A pattern caused by porosity or segregation will change only slightly; however, one caused by diffraction will show a marked change. The radiographs of some specimens will show a mottling from both effects, and careful observation is needed to differentiate between them.
The basic facts of x-ray diffraction are given in "X-Ray Diffraction". Briefly, however, a relatively large crystal or grain in a relatively thin specimen may in some cases "reflect" an appreciable portion of the x-ray energy falling on the specimen, much as if it were a small mirror. This will result in a light spot on the developed radiograph corresponding to the position of the particular crystal and may also produce a dark spot in another location if the diffracted, or "reflected," beam strikes the film. Should this beam strike the film beneath a thick part of the specimen, the dark spot may be mistaken for a void in the thick section. This effect is not observed in most industrial radiography, because most specimens are composed of a multitude of very minute crystals or grains, variously oriented; hence, scatter by diffraction is essentially uniform over the film area. In addition, the directly transmitted beam usually reduces the contrast in the diffraction pattern to a point where it is no longer visible on the radiograph.
The mottling caused by diffraction can be reduced, and in some cases eliminated, by raising the kilovoltage and by using lead foil screens. The former is often of positive value even though the radiographic contrast is reduced. Since definite rules are difficult to formulate, both approaches should be tried in a new situation, or perhaps both used together.
It should be noted, however, that in same instances, the presence or absence of mottling caused by diffraction has been used as a rough indication of grain size and thus as a basis for the acceptance or the rejection of parts.
SCATTERING IN 1- AND 2-MILLION-VOLT RADIOGRAPHY
Lead screens should always be used in this voltage range. The common thicknesses, 0.005-inch front and 0.010-inch back, are both satisfactory and convenient. Some users, however, find a 0.010-inch front screen of value because of its greater selective absorption of the scattered radiation from the specimen.
Filtration at the tube offers no improvement in radiographic quality. However, filters at the film improve the radiograph in the examination of uniform sections, but give poor quality at the edges of 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, for example, 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 against backscattered radiation.
Lead filters are most convenient for this voltage range. When thus used between specimen and film, 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 million- volt x-rays, specimens of uniform sections may be conveniently divided into three classes. Below about 11/2 inches of steel, filtration affords little improvement in radiographic quality. Between 11/2 and 4 inches of steel, the thickest filter, up to 1/8-inch lead, which at the same time allows a reasonable exposure time, may be used. Above 4 inches of steel, filter thicknesses may be increased to1/4 inch of lead, economic considerations permitting. It should be noted that in the radiography 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 filters can 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 to 1-inch) lead at the anode. Unless this is done, radiation striking the walls of the x-ray room will scatter back in such quantity as to seriously affect the quality of the radiograph. This will be especially noticeable if the specimen is thick or has parts projecting relatively far from the film.
Multimillion-Volt Radiography
Techniques of radiography in the 6- to 24-million-volt range are difficult to specify. This is in part because of the wide range of subjects radiographed, from thick steel to several feet of mixtures of solid organic compounds, and in part because the sheer size of the specimens and the difficulty in 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 front and back lead screens up to at least 0.030 inch. One problem encountered with screens of such great thickness is that of screen contact. For example, if a conventional cardboard exposure holder is supported vertically, one or both of the heavy screens may tend to sag away from the film, with a resulting degradation of the image quality. Vacuum cassettes are especially useful in this application and several devices have been constructed for the purpose, some of which incorporate such refinements as automatic pre programmed positioning of the film behind the various areas of a large specimen.
The electrons liberated in lead by the absorption of multi megavolt x-radiation are very energetic. This means that those arising from fairly deep within a lead screen can penetrate the lead, being scattered as they go, and reach the film. Thus, when thick screens are used, the electrons reaching 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 front screen (about 0.005 inch) is used, and the back screen is eliminated. This necessitates a considerable increase in exposure time. Naturally, the applicability of the technique depends also on the amount of backscattered radiation involved and is probably not applicable where large amounts occur.