Radiography in Modern Industry - Kodak

The quality of the radiation necessary to obtain an appreciable intensification from lead foilscreens depends on the type of film, the kilovoltage, and the thickness of the material throughwhich the rays must pass. (See Figure 25) In the radiography of aluminum, for example, using a0.005-inch front screen and a 0.010-inch back screen, the thickness of aluminum must be about 6inches and the kilovoltage as high as 160 kV to secure any advantage in exposure time with leadscreens. In the radiography of steel, lead screens begin to give appreciable intensification withthicknesses in the neighborhood of 1/4 inch, at voltages of 130 to 150 kV. In the radiography of11/4 inches of steel at about 200 kV, lead screens permit an exposure of about one third of thatwithout screens (intensification factor of 3). With cobalt 60 gamma rays, the intensification factorof lead screens is about 2. Lead foil screens, however, do not detrimentally affect the definition orgraininess of the radiographic image to any material degree so long as the lead and the film arein intimate contact.Figure 26: Upper area shows decreased density caused by paper between the lead screenand film. An electron shadow picture of the paper structure has also been introduced.Lead foil screens diminish the effect of scattered radiation, particularly that which undercuts theobject (See "Scattered Radiation"), when the primary rays strike the portions of the film holder orcassette outside the area covered by the object.Scattered radiation from the specimen itself is cut almost in half by lead screens, contributing tomaximum clarity of detail in the radiograph; this advantage is obtained even under conditionswhere the lead screen makes necessary an increase in exposure. A more exhaustive discussionof scattered radiation will be found in "Scattered Radiation".In radiography with gamma rays or high-voltage x-rays, films loaded in metal cassettes withoutscreens are likely to record the effect of secondary electrons generated in the lead-covered backof the cassette. These electrons, passing through the felt pad on the cassette cover, produce amottled appearance because of the structure of the felt. Films loaded in the customary leadbackedcardboard exposure holder may also show a pattern of the structure of the paper that liesbetween the lead and the film (See Figure 26.). To avoid these effects, film should be enclosedbetween double lead screens, care being taken to ensure good contact between film andscreens. Thus, lead foil screens are essential in practically all radiography with gamma rays ormillion-volt x-rays. If, for any reason, screens cannot be used with these radiations, use alightproof paper or cardboard holder with no metal backing.Radiography in Modern Industry 40