Radiography in Modern Industry - Kodak

Chapter 5: Radiographic ScreensWhen an x-ray or gamma-ray beam strikes a film, usually less than 1 percent of the energy isabsorbed. Since the formation of the radiographic image is primarily governed by the absorbedradiation, more than 99 percent of the available energy in the beam performs no usefulphotographic work. Obviously, any means of more fully utilizing this wasted energy, withoutcomplicating the technical procedure, is highly desirable. Two types of radiographic screens areused to achieve this end--lead and fluorescent. Lead screens, in turn, take two different forms.One form is sheets of lead foil, usually mounted on cardboard or plastic, which are used in pairsin a conventional cassette or exposure holder. The other consists of a lead compound (usually anoxide), evenly coated on a thin support. The film is placed between the leaves of a folded sheet ofthis oxide-coated material with the oxide in contact with the film. The combination is supplied in asealed, lightproof envelope.Lead Foil ScreensFor radiography in the range 150 to 400 kV, lead foil in direct contact with both sides of the filmhas a desirable effect on the quality of the radiograph. In radiography with gamma rays and withx-rays below 2,000 kV, the front lead foil need be only 0.004 to 0.006 inch thick; consequently itsabsorption of the primary beam is not serious. The back screen should be thicker to reducebackscattered radiation. Such screens are available commercially. The choice of lead screenthicknesses for multimillion-volt radiography is much more complicated, and the manufacturers ofthe equipment should be consulted for their recommendations.Effects of Lead ScreensLead foil in direct contact with the film has three principal effects: (1) It increases the photographicaction on the film, largely by reason of the electrons emitted and partly by the secondary radiationgenerated in the lead. (2) lt absorbs the longer wavelength scattered radiation more than theprimary. (3) It intensifies the primary radiation more than the scattered radiation. The differentialabsorption of the secondary radiation and the differential intensification of the primary radiationresult in diminishing the effect of scattered radiation, thereby producing greater contrast andclarity in the radiographic image. This reduction in the effect of the scattered radiation decreasesthe total intensity of the radiation reaching the film, thereby lessening the net intensification factorof the screens. The absorption of primary radiation by the front lead screen also diminishes thenet intensifying effect, and, if the incident radiation does not have sufficient penetrating power, theactual exposure required may be even greater than without screens. At best, the exposure time isone half to one third of that without screens, but the advantage of screens in reducing scatteredradiation still holds.Figure 25: Effects of kilovoltage on intensification properties of lead screens.