BIOLOGICAL SYSTEMS 131 



meaning of a hit. Lea (28, pp. 66, 67) defines a hit as the production of 

 ionization within the target. From the point of view of our knowledge 

 of the elementary processes of radiation chemistry as they relate to 

 organic compounds such a definition appears unnecessarily restrictive. 

 Production of excitation within the biological particle, depending on the 

 locale of such excitation, can produce chemical change just as ultraviolet 

 radiation might. The number of such primary excitations within the 

 target always exceeds the number of primary ionizations. Thus, the 

 conclusion, based on preoccupation with ionization processes, that in 

 many cases a single hit is all that is necessary for production of a lethal 

 effect is fundamentally misleading and obviously in error. If our ideas 

 of the elementary processes of the radiation chemistry of organic com- 

 pounds are correct, such a computation emphasizes a fact not otherwise 

 apparent; namely, in the target theory there is no special virtue in a 

 single hit if the hit is presumed to be in the organic material itself. 

 Indeed, the best data so far used to support that notion offer clear 

 evidence that it cannot be correct for targets of the size assumed. 



On the other hand, there is almost inevitable formation of HO2 (via 

 reaction 3 or 19) in the ambient layer whenever primary ionization 

 occurs in the biological particle. Such HO2 may be more virulent in its 

 effects than either a primary ionization or a primary excitation. Under 

 such circumstance, we may expect that ionization deep within the 

 particle may be more probably associated with damage than would 

 similar excitation. 



A question that must arise regarding application of Fig. 1 to any 

 particular case concerns the volume ratio of regions A -{- B to C. This 

 is a matter regarding which the chemist certainly can make no general 

 statement. Another question concerns a possibility that this same ratio 

 may be a function of particle size ; for example, the larger the particle the 

 greater is the probability that a significant portion of it (that is, C) is 

 effectively inert to the radiation. From the purely chemical viewpoint 

 such an assumption may be justifiable in a homologous series. I am 

 unable to judge its worth biologically. However, it may be profitable 

 to examine the more obvious consequence of this assumption. That 

 consequence simply is that the larger the particle the greater becomes 

 the probability of an ineffective hit. The further consequence is that 

 with large particles an increasingly large negative deviation of the 

 calculated size (as determined from naive target theory) from the true 

 geometric size (as determined from electron diffraction microphotographs 

 of the dry material) is to be expected. 



The significance of elementary processes in radiation chemistry for 

 radiobiological reactions can be summed up by consideration of "target 

 size" as affected by each one of the elementary processes. 



