T.O. 33B-1-16-26.1.1.2.1WARNINGExposure to excessive X or gamma radiation is harmful to human beings. Whilemost X-ray equipment is designed to minimize the danger of exposure to direct orstray radiation, certain precautions must be observed. Radiation protectionrequirements are discussed in Section 9.The most distinguishing characteristic of X-rays is their short wavelength. The penetrating ability of X-rays is directlyproportional to their energy, which in turn, is inversely proportional to their wavelength; that is, the shorter thewavelength, the higher the energy; the longer the wavelength, the lower the energy. Short wavelength X-rays arecommonly described as "hard" while long wavelength X-rays are referred to as "soft."6.1.2 PropertiesofX-RaysandGammaRays.There are several properties which X-rays and gamma rays possess which make them useful for radiographicinspection. X-rays and gamma rays are the same form of energy as visible light; both are part of the electromagneticspectrum. 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 befocused, but the techniques are so cumbersome that in normal NDI applications X-rays are not focused. Although theproperties of X and gamma rays and visible light are theoretically similar, the differences in application make it mostconvenient to consider X and gamma rays as being different, since their observable effects are quite different from thoseof light. This is noted particularly in the ability to penetrate matter. Some general properties of X and gamma raysmay 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 smallerdegree.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.
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