72 



RADIATION BIOLOGY 



the total number of particles and photons doubles, on the average, 

 after each step of the phenomenon, as shown schematically in Fig. 

 1-43. 



®x© 



Fig. 1-43. Diagram of a cascade-shower 

 process. Dotted lines indicate the 

 paths of the X rays; solid lines indicate 

 the tracks of the electrons and positrons. 



Fig. l-i4. Stereoscopic cloud-chamber pic- 

 ture of a shower, initiated by an electron, 

 which develops as the radiation passes 

 through successive layers of lead. (Gent- 

 ner et al., 1940.) 



The finer and finer subdivision of the total energy available, into 

 packets of comparable size, stops when the energy of the electrons and 

 positrons reduces to the level at which it is spent mostly in inelastic 

 rather than in radiative collisions. 



3-3. ACTIVATION BY NEUTRONS 



Neutrons traversing a material interact almost exclusively with the 

 atomic nuclei. Nuclear collisions occur here and there throughout the 

 material, according to the general rules indicated in Sect. 2-5. 



The collisions result in the production of secondary radiations which 

 may include charged corpuscular radiations, y rays, and neutrons of lower 

 energy than the incident ones. In any event, the great majority of 

 chemical activations result from the action of charged particles since 

 the secondary y rays and neutrons also act only indirectly. A delayed 

 emission of secondary radiation results when the capture of a neutron 



