Hi Steemians, today we will continue to give some information about electron beam generation.
"There are many technological areas for using electron beam for example, material fabrication, sterilization, etc. The electron beam is also used in the high energy and high energy fields (LHC), which are more exotic areas than the mentioned technological fields. Certain sources which are thermionic emission [1] and photoelectric emission [2], used for electron beam generation."
Thermionic emission is given in the previous work.
Today, the work will be about photoelectric emission.
------Photoelectric Emission------
A photon with a high energy from the work function removes an electron when it hits the material, which is called photoelectric emission [2]. By using the frequency of the photon, which causes emission, there is an electron extraction and formula is given by,
hv = eφwork + K.E.
h - Planck's constant, φwork - work function, v - frequency of the photon, K.E. - kinetic energy
Figure-1: Left - No electron extraction, Middle - electron extraction with low kinetic energy, Right- electron extraction with high kinetic energy.
The more energy the photon has, the higher the kinetic energy of the resulting electron. That is why a photon can emit an electron. The photon energy, which has more energy than the work function, can actually be seen as missing. In some methods, two-photon absorption [3] can also be achieved and this two-photon can emit an electron. Quantum efficiency [4] is the most important indicator for electron beam generation in photoelectric emission. Quantum efficiency is related to how much electron emission is from incoming photons. Therefore, quantum efficiency is a function of photon energy. Photons for metals are in the ultraviolet energy range. However, since the metals show a good reflectivity in the ultraviolet wavelength, the electron emission rate is low. This reduces the quantum efficiency.
Table-1: Quantum efficiency and the wavelength of photon which is used to exracting the electrons for some metal and semi-conductor metals.
It is possible to increase the quantum efficiency by using semiconductor metals instead of metals. Because the rate of penetration of photons in semiconducting materials is higher. The quantum efficiencies of metals and semi-conductor materials in Table-1 are compared. Cs2Te is the most widely used semi-conductor photocathode material in electron beam production.
------Comparison Between Thermionic Emission and Photoelectric Emission------
When you are working with the thermionic emission, you need to struggle with really high temperatures about 2500K. However, for photoelectric emission, you do not need to heat the cathodes. By using laser also you can heat the cathode and get some electrons but this is not the aim. For example, without enough photon energy, the cathode will be also heated but this is unwanted case. Because there is a problem with the high temperatures. With thermionic emission you cannot control the number of the electron which is extracted from the material. However, using the one photon extract one electron feature, the number of extracted electron can be controlled easily. Controlling extracted electron is really important to make a computational calculations how many electrons hitted in the collider and what the probability is.
Nowadays, for generation of electron beam, photoelectric emission is used. There are numbers of types of laser system. In our lab, we developed a novel fiber laser system which is ablation-cooled method and published in 2016 in Nature. In the next article, we will try to explain the fiber laser components then we will move on ablation-cooled fiber laser systems as well.
Have a curious day...