BIOLOGICAL SYSTEMS 125 



The fate of the electron in an aqueous system is determined by the 

 electron affinity of the various species present. Magee and Burton (18) 

 have shown that the electron must, with high probability, be thermalized 

 before it can be captured. Furthermore, the threshold energy for capture 

 must be practically zero in order that negative-ion formation compete 

 effectively with capture by positive ions. The threshold energy for 

 capture by water is practically zero; the electron affinity of the hydroxyl 

 ion plus its heat of solvation makes the reaction 



H2O + aq + e -> H + OH--aq (3) 



of significant importance. In general, it is improbable (though not 

 impossible) that some dissolved or suspended species can capture thermal 

 electrons to form negative ions. Since water preponderates, we may 

 once again expect that on this basis alone it will provide the major trap 

 for free thermal electrons. Possibility of the competitive reaction 



H3O+ + e -^ H + H2O (19) 



must not be ignored. However, its effect, like that of reaction 3, is to 

 yield free hydrogen atoms in the ambient layer and to increase the local 

 pH of the solution. / 



Biological Systems 



The major conclusion from our brief consideration of elementary 

 physical processes is that in aqueous systems of interest to radiobiologists 

 the initial radiation chemical processes are much the same as they are in 

 pure water. We may now review the major important features. 



DISTRIBUTION OF EFFECTS 



For the most part the radiant energy affects the water itself. The 

 first chemically important elementary processes are reactions 1, 2, and 

 3. Reaction 3, or tts equivalent {19), ynay occur in the aqueous layer even 

 when the primary effect of the radiation is in the biological particle itself. 

 The distribution of primary products depends on the nature of the 

 incident radiation. For particles of the same charge and energy, velocity 

 and mass are related by the expression 



velocity oc mass~^-^ 



The frequency of production of excited molecules or ions by action of 

 the impinging particle is inversely related to its velocity. Thus, slow 

 particles produce reactions such as (1) relatively close together, whereas 

 fast particles tend to produce a relatively isotropic distribution of free 

 hydrogen atoms and hydroxyl radicals. High-energy gamma and x-radi- 



