-Surface Interaction Measurements

摘要

The report describes the performance of an instrument designed to detect molecular flow rates in the presence of the relatively high background gas noise of a space environment chamber. The technique of molecular beam modulation employed consists of three primary elements: (1) the mechanical beam modulator, (2) a quadrupole mass spectrometer, and (3) a lock-in amplifier. Molecular beam intensities of about ten billion molecules/sec-sq.cm are distinguishable from noise at a background pressure of 0.0000002 torr, which corresponds to about one hundred trillion molecules/sec-sq.cm. The detector performance for molecular beams of He, A, N2, and CO2 is reported. This high sensitivity makes possible the use of molecular beam techniques for direct measurements of gas condensation rates. (Author)