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.
