Radiography in Modern Industry - Kodak

narrow angle would be very thick, especially for cobalt 60 radiation, and hence difficult to makeand to position.The technique is generally limited to thin specimens or cases where a large source-film distancecan be used. The motion of the tube gives rise to a geometrical unsharpness that can betroublesome for thick specimens or short source-film distances. This unsharpness (U m ) can becalculated from the formula:where t is the thickness; w is the width of the radiation beam at the source side of the specimen,measured in the direction of motion of the tube; and d is the source-specimen distance.If the permissible motion unsharpness can be estimated, the formula can be rearranged to givethe maximum width (in the direction of motion) of the beam at the source side of the specimen:In the above formulae, distances are usually measured in inches and Um is also in terms ofinches. Thus, care must be exercised in comparing values so obtained with values of geometricunsharpness (U g ) calculated from the formula:Because focal spot sizes are usually specified in millimetres, values of U g , from the formulaabove are usually in millimetres also.Since the exposure time T that is required for a single exposure of the specimen is usually knownfrom experience or from an exposure chart of the material, the required rate of travel V of the tuberelative to the specimen can be calculated:where w is the width of the beam (in the direction of travel) at the source side of the specimen.For best results with this technique, the motion of the radiation source must be smooth anduniform. Any unevenness of motion results in parallel bands of overexposure and underexposureat right angles to the direction of motion.Rotary Radiography of Annular SpecimensAnnular specimens often present economic problems if they must be radiographed in quantity.Placed flat on a film, they are wasteful of film area, of exposure and setup time, and of filingspace. Great economies of time and money can be achieved by the use of a variation of thetechnique described above.Film is wrapped around a circular, cylindrical, lead-covered mandrel, and covered with a light tightcovering that is also rugged enough to protect the film from abrasion. Annular specimens are thenslipped on the mandrel, over the film in its protective cover. Clamping means may be required toprevent rotation of the specimens with respect to the mandrel. The "loaded" mandrel is placedbehind a lead shield containing a narrow slit at least as long as the mandrel (See Figure 86).During the course of the exposure, the mandrel is rotated behind the slit, each part of thespecimens being radiographed in turn by the thin "sheet" of radiation passing through the slit. It isnot necessary that radiography be completed in a single turn of the mechanism. Several rotationscan be used but it is important that it be an integral number of turns; otherwise there will be aRadiography in Modern Industry 143