Description Of Major Components - Veeco MS40 Operation And Maintenance Manual

Portable automatic leak detector

Table of Contents

3.4

VACUUM SYSTEM

Mass Spectrometer: The mass spectrometer is the component of the

vacuum system that detects the helium tracer gas present in the test object.

The unit ionizes the gas molecules, separates the helium ions from other

gas ions, and converts the helium ions into an electrical current that

represents the size of the leak. The spectrometer consists of three parts:

Ion Source Assembly: To permit neutral atoms and molecules of gas

to be separated by a magnetic mass spectrometer, they must first be

ionized in the Ion Source. Inside of the ion source, there is an

electrically heated filament that provides a source of electrons. As

these electrons collide with gas molecules, they split off electrons from

the gas molecules and create positively charged gas ions. These

ions then encounter the electrostatic fields, established within the ion

source by the ion repeller, the ion chamber, the focusing plates and the

object plate. The end-result of this arrangement is to accelerate the gas

ions into a well defined, mono-energetic beam.

Mass Spectrometer Magnets: Since the gas entering the

spectrometer contains atoms and molecules of different kinds, the ion

beam will likewise contain a variety of ions including nitrogen, oxygen,

carbon dioxide, and, if a leak has been found, helium. Magnets are

mounted on the outer portion of the spectrometer, outside of the vacuum

system, and are used to create magnetic fields that are perpendicular to the

beam of gas ions. A magnetic field causes the ions to deflect off their

current trajectories, the amount of deflection depends upon the mass of the

gas ion. Lighter ions such as hydrogen are deflected to a greater extent

than heavier ions such as oxygen.

Two identical magnetic fields are used in the MS-40's mass spectrometer.

The first magnetic field is organized so that only helium has the right

mass to pass through the field and through the narrow opening in the

baffle on the far side of the magnetic field. Most of the other ions will be

deflected in a different direction (they will be either too heavy or too

light) and will be intercepted by the baffle. A few of the non-helium ions

Vacuum Instrument Corp.

will be able to pass through the baffle (due to random scattering and

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