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A getter is a deposit of reactive material that is placed inside a vacuum system to complete and maintain the vacuum. When gas molecules strike the getter material, they combine with it chemically or by adsorption. Thus the getter removes small amounts of gas from the evacuated space. The getter is usually a coating applied to a surface within the evacuated chamber.
A vacuum is initially created by connecting a container to a vacuum pump. After achieving a sufficient vacuum, the container can be sealed, or the vacuum pump can be left running. Getters are especially important in sealed systems, such as vacuum tubes, including cathode-ray tubes (CRTs), vacuum insulating glass (or vacuum glass) and vacuum insulated panels, which must maintain a vacuum for a long time. This is because the inner surfaces of the container release adsorbed gases for a long time after the vacuum is established. The getter continually removes residues of a reactive gas, such as oxygen, as long as it is desorbed from a surface, or continuously penetrating in the system (tiny leaks or diffusion through a permeable material). Even in systems which are continually evacuated by a vacuum pump, getters are also used to remove residual gas, often to achieve a higher vacuum than the pump could achieve alone. Although it is often present in minute amounts and has no moving parts, a getter behaves in itself as a vacuum pump. It is an ultimate chemical sink for reactive gases.
Getters cannot react with inert gases, though some getters will adsorb them in a reversible way. Also, hydrogen is usually handled by adsorption rather than by reaction.
Types
To avoid being contaminated by the atmosphere, the getter must be introduced into the vacuum system in an inactive form during assembly, and activated after evacuation. This is usually done by heat. Different types of getter use different ways of doing this:
; Flashed getter: The getter material is held inactive in a reservoir during assembly and initial evacuation, and then heated and evaporated, usually by induction heating. The vaporized getter, usually a volatile metal, instantly reacts with any residual gas, and then condenses on the cool walls of the tube in a thin coating, the getter spot or getter mirror, which continues to absorb gas. This is the most common type, used in low-power vacuum tubes.
; Non-evaporable getter (NEG): The getter remains in solid form.