IONIZATION AND BIOLOGICAL EFFECTS 101 



the latter) and those primary photons which have been scattered without 

 loss of energy. ^° This process has not been mentioned so far because 

 no ionization results from it. The main effect is a deviation of some 

 photons from their straight-line paths with a resultant widening of the 

 radiation beam, and consequently a decrease of the intensity of radiation 

 along the axis. 



SECONDARY PHENOMENA 



The interactions of radiation and (organic) matter just described 

 lead to some very important results. 



A. It is evident that monochromatic radiation becomes polychromatic 

 (heterogeneous) after traversing even a small thickness of matter, on 

 account of the presence in the transmitted beam of photons of lower 

 energy. 



B. The proportion of lower-energy photons in the transmitted beam 

 increases with the thickness of the material, that is, the radiation becomes 

 gradually softer. But, since the lower the photon energy the more 

 easily it is transferred to electrons and thence utilized in producing ions, 

 the softer components of the radiation do not travel so far in the material 

 as the primary radiation. Accordingly, beyond a certain thickness the 

 energy distribution of the photons in the transmitted beam {i.e., the 

 quality) remains substantially constant, since the photons of lower 

 energy are eliminated by the very layer of matter in which new ones of 

 the same energy distribution are produced. When this condition is 

 reached, it is said that the radiation is in "equilibrium with its second- 

 aries." In the end, however, provided the thickness of material is large 

 enough, the "degeneration" of the radiation is complete and all its energy 

 is finally transferred to ions. 



C. Since secondary photons are emitted in all directions, a material 

 body traversed by a beam of X-rays acts in some respects as a new source 

 of radiation. The most important difference is in the distribution of the 

 rays around the new source. The part of the secondary radiation which 

 is due to the photoelectric effect, being extremely soft, is utilized in situ 

 almost entirely. What comes out of the material body as secondary 

 radiation, therefore, is made up very largely of photons liberated by the 

 Compton effect. However, as already stated, these photons are pro- 



1" It might be well to mention at this point that most authors think of the Compton 

 effect as a scattering process, whereby the photon collides with an electron and 

 bounces off with diminished energy (longer wave-length). To distinguish this type 

 of scattering from the one mentioned above in which no change of wave-length 

 occurs, they refer to it as "Compton scattering," or scattering with change of wave- 

 length. The writer feels that for the purpose of this presentation it is preferable to 

 call the photon of lower energy resulting from the Compton collision a secondary 

 photon. Of course, the final result is identical irrespective of the terminology. 



