134 



ELEMENTARY CHEMICAL PROCESSES 



is nil. On the other hand, in B there is a cage effect and that effect 

 becomes bigger the larger the size of the particle. This effect is also 

 shown by a line like 4 in Fig. 3. The probability of lethality of a hit is a 

 maximum in or near the surface. The effect is shown by line 3 in Fig. 4. 

 The conclusion from these considerations is that the relationship 

 between biologically computed target radius and geometric radius even 

 for a spherical particle is not simple. For real particles of such ideal 

 shape the qualitative nature of the relationship must be as shown in 

 Fig. 5. The fact that the computed radius is so nearly like the geometric 



/ 



/ 



A 



Geometric radius 



Fig. 5. Relation between effective radius of target and actual dimensions of biologi- 

 cal entity. 



radius transpires to be an interesting consequence of the elementary 

 processes involved in radiobiological reactions. It is a relationship 

 which has, for some, emphasized the naive features of target theory and 

 really beclouded the processes involved. The happy fact is that investi- 

 gators in the field were actually not led astray in spite of the terminology 

 employed. Lea (28) himself emphasized the lack of a definite boundary. 

 In a study of the eft'ect of deuteron bombardment on bacteriophage, 

 Pollard and Forro (29) have shown that a target exists which is smaller 

 than the phage itself but that nevertheless the computed target size is 

 increased because a deuteron whose path actually misses the phage can 

 nevertheless inactivate it. 



In conclusion one fact not heretofore mentioned, so far as I know, 

 bears repeated emphasis. Free H and resultant HO2 are very probably 

 produced in the ambient liquid around a particle even when the hit, as 

 in x-irradiation, may be directly and exclusively in the particle. It is 



