PRINCIPLES OF RADIOLOGICAL PHYSICS 



109 



Calculations of the build-up factor are rather laborious and are possible 

 at present only in simple geometrical arrangements. Figure 1-68 shows 



40 



cc 30 

 o 



H 

 O 

 < 



CL 



I 



Q 



_J 



3 

 CD 



20 



10 



0, 



80 160 240 



DISTANCE FROM SOURCE, cm 

 Fig. 1-68. Build-up of secondary radiation at increasing distances from a Co*" 7-ray 

 source in water. (Spencer and Fano, 1951.) 



how scattered X rays build up in a particularly simple experiment. The 

 continued increase of the build-up factor is supported by scattered com- 

 ponents which have been deflected by a very small angle and therefore 

 have nearly the same energy and are almost as ''hard" as the primary 

 X rays. 



The travel of scattered X rays sidew^ise beyond the original boundaries 

 of a beam presents particular interest for medical radiology (see Fig. 

 1-48). Fairly extensive data on this subject have been obtained experi- 

 mentally; see, for example, Glasser et al. (1944), Chap. 12. 



4-3d. Energy Distribution. Assuming that the intensity and the spectrum of 

 the X rays flowing at various points within a material are known, there remains 

 the problem of finding where the X-ray energy is dissipated. 



The first step in solving this problem is to calculate how much energy is trans- 

 ferred from the X rays to the material at each point. The transfer includes the 

 whole energy of the photons that are absorbed photoelectrically or by pair pro- 

 duction and a fraction of the energy of Compton scattered photons. This 

 fraction, f{hv), depends on the photon energy hv. Therefore the absorption 

 coefficient adjusted to reckon the actual energy transfer equals 



titr 



y-ph + Mpair + j{hv)lXs 



(44) 



To be accurate, the energy absorption by photoelectric effect must also be dis- 

 counted -by a suitable factor, g{hv), to take into account the energy fraction that 

 returns to X rays locally by fluorescence. Therefore Eq. (44) should actually 



