10 PHYSICAL PROCESSES IN LIVING MATTER 



are about 600 times as numerous as the atoms which directly interact 

 with radiation. The effect of radiation here, then, is just the effect of 

 randomlj^ originating ionizing recoil electrons. We can distinguish a few 

 important quantum energies in the whole spectrum as marking different 

 regimes of radiation : 



1. Near 4-5 ev. This ultraviolet region is marked by strong selective 

 absorption in specific atomic and molecular structures. Only the direct 

 action of the quantum is important here, as indeed for lower-frequency 

 visible light, where the biological effects of radiation are the most 

 important of all — photosynthesis. 



2. Up to 50 or 60 kev. In this region of soft and medium x-rays, the 

 principal process of energy transfer is through the photoelectric absorp- 

 tion of the quantum by an inner electron of some atom. All the quantum 

 energy appears in the ejected electron, mostly as its kinetic energy, but 

 partly in the potential energy gain of ionizing the inner atomic shell. 

 Here specific heavy atoms may somewhat affect the probability of the 

 process, though, of course, the photoelectrons are responsible for the 

 great bulk of all energy transferred to the tissue. 



3. Up to about 20 mev. In this domain of gamma rays, high-energy 

 therapy machines, and the betatron, the principal transfer process is 

 the Compton process, in which the gamma ray is absorbed by an 

 essentially free electron, and both a recoil electron and a secondary 

 gamma emerge. The recoil electrons have a wide range in energy but 

 are invariably fast from the point of view of their stopping effects. Only 

 electron density counts in this region; no specific atomic effects are to 

 be expected. 



4. Above 20 mev, up to about 100 mev. Here the formation of positron- 

 electron pairs is more important than the Compton effect. This essen- 

 tially means that the quantum energy is converted to that of two elec- 

 trons, of widely distributed, very roughly equal energies. 



5. Beyond 100 mev. Here the cascade region is reached. A single 

 quantum absorbed will lead to a whole chain of new quanta and electrons, 

 dividing the energy up among many fast electrons in the end. The 

 spatial distribution of the energy transferred will be markedly different 

 from that in the other regions; in general the depth dose will exceed the 

 entry dose. On the cellular, fundamental level, of course, all these 

 radiations above the ultraviolet should have qualitatively similar effects : 

 those of fast secondary electrons. But much fundamental dosimetric 

 work still remains in the high-energy field, especially with multicellular 

 organisms. 



