4.1.1 Sector field mass spectrometers
Because of their simple, robust design, sector field mass spectrometers are used for helium leak detectors, where only little demands are placed on resolution.
The operating principle of sector field mass spectrometers is shown in Figure 4.3.
Figure 4.3:
Operating principle of a sector field mass spectrometer
Neutral gas molecules are ionized in an ion source through electron bombardment. The electrons thus generated are accelerated into the magnetic sector field with the aid of an electrical voltage. The magnetic field is homogeneous in the area of the trajectories of the ions and is positioned perpendicular to the image plane. Helium ions having a mass of 4 amu are able to pass through a slot to reach the detector. All other molecules are unable to pass through the slot and are re-neutralized. The ion current measured for helium is proportional to the helium partial pressure.
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