X-ray Production

Key Terms

beam quality

beam quantity

bremsstrahlung (brems) interactions

characteristic cascade

characteristic interactions

continuous emission spectrum

discrete emission spectrum

half-value layer (HVL)

x-ray emission spectrum

Introduction

This chapter examines the anode target interactions at a micro level. To this point the focus has been on the use of electricity and electrical devices to manipulate the flow of electricity for the purpose of x-ray production. This chapter focuses on the interactions in the anode target that result in x-ray photons as well as the properties, characteristics, and factors that influence the nature of the x-ray beam. The radiographer’s actions at the control panel directly determine the nature and makeup of the x-ray beam. The nature and makeup of the beam (in conjunction with patient, image receptor, and processing characteristics) determines image quality. Several complex and overlapping processes result in the radiographic image. The x-ray machine is a tool in this process. An understanding of this tool is one factor that determines the skill of the radiographer.

Figure 6-1 illustrates the inside of the x-ray tube. Exposure factors have been selected; electricity has traveled to the anode, cathode, and filament; and electrons have been boiled off of the filament and are streaming across to the anode at tremendous speeds. The filament electrons penetrate the face of the target to a depth of approximately 0.5 mm, interacting with the tungsten target atoms in their path. These filament electrons interact with target atoms to produce x-rays in two ways: characteristic interactions and bremsstrahlung (brems) interactions. It should be noted that most of the interactions (approximately 99%) do not result in x-rays but produce only heat. Keep in mind also that there are thousands of interactions taking place inside the target at once, but for the sake of explanation we focus on a single event in each case. Refer to the first part of Chapter 2 for a refresher on the general structure of the atom.

FIGURE 6-1 **X-ray Tube.**
A general-purpose x-ray tube.

Critical Concept 6-1

Target Interactions

There are two interactions in the target that produce x-rays: brems and characteristic. X-rays are produced when filament electrons interact with target atom electrons or nuclei. Ionization of target atom electrons leads to release of characteristic x-ray photons, whereas interactions with target nuclei produce brems photons. During a single exposure thousands of such events take place. However, most of the interactions in the target only result in heat (approximately 99%).

Characteristic Interactions

Characteristic interactions involve the filament electron and an orbital electron of a target atom. In general, a filament electron enters a target atom, strikes an orbital electron, and if its energy is greater than the binding energy of the orbital electron it is removed from orbit. Recall from Chapter 2 that orbital shells fill from the shell nearest the nucleus outward and a vacancy in a shell makes the atom unstable. To correct this situation, outer-shell electrons drop to fill inner-shell vacancies. To do so the outer-shell electron must expend some of its potential energy. This energy is given off as a characteristic x-ray photon (Figure 6-2). Note that when the first orbital electron drops to fill the vacancy, it in turn leaves another. This second vacancy is also filled by an outer-shell electron that again must give up some of its energy, producing another characteristic photon. This process of outer-shell electrons filling inner-shell vacancies continues down the line, creating a cascading effect called a characteristic cascade. Each time an orbital electron moves to a lower orbit, a characteristic photon is produced. This process is not necessarily orderly. If a filament electron removes a K-shell electron from an atom, the most likely electron to fill the vacancy is an L shell because of proximity. But any outer-shell electron can fill the K-shell vacancy; it is just not as likely to do so. Notice that the removal of the orbital electron established the environment for x-ray production and it is the expending of energy during the cascade that produces characteristic x-rays.

FIGURE 6-2 **Characteristic Interaction.**
A characteristic interaction event. Note that as outer-shell electrons fill inner-shell vacancies, their excess energy is released as characteristic x-ray photons.

Critical Concept 6-2

Characteristic Interactions

Characteristic photons are created when orbital electrons of target atoms are removed from their shell and outer-shell electrons fill inner-shell vacancies. To fill the vacancy, the outer-shell electron gives up some of its potential energy as a characteristic photon.

Characteristic photons are so called because their energy is “characteristic” or dependent on the difference in binding energy between the shells involved. The electron shells of each element have specific binding energies. Table 6-1 presents binding energies that can be assumed for tungsten. A tungsten atom has 74 electrons orbiting its nucleus in six different shells. The filament electron may interact with any of them, but medical imaging generally focuses on K-shell (innermost shell) interactions because they are the highest energy and the most useful for imaging purposes. Recall from Chapter 2 that each orbital electron is held in orbit by a binding energy and the closer the orbit, the stronger the bond. K-shell electrons in tungsten have the strongest binding energy at 69.5 kiloelectron volt (keV). For a filament electron to remove this orbital electron, it must possess energy equal to or greater than 69.5 keV. For all practical purposes using a general purpose x-ray machine, if a radiographer selects a kVp less than 70 on the control panel, there will be no photons produced from K-shell interactions.

TABLE 6-1

Binding Energies for Tungsten

K shell	69.5 keV
L shell	12.1 keV
M shell	2.82 keV
N shell	0.6 keV
O shell	0.08 keV
P shell	0.008 keV

The characteristic photon is named for the shell being filled in each case. If an outer-shell electron is filling a K-shell, regardless of where that filling electron is coming from, the photon produced is called K characteristic. If an electron is filling an L–shell, the resulting photon is called L characteristic, and so on. To find the energy of a characteristic photon, the radiographer needs to know the target element (in this case, tungsten) and the shells involved. Using this information, the radiographer subtracts the binding energy of the farther shell from the closer shell.

Again, with characteristic interactions, to remove an orbital electron, the filament electron must have kinetic energy equal to or greater than the binding energy of the electron with which it interacts. If, for example, a filament electron has 50 keV of kinetic energy and strikes a tungsten K-shell electron (binding energy = 69.5 keV), it does not have the energy to remove it. The result of this type of interaction is heat production and, as noted earlier, this happens most of the time. The K-shell electron absorbs the kinetic energy from the filament electron and reemits it as heat energy. This also happens with any other orbital electron regardless of shell if the filament electron does not have sufficient energy to remove it. In such cases the filament electron, having lost all of its kinetic energy, then drifts away to fill a vacancy in another atom or become

Math Application 6-1

Characteristic Photons

To find the energy of a characteristic photon, one must know the shell-binding energies of the element and the shells involved. The filament electron must possess kinetic energy equal to or greater than the shell-binding energy to remove it from orbit. The photon energy is then equal to the difference in the binding energy of the shells involved.

Example:

A filament electron removes a K-shell electron and an L-shell electron fills the vacancy:

K-shell binding energy = 69.5 keV

L-shell binding energy = 12.1 keV

69.5 – 12.1 = 57.4 keV

The energy of the K-characteristic photon produced is 57.4 keV.

This scenario creates a vacancy in the L shell. An M-shell electron fills the L-shell vacancy:

L-shell binding energy = 12.1 keV

M-shell binding energy = 2.8 keV

The energy of the L-characteristic photon produced is 9.3 keV.

Alternatively, an N-shell might fill the original K-shell vacancy:

K-shell binding energy = 69.5 keV

N-shell binding energy = 0.6 keV

69.5 – 0.6 = 68.9 keV

The energy of the K-characteristic photon produced is 68.9 keV.

part of the current through the tube. If, however, that same filament electron had 100 keV of energy, it would easily remove the K-shell orbital electron and be deflected in a new direction. It would still have 30 keV of energy left and with it interact with another atom.