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RADIATION BIOLOGY 



Figure 1-69 gives some data on the values of the factors/ and g. 



(44') 



1.0 



S^ 0.8 



-S 0.6 

 u. < 

 °v 



y uj 



O Ll) 

 < 



q: 



0.2 



OX)l 



0.1 



10 



100 



1.0 



PHOTON ENERGY, Mev 

 Fig. 1-69. Fraction of the energy of incident X rays which is actually absorbed in the 

 photoelectric effect on the K shell (curves g) or in the Compton effect (curve /). 

 {Courtesy G. R. White.) 



The adjusted absorption coefficient must be multiplied by the intensity I{hv) 

 (energy flow per unit area per unit time) of the X rays with photon energy hv 

 to obtain the rate of energy transferred by these X rays per unit volume of space 

 pel' unit time. 



iitr{hv)I{hv) (45) 



Finally, the energy transfers by all spectral components with different photon 

 energies must be added. The result expresses the energy transfer per unit 

 volume or per unit mass of material depending on whether /x indicates the linear 

 or the mass absorption coefficient. 



Wlien this step of calculation is completed, consideration must be given to 

 where the energy transferred to matter is eventually dissipated into chemical 

 activations. This problem does not offer difficulty in the case of lower energy 

 X rays, e.g., below 100 kev. The secondary electrons generated by these X rays 

 have a very short range, namely a minute fraction of a millimeter in any non- 

 gaseous material. Therefore, in practice, it may be considered that these elec- 

 trons dissipate their energy at the very place where they received it. In other 

 words, the distribution in space of the eventual energy dissipation parallels very 

 closely the distribution of the energy transfer from X rays to matter. 



This parallelism holds in so far as the distribution of the energy transfer varies 

 but little over the range of distances traveled by electrons. The travel of elec- 

 trons all around from their points of origin tends to smooth out sharp variations in 

 the distribution of energy transfers. 



As the X-ray energy increases, the range of the electrons produced increases 

 rapidly. The surface of any material is one of the first places where the dis- 

 crepancy between the distribution of energy dissipation and the distribution of 

 energy transfer becomes apparent. Consider, for example, the skin surface of a 

 patient exposed to a beam of X rays. The density of energy transfer jumps 

 suddenly from a very low value in the air just outside the surface to a much 



