increase the quality or hardness of the x-ray. Quality or hardness refers to the
penetrating ability of x-rays.
d. The target used in these tubes is usually tungsten or tungsten-molybdenum.
This material is used because of certain properties that are desirable for use in the x-
ray tube. For instance, with all the heat produced when the electrons strike the target, it
is desirable to have a material with a high melting point. Tungsten's melting point is
3,370 C. One other desirable property of tungsten is the fact that it produces a usable
characteristic x-ray, which will be discussed later.
e. X-rays are emitted in a broad energy spectrum, ranging from an amount of
energy equivalent to the maximum energy of the accelerated electron down to the
minimum energy x-ray, which can penetrate the window of the x-ray tube. This
spectrum is composed of two components--continuous and characteristic x-rays.
Figure 2-1. X-ray tube.
f. Continuous x-rays exhibit a range of energies because not all of the electrons
striking the target lose all their energy. This continuous emission is known as
bremsstrahlung, or braking radiation, from the German language. Electromagnetic
theory holds that a moving electric charge will radiate energy whenever it is
accelerated. The same is true in cases where a charged particle is decelerated.
g. Characteristic x-rays appear as sharp peaks if superimposed over the
continuous spectrum (Figure 2-2). The wavelengths of these x-ray emissions are
unique characteristics of the element used as the target material. In the x-ray tube, the
accelerated electron occasionally ejects one of the orbital electrons from a shell of one
of the atoms in the target. This loss of a negative charge gives the atom a net positive
charge and thus attracts an electron from an outer shell or a free electron to fill the