X-RAY GENERATORS

X-RAY GENERATORS

Definition. X-ray generators are man-made electronic devices designed to produce X radiation. Many types of X-ray generators may be obtained commercially. X-ray equipment may be either portable or stationary. Portable X-ray generators are used for inspection of test objects that are either impossible or very difficult to transport. Stationary X-ray generators are used in shielded facilities where the objects to be tested can be readily transported to the X-ray equipment.

Basic Requirements for Production of X-Rays. X-rays are produced when some form of matter is struck by a rapidly moving electron. To accomplish this, three basic requirements must be met.

1. Supply Of Electrons

There must be a supply of the electrons. Fortunately, they can be supplied by simply raising the temperature of a suitable material. An electron source is readily obtainable in as much as all matter is generally considered to be composed of electrons and other minute particles. All that is necessary is to sufficiently heat the proper material. As the temperature rises, the electrons become more and more agitated until finally they escape or “boil off” the material, surrounding it in the form of an electron cloud (see Figure 6-5). This is known as thermionic emission. In an X-ray tube the heated material is known as the filament, which is similar to the filament in a light bulb. Just as in a light bulb the filament is heated by passing electrical current through it.

This cloud of electrons simply hovers around and returns to the emitting substance unless some external action or force pulls it away.

2 Movement of the Electrons.

Movement of the emitted electrons is the second step in producing X-rays. This movement is brought about by the repelling and attracting forces inherent in electrical charges. The fundamental law of electrostatics states that like charges repel each other and unlike charges attract each other. Electrons are negative charges, thus repel each other. However, a stronger attracting force is needed to accelerate the electrons to a higher velocity. Therefore, a strong opposite (positive) charge is used to move the electrons from one point to another. It is important that this movement is conducted in a good vacuum, otherwise the electrons collide with air molecules and lose energy through ionization and scattering. In an X-ray tube the anode is given a positive charge with respect to the filament, which is part of the cathode.

3. Impingement of Electrons on a Target.

3.1 Continuous X-Ray Spectrum. Merely generating electrons in a vacuum and setting them in motion is not sufficient to create X-rays. It is necessary also that the electrons strike some target substance. In an X-ray tube the target is the anode. When the electrons bombard the target, they are brought to an abrupt halt. Unfortunately most of the electrons’ kinetic energy is converted into heat which must be dissipated by the target material. Only a small percentage of the energy available in the electron beam is converted into X-ray photons which can have energies ranging from zero to a maximum which is determined by 1) the original kinetic energy of the electrons and 2) by how rapidly the electrons are decelerated. This process produces the continuous portion of the X-ray spectrum and is known either by the German term Bremsstrahlung, meaning braking radiation, or by the term white radiation. X-rays are produced regardless of the material bombarded, whether it is a solid, liquid or gas. In the X-ray tube a solid material is used for the target. The higher the atomic number of the target material the higher the efficiency of X-ray production.

3.2 Characteristic X-Ray Spectrum. In addition to the white radiation, there are several characteristic peaks in a typical X-ray spectrum. These intensity spikes are caused by interaction between the impinging stream of high-speed electrons and the electrons that are bound tightly to the atomic nuclei of the target material. If the atom is considered as a planetary system with the nucleus of protons and neutrons at the center of the system and the electrons moving in orbits around the nucleus, modern physics predicts that the orbital electrons near the nucleus will have very well-defined energies, with electrons in different orbits having different energy levels. If an electron from an external beam collides with one of these orbital electrons with sufficient energy to knock it out of its orbit, an electron from a higher energy level would, after a time, drop down to fill the void and restore atomic stability. When that electron drops to the lower energy level, it gives off a photon with energy equal to the difference in energy levels. Since these energy levels depend strictly upon the particular atom, the radiation emitted is called characteristic radiation. The characteristic radiation emitted by the target material is superimposed upon the continuous spectrum. A typical X-ray spectrum of radiation generated by an X-ray tube would appear as shown in Figure 6-6. The K and L series of characteristic radiation designates the radiation emitted from different electron orbits around the nucleus of the atom. As energy levels increase, electrons are dislodged from the various orbits with the K series being the closest to the nucleus.

Electron Cloud

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