194 RADIATION BIOLOGY 



1-3. RADIATION-CHEMICAL REACTIONS 



The difference in the nature of energy absorption by a material medium 

 from a beam of light, as in photochemistry, and from a beam of high- 

 energy radiation, as in radiation chemistry, is such as to obliterate the 

 two great simplifying features of photochemistry described above. 



In the first place, since the energy of a single particle of the radiation 

 is very great compared to probable excitation energies of the molecules 

 of the medium, the energy transfer occurs not in a single step, but in a 

 great number of them. Thus not a single one of the possible excited 

 species, but invariably a great variety of them, will be produced by each 

 particle. No selectivity is possible. And among the various excited 

 species there will commonly be many, the detailed physical nature of 

 which is not known. In addition to the excited molecules, some, and 

 usually many, different species of ionized molecules will also be present. 

 Thus, in contrast to photochemistry, a complete enumeration of the num- 

 ber and nature of the primary products of irradiation is not possible, and 

 analysis must proceed on less specific, and therefore less satisfactory, 

 terms. In partial compensation for this state of affairs, the very lack of 

 specificity leads to a general similarity (although not identity) between 

 different forms of high-energy radiation in so far as the types and relative 

 numbers of the different species produced are concerned. 



The second great intrinsic difference between radiation-chemical and 

 photochemical reactions lies in the spatial distribution of the primary 

 products. Energy transfer from high-energy radiation to matter 

 devolves on the energy loss of swiftly moving charged atomic particles as 

 they penetrate the medium. The excited (or ionized) molecules are 

 formed more or less close together along particle ''tracks." Even along 

 a track the distribution varies markedly both laterally and longitudinally. 

 This departure from the uniformity of a thermal reaction or even the near- 

 uniformity of a photochemical reaction completely changes the nature of 

 the initial chemical kinetics of the reaction. No really adequate theoret- 

 ical treatment of these kinetics has been developed as yet/ although 

 beginning attempts have been made (Lea, 1947a; Magee, 1951). The 

 difference in spatial distributions of primary products produced by var- 

 ious forms of high-energy radiation is the chief factor responsible for the 

 difference in ultimate chemical effects they may produce. 



1-4. RADIOBIOLOGICAL REACTIONS 



Effects of high-energy radiation on biological materials must derive 

 from primary effects superficially similar to those with simple chemical 



3 The problem is related to that of ret-ombination along the tracks of densely ionizing 

 radiation such as a particles. The latter problem was originally treated many years 

 ago by Jaffe. It has recently been studied by a number of investigators (cf., for 

 example, Gerritsen, 1948). 



