126 ELEMENTARY CHEMICAL PROCESSES 



ation (which produce Compton recoils) and electrons are included among 

 the fast group. On the other hand, alphas, deuterons, and protons of 

 energies usually employed belong in the slow group. Since in hydroge- 

 nous material the major effect of an incident neutron is the liberation 

 of energetic protons, the neutron may also be placed in the slow group. 

 In summation, slow particles produce relatively high densities of 

 ionization and of hydroxyl radicals along the ion track. A beam of fast 

 particles produces a relatively isotropic distribution of a mixture of free 

 hydrogen atoms and hydroxyl radicals. 



pU. EFFECTS 



It may be remarked also that pH is markedly changed in the neighbor- 

 hood of an ion track. In the track itself pH decreases; in the envelope 

 immediately surrounding the track pH increases. The pH values locally 

 attained may be very much less or very much greater than those which 

 are common to biological systems. In the neighborhood of the track of 

 a heavy particle this effect may be very much exaggerated. In a private 

 discussion with the participants in this symposium Franck has pointed 

 out that since biological systems are notoriously sensitive to variations 

 in pH this pH effect itself can have profound significance for radiobiology. 



EFFECT OF OXYGEN 



It is usually maintained that in the radiation chemistry of aqueous 

 systems the principal competitive processes are the diffusion of free H 

 and OH toward each other and their diffusion toward the other reactive 

 species present. Biological systems are marked principally by the 

 presence of impurity, the outstanding example of which is oxygen. 

 When the latter is present, an important fate of the free H atoms is the 

 formation of hydroperoxyl radicals, HO2, as by reaction 16. Although 

 these radicals may themselves react with free OH, reaction 15, we may 

 expect that such a process will have a distinct activation energy and 

 steric factor. Meanwhile, since both H and H2 concentrations are 

 reduced, probability of reactions 8 and 9 decreases. The effect of the 

 presence of oxygen is thus not only to convert a reducing agent, free H 

 atom, into a persistent radical, HO2, but at the same time to increase 

 the life span of free OH radicals. It must he emphasized that reaction 3 

 or 19 very probahly occurs in the water even when the primary ionization 

 effect of the radiation is on the biological material. Thus, there is a high 

 prohahility, if oxygen is present in the system, that the entity HO2 will he 

 formed sufficiently close to a biological particle to have a chemical effect on 

 it even when the initial ionization does not. In such case, ionization of the 

 biological particle is effectively destructive, whereas excitation may not he. 



