When a beam of high-speed electrons or other similar particles strikes on a metal surface, the part of kinetic energy of the moving particles gets transferred to the free electrons on the surface of the metal body. The phenomenon causes electron emission from the surface of the metal body. The Process of electron emission with the help of bombardment of the high-speed electron or other similar particles on a metal surface is known as Secondary Emission.
Actually, there is a potential barrier on the surface of any metal body due to which no free electron can escape from the metal surface in normal condition. The potential barrier created at the surface of the metal body to prevent escaping of any freely moving electron inside the metal body is longer as the surface barrier. Although there are planning of free electrons available inside a metal and body even at room temperature. These free electron move freely inside the metallic system. They move from atom to atom in a random manner but none of them comes out from the metal body at normal condition because of the surface potential barrier. Actually, the electrons well inside the metallic crystal get attracted from all sides due to the presence of positive nuclei surround it. But when a free electron comes at the extreme edge of the metal, there would not be any positive nucleus ahead of it to pull it outwards but at that instant, there would be a plenty of positive nuclei behind it to pull the free electron inward in the metallic crystal. This backward or inward attraction electrostatic force on the electrons on the surface of the metal creates a potential barrier for that free electrons.
But when a free electron at the surface of the metal body gets sufficient kinetic energy, it may overcome the potential barrier (surface barrier ) and gets emitted in front of the body or emitter.
As we have already explained, the emission of electrons can be created by bombardment of high-speed electrons or similar particles on the surface of the metal body. In that case, the kinetic energy, of the bombarding electrons or similar particles, partly or entirely get transferred to the free electrons on the surface of the metal body. As a result, the free electrons get sufficient kinetic energy to overcome the surface barrier of the metal and the free electrons may be emitted from the metal. The entire process as we already told is known as Secondary Emission. This is secondary emission because the electrons here get emitted due to the bombardment of the other electrons on them. The electrons which strike on the metal surface are known as primary electrons and the which get emitted from the metal surface are known as Secondary Electrons.
The intensity of secondary emission depends upon mainly two factors,
- The material by which the metallic emitter is made of.
- The momentum of the high-speed particles strick on the emitter surface.
Here we have tried to draw a conceptual pictorial view of secondary emission. Here S is the source of primary electrons, E is the emitter or cathode, A is the collector electrode or anode. A and E are connected across a battery (B) to make the A of positive potential with respect to E. In the external circuit between A and E, we connect one sensitive Galvanometer (G) to measure the emission current. Here when the primary electrons from source S strike on the surface of cathode E, secondary electrons get emitted and attracted towards positive collector electrode or anode A.
In the field of vacuum tube technology, the secondary emission is an undesirable natural phenomenon during operation of many vacuum tube devices. For example, tetrode values get negative resistance for that secondary emission phenomenon. In some cases, we use the secondary emission process intentionally, such as in electron multipliers, Cathode Ray Tubes, etc.