RADIOGRAPHY 



829 



which differ only slightly may be radiographed satisfactorily by changing the time 

 and keeping the voltage constant. By establishing exposure values for a few different 

 metals, by studying the effect of composition on density, and by becoming familiar 

 with various settings on his own X-ray machine, the operator will soon be able to make 

 a good radiograph of almost any object with one 

 trial exposure. 



This table shows why lead is used as a shield 

 against X rays. Only gold, platinum, and tungsten 

 are greater X-ray absorbers and the last three are 

 expensive whereas lead is cheap. 



2. Size and shape of the object examined are 

 important and are frequently such that they provide 

 the major problem of the work. It is desirable to 

 place the film in its holder as near as possible to the 

 area where defects are likely to occur, and to avoid, 

 in the case of complicated castings and machine 

 parts, interposition of irrelevant sections between 

 film and X-ray tube. Ideal radiographic conditions 

 are those which allow the X-ray beam to pass from 

 the target through the area to be inspected and 

 thence, without interruption, to a film placed per- 

 pendicular to the line of the main beam (Fig. 1). 



Although this condition is often unattainable, 

 the nearest possible approach should be made. In 

 some cases it is better to place the film other than 



Objec-f- 

 ^— Film 

 Fig. 1. — Diagram illustrating 

 operation of X-ray photography. 

 A source of electrons striking the 

 target produces waves which 

 spread out as indicated by the 

 diverging lines. These waves 

 pass through the object to be 

 radiographed, where they are 

 partially absorbed, and thence to 



the film where a permanent 

 perpendicularly, to avoid overlapping of undesired record is produced, 

 shadows on the film (Fig. 2). 



In the case of curved surfaces care should be taken to center the target and film, 

 and the distance should be such as to permit the whole film to be exposed. Some 

 allowance must also be made for the fact that the ends of the film are farther from the 

 target than the center because of their curvature (Fig. 3). 



Overlapping-''' 



Film -''" A rea io be excrmined-' 



Fig. 2.- — Diagram illustrating one possible method of examining a portion of an object hav- 

 ing sections which ordinarily would provide some shielding of X rays. 



This distance defect may be overcome either by increasing the target-film distance 

 or by shortening the X-ray film. In the first the exposure time is increased to com- 

 pensate for the distance, and in the second instance more pictures must be made to 

 cover the total area, so that conditions prevailing at the time must decide which is the 

 better course. 



In cases where the entire film is not covered by the object some provision must be 

 made to keep scattered radiation from "creeping" in around the edges and utterly 



