PRINCIPLES OF RADIOLOGICAL PHYSICS 



113 



It should be remembered, of course, that all the preceding considerations on 

 energy dissipation pertain to the average energy dissipated within sizable portions 

 of matter. Microscopically, the energy is distributed along the tracks of elec- 



v.:./,'.. 



/'■■-/ 



Fig. 1-74. Cloud-chamber picture of the effects of an X-ray beam in air. {Gentner 

 et al., 1940.) 



trons. Cloud-chamber pictures of the ionizations produced by an X-ray beam 

 in a gas (see Fig. 1-74) illustrate this point clearly. 



Systematic collections of data on the energy distribution are not available. 



4-4. NEUTRONS 



The penetration of neutrons may be discussed along the same general 

 lines as the penetration of X rays, since neutrons, like X rays, interact 

 with matter through comparatively few successive elementary processes 

 at fairly widely separated points. Quantitative information on the 

 elementary processes caused by neutrons is less adequate than the corre- 

 sponding information for X rays. On the other hand the study of the 

 flow of neutrons through complicated masses of matter has received a 

 great deal of attention, mostly on account of its applications to the design 

 of nuclear reactors. 



Neutron penetration is limited by collisions with nuclei (see Sect. 2-5). 

 Neutrons usually lose some of their energy in a collision, but outright 

 capture is comparatively infrequent. Final capture may take place 



