62 RADIATION BIOLOGY 



It may be asked whether the chemical and biological effects of any 

 radiation could be regarded as thermal effects taking place after the 

 radiation energy has already been degraded to the thermal range, i.e., 

 into "heat," though not quite down to the ambient temperature (see, 

 for example, the concept of "point heat" introduced by Dessauer, 1923). 

 The answer seems to be negative because the heat conduction, away from 

 the small region where a packet of radiation energy was absorbed and 

 degraded, takes place much faster than a thermochemical reaction. 



Nuclear changes can be produced by radiations of very high potency 

 and also by slow neutrons, since neutrons always end up by being 

 attracted and captured by some nucleus (except for extremely rare dis- 

 integrations; see Sect. 1-lb). These changes do not depend on the 

 extranuclear state of aggregation of the material but take place with 

 unimportant exceptions as though each nucleus were free (see Sect. 2-5). 

 A fairly extensive production of nuclear changes results from the action 

 of neutrons in nuclear reactors (see Sect. 1-3) and also from the action of 

 high-potency radiations. 



However, the direct biological significance of the nuclear effects of 

 radiations is normally overshadowed by the simultaneous and far more 

 extensive occurrence of chemical changes. Accordingly, the production 

 of chemical activations calls for a more detailed discussion than the 

 production of nuclear changes. 



The following sections describe the chains of elementary processes 

 through which ionizing radiations exert their "chemical" action. The 

 action of nonionizing radiations is the object of a few additional notes. 



3-1. ACTIVATION BY CHARGED CORPUSCULAR RADIATIONS 



The over-all action of high-speed charged particles upon a material 

 consists, in the main, of a succession of "fast" inelastic collisions (see 

 Sect. 2-4). The atoms and molecules along and around the path of each 

 incident particle react to the passage of the particle in a manner essen- 

 tially independent of the presence of adjacent atoms. A very large 

 number of inelastic collisions takes place along the path of a charged 

 particle. The energy of the particle is thus progressively used up and 

 transferred to various atoms and molecules within the material. 



A large number of the collisions experienced by a fast charged particle 

 may be regarded as "glancing collisions" affecting the external electrons 

 of atoms (see Sects. 2-4a and c). The energy transferred in each of these 

 collisions is of the order of magnitude required to produce chemical 

 activations, or slightly in excess of that magnitude. 



Therefore glancing collisions against external electrons afford a mecha- 

 nism for degrading high potency radiation directly down to the chemical 

 range. In fact, these collisions constitute the main mechanism of chemical 

 activation by ionizing radiations because, as we shall see, all other 



