90 RADIATION BIOLOGY 



4-lc. Effect of Nuclear Collisions. Particles of sufficiently high energy, of 

 the order of 100 Mev or more, penetrate so deeply into matter that they have a 

 considerable chance of hitting a nucleus squarely. Under these conditions the 

 pattern of distribution of the radiation energy through matter is considerably 

 altered. Secondary high-potency radiations stem from the nuclei that have 

 been hit (see Sects. 2-4d and 2-5). Information on effects of this type was 

 still fragmentary at the time of this writing (1950). 



4-2. ELECTRONS 



Electrons penetrating into matter undergo, like heavy charged par- 

 ticles, a progressive energy loss through inelastic collisions; they also 

 experience repeated deflections, mostly through elastic collisions. On the 

 other hand, these two processes of absorption and scattering have different 

 relative importance for electrons and heavy particles because electrons 

 are much more easily deflected owing to their smaller mass. Since the 

 over-all process of penetration is quite complicated when the deflection 

 effects are paramount, data on the average amount of energy delivered 

 by electron beams to successive layers of various materials are still 

 inadequate. 



Scattering effects become comparatively less important when the elec- 

 tron velocity approaches the velocity of light. For electrons of very high 

 energy, above 1 Mev, the effects of energy dissipation, i.e., of outright 

 absorption, tend to predominate over the effects of scattering, owing to 

 the increased inertia of very-high-speed particles. Therefore the tracks 

 of high-energy electrons are more nearly straight, like the tracks of heavy 

 particles. However, the length of electron tracks greatly exceeds the 

 length of heavy-particle tracks of equal energy, because heavy particles, 

 being slower, dissipate energy at a higher rate (except at extremely high 

 energies, as noted later). 



Photographs of electron tracks of different energies in Fig. 1-47 illus- 

 trate the different importance of scattering effects in different energy 

 ranges. The scattering effects would be much more important if the 

 medium consisted of a high atomic number material rather than of air. 



Finally, the energy loss by electrons due to X-ray emission becomes 

 paramount at sufficiently high energies (that is, beyond 10 to 100 Mev, 

 depending on the atomic number of the material; see Sect. l-4b). This 

 type of energy loss, which is wholly unimportant in the case of heavy 

 particles, sets an upper limit to the penetration of electrons in a material, 

 no matter how high their energy. However, the energy lost by the elec- 

 trons is carried along in this case by the propagation of X rays and by 

 the resulting cascade shower production and thus reaches much farther 

 than the electrons themselves. 



In conclusion the distribution of the energy carried by an electron 

 beam to a material takes place according to different patterns, depending 



