244 EADIATION BIOLOGY 



such a possibility is discussed in Sect. 5-2 in terms of nonlocalized action 

 of ionizing radiation. 



Moreover, the local heating produced when densely ionizing particles 

 penetrate matter should not be overlooked in an analysis on the basis of 

 even the simplest reaction kinetics. This "heating" is actually an 

 extremely complex phenomenon, of course, for the "heated" region is in 

 any transient state a system intermediate between one in thermal 

 equilibrium and a large molecule the vibrations of which are highly 

 excited. It will certainly influence elementary processes which occur 

 during a certain period following passage of the primary particle. Just 

 how to treat such effects is not known, but it is certainly not justifiable to 

 ignore them — as some investigators have done, for example, in consider- 

 ing the reaction of two OH radicals in the "track" of an a particle in 

 water to proceed at a rate corresponding to room temperatures. 



5. GENERAL REMARKS ON APPLICATIONS TO BIOLOGY 



5-1. INDIRECT CHEMICAL EFFECTS OF HIGH-ENERGY RADIATIONS 



ON BIOLOGICAL MATERIAL 



The action of high-energy radiations on biological material has, in 

 many cases, been shown to be either in whole or in part an indirect one. 

 Many examples of such type have been observed — for instance, by 

 irradiation of cells, viruses, enzymes, etc., in aqueous suspension under 

 conditions where transfer of energy from the high-energy radiation is 

 predominantly to molecules of water (cf. Sect. 1-4). 



In the case of pure water,-" this energy transfer results in the formation 

 of hydrogen peroxide at a rate which depends on the concentration of 

 molecular oxygen dissolved in the water. If biological matter is sus- 



20 The effects of various high-energy radiations on water all appear to stem from 

 H and OH radicals formed in primary processes , both from dissociation or predissoci- 

 ation following excitation, and from ionization, as explained in Sect. 4-4. The actual 

 final products, which may include Ho, O2, and H2O2 in the case of pure water and 

 various further possibilities in the case of solutions, and the efficiencies with which 

 they are produced, are determined by the intricate kinetics according to which the 

 primary products recombine, diffuse, react, etc. They depend chiefly on the type of 

 radiation, which governs the density of ionization, on the temperature, and also on the 

 actual purity of the water, and, in principle, on the irradiation intensity. Thus, in 

 the case of a-particle irradiation, many of the OH radicals are produced in regions of 

 high transient local concentration and temperature; the combination of two OH radi- 

 cals to form H2O2 is therefore highly favored, and H2O2 is indeed an abundant product. 

 In the case of /3-particle or 7-ray irradiation, most of the radicals are produced in low 

 local concentrations, general diffusion is favored, and the recombination reaction 

 H + OH -* H2O dominates; thus there is little or no permanent effect. If dissolved 

 air or oxygen is present, however, the very probable reaction H + O2 -^ HO2 in 

 effect "stabilizes" the radicals and makes possible the over-all formation of H2O2 in 

 subsequent stages. 



