Radiography in Modern Industry - Kodak

lead foil screens ran be retained while gaining the advantages of cleanliness, convenience, andscreen contact.Indications for Use of Lead Oxide ScreensFilms packaged in this manner may be used in the kilovoltage range from 100 to 300 kV,particularly in those circumstances in which cleanliness is important and where good film-screencontact would otherwise be difficult to obtain.Fluorescent ScreensCertain chemicals fluoresce, that is, have the ability to absorb x-rays and gamma rays andimmediately emit light. The intensity of the light emitted depends on the intensity of the incidentradiation. The phosphors are finely powdered, mixed with a suitable binder, and coated in a thin,smooth layer on a special cardboard or plastic support.For the exposure, the film is clamped firmly between a pair of these screens. The photographiceffect on the film, then, is the sum of the effects of the x-rays and of the light emitted by thescreens. A few examples will serve to illustrate the importance of intensifying screens in reducingexposure time. In medical radiography, the exposure is from 1/10 to 1/60 as much withfluorescent intensifying screens as without them. In other words, the intensification factor variesfrom 10 to 60, depending on the kilovoltage and the type of screen used. In the radiography of1/2-inch steel at 150 kV, a factor as high as 125 has been observed, and in the radiography of3/4-inch steel at 180 kV factors of several hundred have been obtained experimentally. Underthese latter conditions, the intensification factor has about reached its maximum, and itdiminishes both for lower voltage and thinner steel and for higher voltage and thicker steel. Usingcobalt 60 gamma rays for very thick steel, the factor may be 10 or less.LimitationsDespite their great effect in reducing exposure time, fluorescent screens are used in industrialradiography only under special circumstances. This is in part because they give poor definition inthe radiograph, compared to a radiograph made directly or with lead screens. The poorerdefinition results from the spreading of the light emitted from the screens, as shown in Figure 32.The light from any particular portion of the screen spreads out beyond the confines of the x-raybeam that excited the fluorescence. This spreading of light from the screens accounts for theblurring of outlines in the radiograph.Figure 32: Diagram showing how the light and ultraviolet radiation from a typicalfluorescent screen spreads beyond the x-ray beam that excites the fluorescence.Radiography in Modern Industry 45