Radiography in Modern Industry - Kodak

discontinuities or of segregation in alloys in which the components differ fairly widely in atomicnumber--for example, the examination of aluminum copper alloys (See Figure 103), or in thedetermination of the dispersion of lead in a leaded brass. For such applications, tungsten-target,beryllium-window tubes operating up to 50 kV are useful. The continuous spectrum from x-raydiffraction tubes can also be used. Microradiography has also been accomplished using the whiteradiation from ordinary radiographic tubes operated at a low voltage, although the x-ray intensityobtained at low voltages is severely limited by the thickness of the tube window.When segregation of components that do not differ greatly in atomic number must be detected,the use of the characteristic K x-ray spectrum from a suitable element gives the best results. Kcharacteristic radiation can be obtained by two methods. An x-ray tube with a target of thesuitable material has the advantage of a relatively high intensity of K radiation, but has thedisadvantage of requiring several x-ray tubes of different target materials or a tube withdemountable targets. In addition, a number of elements that emit K radiation that might be usefulcannot be made into x-ray tube targets. Use of K fluorescence radiation, obtained by irradiating asecondary target of a suitable material with the intense continuous spectrum of a tungsten-targettube avoids the disadvantage of the first method and gives a nearly pure K spectrum. However,the intensities available by this method are relatively low and long exposure times are needed.Extremely good contact between specimen and film is necessary if the maximum enlargement ofwhich the film is capable is to be achieved. Good contact can be obtained with a simplemechanical jig that presses the specimen against the film or plate, but best results are probablyobtained with a vacuum exposure holder. This consists of a plate with a milled recess into whichthe photographic material and specimen are placed and over which is put a flexible transparentcover. Evacuation of the recess with a vacuum pump or water aspirator causes the atmosphericpressure on the cover to press the specimen into intimate contact with the film or plate. (SeeFigure 104)Figure 104: Arrangement for microradiography using a vacuum exposure holder. Thespecimen thickness has been exaggerated for the sake of illustrative clarity.Any material placed between tube and specimen must be thin and of low atomic number (forexample, a thin sheet of cellulose derivative) to minimize x-ray absorption, and must have nomarked structure. Certain sheet plastic materials contain chlorine. These should not be used asRadiography in Modern Industry 165