PHYSICAL PRINCIPLES OF CHEMICAL REACTIONS 197 



and an abnormally great density of excitation and ionization immediately 

 adjacent to it (Platzman, 1952a). 



2-3. THE PRIMARY PROCESSES OF RADIOBIOLOGY^ 



The same multiplicity of possible primary processes which may lead to 

 radiation effects in ordinary chemical systems exists also for primary 

 processes in biological systems. Analysis of which primary processes are 

 responsible for a particular biological effect depends intimately on the 

 nature of that effect, of course. Perhaps the cardinal feature character- 

 izing a biological system, insofar as possibly effective primary processes 

 are concerned, is the great size of the structural unit of the material (a 

 single cell in the case of certain studies of radiation killing; a single but 

 very great molecule in the case of radiation-induced gene mutation). 

 Very little work has as yet been accomplished in the examination of how 

 the existence of such large units (which, despite their great dimensions on 

 an atomic scale, have the unique biological degree of organization) must 

 change the analysis of primary processes. It appears that some radio- 

 biological changes may be effected by a single excitation, or by a single 

 ionization, in the unit (or at least by a very few of these processes). On 

 the other hand much evidence indicates (see, for example. Lea, 1946) that 

 some types of radiobiological effects are not caused by isolated excitation 

 or ionization acts. (The unit has such structural stability that it appar- 

 ently is able to recover from such events.) The best method of deaUng 

 with these effects has so far been an analysis in terms of several such 

 acts occurring within the unit (cf. Lea, 1947b) either because of high 

 "density of ionization" (cf. Sect. 4-6) or because of fluctuations from 

 the average spatial distribution of ionization. This is a crude approach 

 which might be justified, in some instances, and a very poor approxima- 

 tion indeed, in others. However, it is very difficult to improve it without 

 more detailed information on the structural properties of the biological 

 unit. The less common, but by no means rare, primary processes involv- 

 ing violent molecular alteration, described in Sect. 2-2, merit examination. 

 Other possibilities may be invoked in special examples; several are pre- 

 sented in Sect. 5-2. 



3. REACTIONS FOLLOWING EXCITATION 

 3-1. ELEMENTARY PROCESSES INVOLVING EXCITED ATOMS 



3-1 a. The Excitation Process; Energy Diagrams. An isolated atom can 

 exist only in certain well-defined states,^ the so-called quantum states of 

 its electronic system. Each of these states has its own characteristic set 

 of values of those properties of an atom which are well defined. Examples 



4Cf. Platzman (1952a). 



5 An elementary knowledge of atomic structure is presumed of readers of this article. 



