Radiography in Modern Industry - Kodak

0.010-inch front screen of value because of its greater selective absorption of the scatteredradiation from the specimen.Filtration at the tube offers no improvement in radiographic quality. However, filters at the filmimprove the radiograph in the examination of uniform sections, but give poor quality at the edgesof the image of a specimen because of the undercut of scattered radiation from the filter itself.Hence, filtration should not be used in the radiography of specimens containing narrow bars, forexample, no matter what the thickness of the bars in the direction of the primary radiation.Further, filtration should be used only where the film can be adequately protected againstbackscattered radiation.Lead filters are most convenient for this voltage range. When thus used between specimen andfilm, filters are subject to mechanical damage. Care should be taken to reduce this to a minimum,lest filter defects be confused with structures in or on the specimen. In radiography with millionvoltx-rays, specimens of uniform sections may be conveniently divided into three classes. Belowabout 11/2 inches of steel, filtration affords little improvement in radiographic quality. Between11/2 and 4 inches of steel, the thickest filter, up to 1/8-inch lead, which at the same time allows areasonable exposure time, may be used. Above 4 inches of steel, filter thicknesses may beincreased to1/4 inch of lead, economic considerations permitting. It should be noted that in theradiography of extremely thick specimens with million-volt x-rays, fluorescent screens (See"Fluorescent Screens") may be used to increase the photographic speed to a point where filterscan be used without requiring excessive exposure time.A very important point is to block off all radiation except the useful beam with heavy (1/2-inch to1-inch) lead at the anode. Unless this is done, radiation striking the walls of the x-ray room willscatter back in such quantity as to seriously affect the quality of the radiograph. This will beespecially noticeable if the specimen is thick or has parts projecting relatively far from the film.Multimillion-Volt RadiographyTechniques of radiography in the 6- to 24-million-volt range are difficult to specify. This is in partbecause of the wide range of subjects radiographed, from thick steel to several feet of mixtures ofsolid organic compounds, and in part because the sheer size of the specimens and the difficultyin handling them often impose limitations on the radiographic techniques that can be used.In general, the speed of the film-screen combination increases with increasing thickness of frontand back lead screens up to at least 0.030 inch. One problem encountered with screens of suchgreat thickness is that of screen contact. For example, if a conventional cardboard exposureholder is supported vertically, one or both of the heavy screens may tend to sag away from thefilm, with a resulting degradation of the image quality. Vacuum cassettes are especially useful inthis application and several devices have been constructed for the purpose, some of whichincorporate such refinements as automatic preprogrammed positioning of the film behind thevarious areas of a large specimen.The electrons liberated in lead by the absorption of multimegavolt x-radiation are very energetic.This means that those arising from fairly deep within a lead screen can penetrate the lead, beingscattered as they go, and reach the film. Thus, when thick screens are used, the electronsreaching the film are "diffused," with a resultant deleterious effect on image quality. Therefore,when the highest quality is required in multimillion-volt radiography, a comparatively thin frontscreen (about 0.005 inch) is used, and the back screen is eliminated. This necessitates aconsiderable increase in exposure time. Naturally, the applicability of the technique depends alsoon the amount of backscattered radiation involved and is probably not applicable where largeamounts occur.Radiography in Modern Industry 59