Radiography in Modern Industry - Kodak

from this source. Since scatter also arises from the specimen itself, it is good practice whereverpossible, to limit the cross an x-ray beam to cover only the area of the specimen that is of interestin the examination.For occasional pieces of work where a cutout diaphragm would not be economical, barium claypacked around the specimen will serve the same purpose. The clay should be thick enough sothat the film density under the clay is somewhat less than that under the specimen. Otherwise,the clay itself contributes appreciable scattered radiation.It may be advantageous to place the object in aluminum or thin iron pans and to use a liquidabsorber, provided the liquid chosen will not damage the specimen. A combined saturatedsolution of lead acetate and lead nitrate is satisfactory.WARNING! Harmful if swallowed. Harmful if inhaled. Wash thoroughly after handling. Use onlywith adequate ventilation.To prepare this solution, dissolve approximately 31/2 pounds of lead acetate in 1 gallon of hotwater. When the lead acetate is in solution, add approximately 3 pounds of lead nitrate.Because of its high lead content this solution is a strong absorber of x-rays. In masking withliquids, be sure to eliminate bubbles that may be clinging to the surface of the specimen.One of the most satisfactory arrangements, combining effectiveness and convenience, is tosurround the object with copper or steel shot having a diameter of about 0.01 inch or less (SeeFigure 37). This material "flows" without running badly. It is also very effective for filling cavities inirregular objects, such as castings, where a normal exposure for thick parts would result in anoverexposure for thinner parts. Of course, it is preferable to make separate exposures for thickand thin parts, but this is not always practical.In some cases, a lead diaphragm or lead cone on the tube head may be a convenient way to limitthe area covered by the x-ray beam. Such lead diaphragms are particularly useful where thedesired cross section of the beam is a simple geometric figure, such as a circle, square, orrectangle.FiltersIn general, the use of filters is limited to radiography with x-rays. A simple metallic filter mountedin the x-ray beam near the x-ray tube (See Figure 38) may adequately serve the purpose ofeliminating overexposure in the thin regions of the specimen and in the area surrounding the part.Such a filter is particularly useful to reduce scatter undercut in cases where a mask around thespecimen is impractical, or where the specimen would be injured by chemicals or shot. Of course,an increase in exposure or kilovoltage will be required to compensate for the additionalabsorption; but, in cases where the filter method is applicable, this is not serious unless the limitof the x-ray machine has been reached.The underlying principle of the method is that the addition of the filter material causes a muchgreater change in the amount of radiation passing through the thin parts than through the thickerparts. Suppose the shape of a certain steel specimen is as shown in Figure 38 and that thethicknesses are 1/4 inch, 1/2 inch, and 1 inch. This specimen is radiographed first with no filter,and then with a filter near the tube.Radiography in Modern Industry 53