Properties of X-Rays and Gamma Rays

Properties of X-Rays and Gamma Rays

WARNING

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.

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:

1. They are invisible to humans.
2. They propagate in straight lines in free space.
3. In special cases they are reflected, diffracted, refracted, and polarized as is light, but to a much smaller degree.
4. They propagate at a velocity of 3×108 meters per second as does light.
5. They consist of transverse electromagnetic vibrations as does light.
   f. X-rays have energies between roughly 1KeV and 50MeV.
6. X-rays for NDI are produced by the interaction of high-energy electrons or ions with matter.
7. Gamma rays are produced in nuclear transformations, such as radioactive decay.
8. X-rays and gamma rays expose (darken) photographic film.
9. They stimulate fluorescence and phosphorescence in some materials.
10. They are capable of ionizing gases and changing the electrical properties of some liquids and solids.
11. They are able to damage and kill living cells and to produce genetic mutations.
12. They are differentially absorbed or scattered by different media.
13. X-rays may be diffracted by the crystalline, structure of materials which acts like a grating.
14. They do not affect fuel cells or munitions.

All of these properties contribute in some degree to the understanding of the radiographic process. Most important of these in terms of usefulness to NDI are the differential absorption of radiation in matter and the ability of radiation to expose film. In the remainder of this chapter the term “X-rays” will be more prevalent since that form of radiation is most used. Except where noted the discussion will also apply to gamma rays.

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