Ein Molekularstrahldetektor mit Elektronenstoßionisierung und Vierpol-Massenfilter

Abstract

A molecular beam detector is described which ionizes by electron bombardment. The ions are then separated in a quadrupole mass filter and detected by a multiplier. The special advantages of the mass filter allow a high overall transmission so that every 1000th molecule of the beam is measured as an ion. The ion current due to the residual gas is reduced by a factor of 10⁻⁶ for all masses >45.

These results were achieved without separately pumping or baking out the ionisation chamber. Since magnets are not used the detector is comparatively light and small in size.

The smallest detectable beam was found to have a current density of 1,8·10⁻⁶ molecules/sec mm² corresponding to 4·10³ molecules/cm³, at a vacuum pressure of 8·10⁻⁷ torr in the apparatus, and using a time constant of 1,25 sec.