Exposure to excessive X or gamma radiation is harmful to human beings. While
most X-ray equipment is designed to minimize the danger of exposure to direct or
stray radiation, certain precautions must be observed. Radiation protection
requirements are discussed in Section 9.
The most distinguishing characteristic of X-rays is their short wavelength. The penetrating ability of X-rays is directly
proportional to their energy, which in turn, is inversely proportional to their wavelength; that is, the shorter the
wavelength, the higher the energy; the longer the wavelength, the lower the energy. Short wavelength X-rays are
commonly described as "hard" while long wavelength X-rays are referred to as "soft."
Properties of X-Rays and Gamma Rays.
There are several properties which X-rays and gamma rays possess which make them useful for radiographic
inspection. X-rays and gamma rays are the same form of energy as visible light; both are part of the electromagnetic
spectrum. Like light, both are refracted when they pass through glass, such as a lens, or any other medium. However,
the amount of refraction of X or gamma rays using visible-light optics is so slight as to be unnoticeable. X-rays can be
focused, but the techniques are so cumbersome that in normal NDI applications X-rays are not focused. Although the
properties of X and gamma rays and visible light are theoretically similar, the differences in application make it most
convenient to consider X and gamma rays as being different, since their observable effects are quite different from those
of light. This is noted particularly in the ability to penetrate matter. Some general properties of X and gamma rays
may be summarized as follows:
a. They are invisible to humans.
b. They propagate in straight lines in free space.
c. In special cases they are reflected, diffracted, refracted, and polarized as is light, but to a much smaller
d. They propagate at a velocity of 3x108 meters per second as does light.
e. They consist of transverse electromagnetic vibrations as does light.
f. X-rays have energies between roughly 1KeV and 50MeV.
g. X-rays for NDI are produced by the interaction of high-energy electrons or ions with matter.
h. Gamma rays are produced in nuclear transformations, such as radioactive decay.
i. X-rays and gamma rays expose (darken) photographic film.
j. They stimulate fluorescence and phosphorescence in some materials.
k. They are capable of ionizing gases and changing the electrical properties of some liquids and solids.
l. They are able to damage and kill living cells and to produce genetic mutations.
m. They are differentially absorbed or scattered by different media.
n. X-rays may be diffracted by the crystalline, structure of materials which acts like a grating.