PERFORMANCE OF A DIRECTIONAL DETECTOR OF MOLECULAR DENSITY

A directional detector of molecular density was developed and applied to the measurements of angular distributions of desorbed molecules from Mo surfaces and outgassing rates from small test surfaces. The detector is composed of a mass spectrometer installed in a detector chamber and a collimator set on the entrance of the detector chamber. The collimator has two apertures and a pumping surface between them. The performance of the collimator is analyzed and compared with experiment data. The ratio of the incident flux intensities of molecules to the detector coming directly from a test surface to the one of chaotic background molecules, collimation efficiency, depends on the pumping speed of the collimator and the bell jar. When the pumping speed of the collimator is increased by cooling the inner wall and/or deposition of evaporated Ti on it, the collimation efficiency can be increased by the factor of 10**2.