PRINCIPLES OF RADIOLOGICAL PHYSICS 



107 



tions of X-ray attenuation obtain only in special experimental arrange- 

 ments. Under conditions of practical interest, such as in the penetration 

 of protective barriers or a patient's body, most of the scattered X rays 

 travel on mixed with the incident beam. The intensity of this radiation 

 must therefore be added to the unaltered fraction of the intensity of the 

 incident beam at any point within or beyond an absorber. 



.a 

 o 



>-" 



to 



< 



0.01 



12 3 4 



Cu ABSORBER THICKNESS, mm 

 Fig. 1-66. Narrow-beam absorption curve in copper for non-monochromatic X rays 

 from a tube operated at 120 kv. (Courtesy F. H. Day.) 



Besides the scattered X rays there must also be considered the second- 

 ary X rays which radiate away from an atom by "fluorescence" (see 

 Sect. 2-3), following the photoelectric absorption of an X-ray photon. 

 Table 1-10 gives data on the "fluorescence yield" of various elements, 



Table 1-10. Fluorescence Yields in the K Series 

 (Compton and Allison, 1935, p. 489) 



i.e., on the probability that the photoelectric absorption of a photon will 

 be followed by the emission of another photon of characteristic energy. 



Scattered and secondary X rays accumulate to an increasing extent as 

 the penetration proceeds to deeper and deeper layers of a material. 

 (Notice, however, that these X rays have lower energy photons and 

 therefore are generally "softer" than the incident, or "primary," X rays.) 

 Therefore the semilogarithmic plot of the intensity transmitted through 



