Radiography in Modern Industry - Kodak

Definite rules as to filter thicknesses are difficult to formulate exactly because the amount offiltration required depends not only on the material and thickness range of the specimen, but alsoon the distribution of material in the specimen and on the amount of scatter undercut that it isdesired to eliminate. In the radiography of aluminum, a filter of copper about 4 percent of thegreatest thickness of the specimen should prove the thickest necessary. With steel, a copper filtershould ordinarily be about 20 percent, or a lead filter about 3 percent, of the greatest specimenthickness for the greatest useful filtration. The foregoing values are maximum values, and,depending on circumstances, useful radiographs can often be made with far less filtration.In radiography with x-rays up to at least 250 kV, the 0.005-inch front lead screen customarilyused is an effective filter for the scatter from the bulk of the specimen. Additional filtrationbetween specimen and film only tends to contribute additional scatter from the filter itself. Thescatter undercut can be decreased by adding an appropriate filter at the tube as mentionedbefore (See also Figures 40). Although the filter near the tube gives rise to scattered radiation,the scatter is emitted in all directions, and since the film is far from the filter, scatter reaching thefilm is of very low intensity.Further advantages of placing the filter near the x-ray tube are that specimen-film distance is keptto a minimum and that scratches and dents in the filter are so blurred that their images are notapparent on the radiograph.Grid DiaphragmsOne of the most effective ways to reduce scattered radiation from an object being radiographed isthrough the use of a Potter-Bucky diaphragm. This apparatus (See Figure 42) consists of amoving grid, composed of a series of lead strips held in position by intervening strips of a materialtransparent to x-rays. The lead strips are tilted, so that the plane of each is in line with the focalspot of the tube. The slots between the lead strips are several times as deep as they are wide.The parallel lead strips absorb the very divergent scattered rays from the object beingradiographed, so that most of the exposure is made by the primary rays emanating from the focalspot of the tube and passing between the lead strips. During the course of the exposure, the gridis moved, or oscillated, in a plane parallel to the film as shown by the black arrows in Figure 42.Thus, the shadows of the lead strips are blurred out so that they do not appear in the finalradiograph.Figure 42: Schematic diagram showing how the primary x-rays pass between the leadstrips of the Potter-Bucky diaphragm while most of the scattered x-rays are absorbedbecause they strike the sides of the strips.Radiography in Modern Industry 57