Figure 6-28. Sources of Scatter Radiation

Table 6-13. Correlation between Beam Divergence and Crack Detectability

TM-1-1500-335-23 Nondestructive Inspection Methods Manual
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T.O. 33B-1-1 6-61 scatter can be the area around the test or other objects that may be in the radiation beam. This is usually referred to as reflective scatter. A second source is scatter radiation from objects behind the film. This is usually referred to as backscatter radiation. The third source of scatter radiation is the test object itself. This scatter can obliterate the object's edges and is called "undercutting." The amount of scatter radiation is affected by the radiation energy and the atomic number of the element doing the scattering. Low atomic number materials scatter radiation to a greater degree than materials with high atomic numbers. Figure 6-28. Sources of Scatter Radiation. 6.7.2.11.1 Several techniques can be used to reduce the amount of scatter. Radiographic cones or masks can be made of lead or other high absorbing materials to reduce the radiation area to only the area necessary for exposure. Lead in many different forms can be placed behind the X-ray film and test object to reduce excessive backscatter. Lead foils may be placed between the test object and the X-ray film to absorb some of the scattered radiation before they expose the film. These act as scatter filters since they permit the higher energy image forming radiation to be transmitted to the film and at the same time absorb the lower energy scattered radiation. Their function is described more fully in the section entitled films, film holders, and screening, but note that filters in this position will reduce subject contrast. In some cases, the scatter problem can be of such a magnitude that special techniques must be applied. Masking the part is often required because of large variation in thickness-and thus differences in absorption lead to scatter from excessive radiation transmitted through thin sections. Figure 6-29 shows how a lead sheet could be used for masking. In this case, the object was a steel hub without the lead sheet (1 /8 inch thick) definition was poor due to internal scatter.

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