PRINCIPLES OF RADIOLOGICAL PHYSICS 



111 



higher value in the tissue just inside the surface. The flow of electrons propelled 

 by the X rays varies rapidly near the surface, without any sudden jump. There- 

 fore the density of energy dissipation does not vary in proportion to the density 

 of energy transfer. Since the flow of electrons is lower near the skin surface 

 than farther in, the energy dissipation also remains abnormally low throughout a 

 layer of tissue comparable to the range of the electrons. 



Thus the energy dissipation rises from a low level at the surface to a higher 

 level farther in, which tends to equal the level of energy transfer from the X rays 

 to the material. Figure 1-70 illustrates this "transition effect" near the surface. 



Ul 



o 



< 



cc 



DEPTH 

 Fig. 1-70. Qualitative plot of the energy dissipated by a beam of X rays at increas- 

 ing depths within a material. 



When the distribution of energy dissipation becomes parallel to the distribution 

 of energy transfer, we say that "electronic equilibrium" has been achieved. 

 This equilibrium is maintained in deeper layers of the material where the energy 

 dissipation decreases in proportion to the attenuation of the X-ray flow. 



The attainment of electronic equilibrium proceeds more and more slowly for 

 higher and higher energies of the X rays. The range of the electrons propelled 

 by the X rays increases rapidly and eventually becomes comparable to the own 

 range of penetration of the X rays (see Fig. 1-71). Moreover, the electrons flow 



0.1 f- 

 O.Oi 

 0.001 



10 



ENERGY, Mev 

 Fig. 1-71. " Relative range " of electrons and photons of equal energy, in water, as 

 a function of the energy. The quantity plotted is the product of the narrow-beam 

 absorption coefficient of the X rays (which may be regarded as the reciprocal of a 

 "range") and of the "true" range of electrons. {Courtesy G. R. White.) 



to an increasing extent in the same direction as the X rays. Eventually no form 

 of electronic equilibrium remains possible at the highest energies when the cascade 

 shower mechanism becomes effective. 



