BIOLOGICAL SYSTEMS 



133 



2. Free radicals as well as sharp variations of pH in the region D, 

 both resultant from hits in that region, ionization transferred to that 

 region, or electron capture therein, may (but do not necessarily) produce 

 a lethal effect in the region A. The effect is to make the computed 

 particle size larger than the geometric size. This contribution of the 

 region D is shown cjualitatively by the difference between lines 1 and 2 

 in Fig. 3. The probability of lethality is shown by Fig. 4. 



-r r 



Distance 



Fig. 4. Lethality of a hit as a function of distance from center of target on basis of 

 various modifications of target theory. 



3. The existence of a region C decreases the biologically computed 

 size below the geometric size. If the ratio C/(A + B) is constant, line 

 3 in Fig. 3 shows this effect. If the ratio increases with size, line 4 shows 

 the effect. The probability curve now has no simple shape but depends 

 on the distribution of C relative to A and B. If the ratio C/(A + B) is 

 a constant greater than zero and C is isotropically distributed, the 

 probability of lethality of a hit within the particle is less than unity and 

 the effectiveness of a hit within the diffusion distance in the ambient 

 layer T> may be decreased. The effect is shown roughly by line 2 in 

 Fig. 4. 



If the ratio C/(A + B) increases with geometric size, the probability 

 of an effective hit within the biological particle simply decreases with 

 size. This statement means that for a large particle the plateau of line 

 2 in Fig. 4 would be lower than for a small particle. 



4. The region A is confined to the surface of the particle. A hit within 

 that region gives a rupture (or other decomposition) and the cage effect 



