60 RADIATION BIOLOGY 



living matter. What general information does physics contribute regard- 

 ing the nature and the production of such disturbances by radiation? 



We may think of two kinds of permanent or semipermanent changes 

 in the structure of matter, namely, "chemical" changes in which some 

 atoms are regrouped to form different molecular or colloidal aggregates, 

 and "nuclear" changes in which the nuclear constituents are regrouped 

 to form different nuclei. Chemical changes do not affect the over-all 

 crude chemical composition of a material, but nuclear changes do. 



Some changes of structure release energy, others require a net addition 

 of energy. In either case, however, a structural transformation can 

 proceed only through an intermediate structure whose energy is con- 

 siderably higher than that of both the initial and the final structure. 

 Therefore a rapid transformation must be "activated" by a temporary 

 loan of energy. Radiations act on matter, in the main, just by carrying a 

 flow of energy and distributing it so as to produce a large number of 

 activations. 



Chemical changes require activation energies of the same order of 

 magnitude as the energies of chemical bonds, which is also the order of 

 magnitude of the binding energies of external atomic electrons. The 

 bond energies range from about 1 to about 10 ev. The activation energy 

 is effectively supplied to an atom within matter by shaking the atom as a 

 whole or, more frequently, by shaking its external electrons. 



An excitation of external electrons brings about a change of the 

 chemical forces which bind an atom within a molecule. This change of 

 forces may or may not lead to a regrouping of the atom in a different 

 molecular arrangement. Thus the excitation of external electrons 

 appears as an indirect method of inducing chemical changes. (The 

 excitation of internal electrons does not even influence chemical bonds 

 directly.) 



Excitation of the motion of an atom with respect to its neighbors by a 

 few electron volts constitutes a more direct way of tearing up chemical 

 bonds. However, whole atoms, because of their high mass, respond to 

 the action of a radiation much less easily than electrons (see, however, 

 Sects. 3-lb and c). 



The chance of inducing a chemical change improves if an atom receives 

 an amount of activation energy somewhat in excess of the minimum 

 requirement. However, a large excess of activation energy increases the 

 chance of a chemical change no further because the excess energy leaves 

 the atom in short order, for example, through the emission of an electron. 



The actual occurrence of one or another chemical change after activa- 

 tion energy has been provided raises problems of molecular mechanics 

 which are treated in later chapters of this collection. Here we are con- 

 cerned with the distrihntion of activation energy, both as regards its mecha- 

 nism and its eventual location in space. 



