T.O. 33B-1-16-416.5.3.1 ScatterRadiation.6.5.3.1.1 Description.When high-energy electromagnetic radiation bombards matter, some of the radiation photons are scattered by electrons,a process called the Compton effect. If the photon has a greater quantity of energy than necessary to eject an electronfrom its orbital path, it continues to travel with a loss of energy at some angle to its original path. The photon energymust be reduced to a very low value before complete annihilation is possible by photoelectric absorption. In low atomicnumber materials the photon direction is changed with little loss of energy and its energy must be reduced to a very lowenergy to be absorbed completely. Thus, a single photon may be scattered many times, losing all semblance of itsoriginal path. If this scattered photon strikes the film, it reduces the image definition since it exposes the film at a spotother than directly under the point where it first entered the test material. High atomic number materials rob thephoton of greater amounts of its original energy and also have much higher photoelectric absorption values. Thesemore quickly reduce the photon energy to the point where the photon is completely absorbed. For these reasons, lowatomic number materials transmit larger quantities of scattered radiation than high atomic number materials. In actualradiographic practices, low atomic number materials should be removed from the beam to the extent possible, toprevent scattering of the primary beam. Wood, concrete or other low atomic number materials in the radiation beamshould be covered with lead or a high atomic number material to reduce the scatter. In actual practice this means thattables, floors or walls that are behind/beside and close to the test part should be covered with lead.6.5.3.1.2 ScatterBuildUp.The scattering is due to photon collision with electrons in their path. As material thicknesses increase up to a criticalthickness, the amount of scattered radiation emanating from the material increases. If additional thicknesses ofmaterial are added, the scattered radiation generated in these added layers have insufficient energy to penetrate thematerial between them and the film. The amount of scattered radiation emanating from the back of a part beinginspected increases with part thickness up to a total which varies with radiation energy. Since absorption due to theCompton effect decreases with increasing radiation energy, less scattering occurs at higher radiation energy levels.Build-up scatter radiation can introduce contrast problems in the radiography of low atomic number materials such asgraphite, plastics, and magnesium. A simple test to reveal the scatter build-up in a test specimen can be made. Choosea radiation source-to-film distance of three or four feet, make two identical exposures — one with the test specimen atthe X-ray source and one with the specimen at the film. Differences in the densities after processing can be credited toscatter radiation.6.5.4 DiffractionPatterns.In the radiography of very coarse grain structure materials, such as Inconel and cast irons, diffraction patterns are oftenrevealed in the radiographic image. These patterns are due to the selective diffraction and absorption by the atoms of adefinite pattern in the crystal structure. The definitive pattern of the atoms of a crystal can be aligned with the X-raybeam at a particular angle so that the radiation is altered in its direction of travel and concentrated upon the film as alinear indication. These crystalline diffraction patterns are superimposed upon the radiographic image and makeinterpretation very difficult. Often these dense, sharp lines caused by the crystal diffraction are interpreted as internalcracks. If uncertainty exists as to interpretation of a particular indication, a second radiograph can be made at aslightly different angle (less than 10 degrees difference). It is unlikely the crystal causing the diffraction pattern wouldbe located to precisely the same relative position as to cause the diffracted line to strike the film in the same relativeposition. Changes in radiation energy will also affect diffraction patterns. Often by changing the operating kilovoltagethe problem of diffraction patterns can be reduced.6.5.5 MaterialContrast.6.5.5.1 MaterialContrastFactor.In consideration of the above discussion on radiation absorption, the most important variable that can be controlled bythe radiographer in industrial X-ray inspection is the kilovoltage. The amount of radiation absorbed by the part beinginspected depends on the atomic number, density, and thickness of the material. The radiographer cannot change thesefactors, but can change the energy of radiation. In the attenuation equation, ln (-mx) = IT/I0, it can be visualized that the
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