Figure 3-7. Production of x-rays by electrons striking the glass wall of the tube.
c. Gas tubes were used until 1913 when W. D. Coolidge introduced the "hot
cathode" tube. With the Coolidge tube, electrons are obtained by heating a wire
filament to incandescence by a low voltage current. At incandescent temperatures, the
filament emits electrons by a process known as thermionic emission. Gas is
undesirable; consequently, the best possible vacuum is used. In the hot cathode tube,
the number of electrons (and consequently the quantity of x-rays) is easily regulated by
adjusting the temperature of the filament. In addition, electron speed (and consequently
the quality of x-rays) is easily regulated by adjusting the strength of the electric field
between the cathode and anode.
d. Other major advances in the development of x-ray tubes are the rotating
anode and the grid-controlled tube. Both are described later in this lesson.
3-10. TARGET MATERIAL AND ANODE CONSTRUCTION
a. Since early in the development of x-ray tubes, tungsten has been used as the
material for target construction. One reason for this is its high atomic number (74). As
the atomic number of the target increases, so does the energy of its characteristic
radiation. As described previously, the energy of characteristic radiation produced in
tungsten reaches 69.5 keV, a value that is considered useful in conventional
radiography. A target with a higher atomic number would produce more energetic
characteristic radiation and, therefore, would seem better suited for x-ray production.
However, target material must meet other requirements in addition to the high atomic