A high precision calibration system for the simulation of cometary gas environments

For the calibration of comet mass spectrometers a novel calibration system was designed to simulate the neutral gaseous atmosphere of comets. The facility consists of a fully automated three-chamber ultra-high vacuum system with a separate gas mixing unit. The molecular beam technique is used for the simulation of the dynamic environment of comets. The gas mixing unit allows one to produce molecular beams and static environments of pure water vapour, of mixtures of water vapour and minor gas constituents and also of a wide range of other gases which are expected in the environment of comets. The uniqueness of this system lies in its emphasis on accuracy and long term stability both in the static and in the molecular beam mode. Of particular interest is the determination of the absolute pressure for all gases in the range below 10 -6 mbar and the determination of the molecular beam characteristics. Although it is optimized for cometary environments, the calibration system is, by virtue of its versatility, well suited for the calibration of various space instruments over an extended parameter range.