372 



ENERGY LOSS AND BIOLOGICAL EFFECTS 



workers (40) have applied the target hypothesis to this problem. More 

 quantitative experiments are needed to uncover the mechanism of action 

 of poisonous agents applied externally to living cells. 



Diffusion Model and Dependence of Biological 

 Effects on Specific Ionization 



The above considerations introduce two additional constants in the 

 radiation survival formulas. One reason for allowing these additional 

 constants in the formulas was to allow variation of the biological effec- 

 tiveness of radiations at different environmental conditions and at 



B, 



D, 



Cue -OHe PO «! Po a^ 



Fig. 8. Experimental data on the relative biological effect of various radiations on 

 haploid and diploid yeast cells. Along the horizontal axis the rate of energy loss is 

 plotted, and along the vertical axis the relative biological effect refers to 200-kev 

 x-rays. Four experimental points were obtained with high-energy deuterons, two 

 with high-energy helium ions, and two with polonium alpha particles. [From (17).] 



different mean rates of energy loss. From data obtained under widely 

 varying conditions, one may possibly determine the constants jS, p, a. 

 As an example of the type of data one obtains, consider the comparison 

 of the relative effectiveness of deuteron ions, x-rays, helium ions, and 

 low-energy alpha particles on yeast cells, as shown in Fig. 8. The curves 

 show an increase in relative biological effectiveness paralleling an 

 increase in rate of energy loss of the radiations used. These data them- 

 selves are not striking, since the majority of the experiments available 

 show such an effect. 



