What is Secondary Emission?
The phenomena of emission of electrons from the surface of solid metallic material by the action of a stream of charged particles are the secondary emission or secondary electron emission. Caesium oxide is an excellent target material that holds the ability to release ten times more current of secondary emission than the primary current of an applied beam of charged particles.
Basically, there are four major types of electron emission, which are as follows:
- Thermionic Emission
- Photoelectric Emission
- Secondary Emission
- Field Emission
In our previous content, we had a discussion on thermionic emission is the emission of charged particles from the surface of the metal when heat is applied to it.
In photoelectric emission, the release of electrons from the surface of metal takes place by the action of light. As against, field emission is a phenomenon where the charged particles move when placed in an electrostatic field. In field emission, the electrons are attracted towards the positive electrode because of the existing electric field.
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How secondary emission takes place?
We know that electrons in an atom orbit the nucleus consisting of neutrons and protons. The nucleus bonds the electrons by a force of attraction.
So, when a beam of charged particles is focused towards the metal surface, then they (electrons or ions) transfer their kinetic energy to the electrons that are bonded together. Thus, the electrons get free. These electrons that get freed by the action of primary electrons are known as secondary electrons.
Once the electrons are freed by the action of the applied electron beam, then this causes the generation of current. This current is what we call the secondary emission current.
Now let us understand this phenomenon in a detailed manner.
Basically, every metallic surface has some specific potential barrier, due to which, in normal conditions, no free electrons are able to escape from the surface of the metal. This potential barrier prevents escaping of electrons from the surface of the metal at room temperature.
The reason for the existence of this surface barrier is the electrostatic force of attraction existing between the nucleus and free electrons. But when a beam of electrons strikes the surface of the metal, then the incoming electron beam transfers its kinetic energy to the electrons present on the surface of the metal. Further, by gaining sufficient kinetic energy, the electrons overcome the barrier potential. Hence, they get emitted from the surface of the metal.
Factors affecting Secondary Electron Emission
The two major factors that influence the emission of secondary electrons from the surface of metal are:
- The material matter which is prone to secondary emission and
- The momentum possessed by the bombarding beam strikes the surface.
- The number of primary electrons that bombards the surface. As when there is a large number of free electrons, then the emission of more secondary electrons takes place.
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Secondary Emissive Materials
The commonly used in secondary emissive materials are as follows:
- Beryllium oxide (BeO)
- Alkali antimonide
- Magnesium oxide (MgO)
- Lead Oxide (PbO)
- Gallium Phosphide (GaP)
- Gallium Arsenide Phosphide (GaAsP)
This phenomenon is mainly used in photomultiplier tubes where a weak electric current is required to be detected, like in radiation detectors. Also, it is used in image intensifiers that are used to generate a brighter version of a fainted object as output.
Further, it is regarded as an unwanted side effect where electrons from the cathode strike the anode, thereby causing parasitic oscillation.