Second principle mechanism of ionization

The second type of ionization is known as the Compton effect (sometimes referred to as Compton Scatter). In this form of interaction, the initial photon energy is higher than that of the Photoelectric effect. The primary difference is that not all of the photon energy will be utilized in liberating and accelerating an electron. There is also energy left over to cause further ionization.

The Compton effect may occur when photon energies range from approximately 50 Kev to 3 Mev. Notice that Compton effect overlaps that of the Photoelectric effect. At relatively low energies, the Photoelectric effect is the dominant form of interaction, and it becomes less predominant as energy levels increase. It has been determined that the Compton effect starts slowly and becomes more dominant at energies above 100-150 Kev.

In the Compton effect process of ionization, not all of the photon energy is absorbed during the liberation of the electron. This excess energy takes on the form of a new photon having longer wavelength (less energy) than that of the original photon. In addition, the new photon moves through the material in a new path. This is where the term scatter derives from.

So what happens with this new photon?

The new photon will continue to interact with the material and its energy may be absorbed in the same manner as the original photon. The photon may continue to go through several Compton effect actions depending on its original energy, and eventually it will go through the Photoelectric effect as the energy diminishes.

It should be noted, that the change in the direction of the new photon due to Compton effect is dependent on the energy of the photon. The higher the energy of the photon, the smaller the change in direction resulting from ionization. Keep in mind that after Compton effect, the path of the resulting photon is never the same as the original. Relatively low energy photons may result in a direction that is completely opposite the original direction.


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