Chapter 21 Production of X-rays

21.1 Aim

The aim of this chapter is to consider the mechanisms by which X-rays are produced at the target of the X-ray tube. The concept of the X-ray spectrum will be introduced and this chapter will act as a foundation for Chapter 22 where the factors affecting the X-ray spectrum will be considered.

21.2 Interactions of electrons with matter

In Chapter 30 we will discuss the construction of the X-ray tube and the functions of its various components. In this chapter we will look in detail at the processes involved at subatomic level in the target of the X-ray tube to allow the production of X-rays.

Electrons are produced at the filament of the X-ray tube, accelerated towards the target of the tube and then made to collide with the target atoms. There are a number of ways in which high-energy electrons from the filament of the X-ray tube may lose their energy when they collide with the atoms of the target. These are shown in Figure 21.1:

• Electrons from the filament lose energy because of interactions between them and the outer electrons surrounding the atoms of the target material (see electron 1 in Fig. 21.1).

• Electrons from the filament lose energy because of interactions between them and the nuclei of the atoms of the target material (see electrons 2 and 3 in Fig. 21.1).

• Electrons from the filament lose energy because of interactions between them and individual inner electrons of the target atoms (see electron 4 in Fig. 21.1).

Figure 21.1 Diagrammatic representation of possible interactions between electrons from the filament and target atoms in an X-ray tube. An explanation of the interactions is given in the text.

The interactions between electrons and target atoms may be elastic where there is conservation of kinetic energy or inelastic collisions in which kinetic energy is lost (see Section 3.3). It is also important to realise that an electron from the filament may experience many interactions (typically about 1000) before it is brought to rest within 0.25–0.5 mm of the target surface.

21.3 Interactions between electrons from the filament and the outer electrons of the target atoms in the x-ray tube

As you will see in Chapter 30, the main material of the X-ray tube target is tungsten, which has an atomic number of 74. Thus each target atom is surrounded by 74 electrons in various orbitals. These electrons have the effect of deflecting the approaching electron from the filament from their original path because of the electrostatic repulsion between them and the filament electron. Such deflections from the electron’s original path are small and so the loss of kinetic energy is small. The kinetic energy lost by the electron is emitted as a photon of electromagnetic radiation. The energy of this photon is such that it falls into the infrared part of the spectrum and so heat is produced in the target material. If we consider the statistical probability of this process occurring rather than the other two processes identified in Section 21.2, we can see that it has a very high probability of occurring.

This is because the other two processes involve an interaction between either the incoming electron and the nucleus or the incoming electron and the inner orbital electrons. The area occupied by the nucleus and the inner electron orbitals is very small compared to the size of the whole tungsten atom and so an incoming electron has a much greater chance of interacting with the whole atom. For the above reasons, about 95–99% of the energy produced at the target of the X-ray tube is in the form of heat.

21.4 Interactions between electrons from the filament and the nuclei of the target atoms in the x-ray tube

As mentioned at the beginning of this chapter, the interactions of electrons from the filament with the nuclei of the target atoms can be elastic interactions or inelastic interactions. As each of these results in a different outcome, they will now be considered separately.

21.4.1 Elastic collisions with the nuclei

The electron has a relatively low mass and a negative charge while the tungsten nucleus has a larger mass (about 334 000 times that of the electron) and a positive charge (74 times that of the electron). Thus the electron is attracted to the more massive tungsten nucleus.

As shown in Figure 21.2, the closer the electron travels to the nucleus, the more it is deflected from its original path by the attraction of the positive nucleus. Because the mass of the electron is so tiny compared to that of the nucleus, the amount of energy it transfers to it is proportionately small.

Figure 21.2 The deflection of an electron from the filament when it interacts with the nucleus of a target atom in the X-ray tube: (A) a small deflection; (B) a large deflection.

These events therefore serve only to produce very tortuous paths for the electrons within the tungsten, without transferring much energy to the tungsten. The number of such events compared to the number of inelastic collisions discussed in the next section increases with the atomic number of the target material and so they are very frequent in tungsten.

21.4.2 Inelastic collisions with the nuclei – production of Bremsstrahlung radiation

Classical physics concluded that when a charged particle changed its velocity, this always resulted in the emission of electromagnetic radiation. However, we have already seen that this is not the case, as the electrons in the Bohr model of the atom are continuously changing their velocity (see Sect. 18.4). We have also seen in the previous section that electrons can undergo elastic collisions with the nucleus which result in no electromagnetic radiation being produced. As shown in Figure 21.2, the electron must accelerate towards the nucleus during its deflection but, despite this acceleration, a quantum of electromagnetic radiation is emitted in only a small percentage of cases. When such a quantum is emitted, the kinetic energy of the electron is suddenly reduced by an amount equal to the energy of the quantum, and so the electron is suddenly slowed down or braked.

This is an example of an inelastic interaction since the total kinetic energy of the electron and of the nucleus is not conserved because some energy is removed by the emission of the quantum of radiation. The energy of the quantum may be in the X-ray part of the electromagnetic spectrum and the radiation is known as braking or Bremsstrahlung radiation. The exact energy of this quantum will vary and may be very small if the electron does not lose much energy in its interaction with the nucleus or it may be up to the total kinetic energy of the electron if it is involved in a direct collision with the nucleus (see electron 2 in Fig. 21.1).

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