L. H. GRAY 



rate in that the duration of exposure has no influence on the degree of bio- 

 logical damage sustained by roots exposed to less than about 50 rad. The dose 

 required to produce three different levels of biological damage is shown as a 

 function of duration of exposure in curves A, B, and C, oi^ Figure 4 (b). A 

 logarithmic plot of minimum growth rate relative to that of control roots 

 (Figure 5) shows that the dose-response relationship is exponential in the case 

 of 12- and 24-hour exposures, but sigmoidal for shorter exposures. The 

 shapes of the sigmoidal curves for both X and y radiation are dose-rate 

 dependent, but the exponential curve is dose-rate independent, as would be 

 expected for an injury indiiced by a single particle. 



10 



Figure 4. 



100 200_ 300 AOO 500 600 rad. 



Dose 



— A 08 



B 0-5 



0-2 



U 8 12 16 20 2A 



Duration of exposure hr 



Influence of duration of exposure on y-ray 

 damage to roots 



The range in duration of exposure studied in biological experiments has 

 generally not been large enough to reveal the transition from dose-rate 

 dependence to dose-rate independence illustrated in Figure 4 (b). In view of 

 the particle nature of ionizing radiation, dose-rate independence is, however, 

 to be expected on physical grounds for the reasons given earlier whenever cells 

 are exposed to sufficiently small doses at low dose rates. The doses and dose 

 rates to which living organisms are exposed on the count of background 

 radiation (0 • 1 rad in a year) would be likely to fall within the dose-rate 

 independent range. 



When two different qualities of radiation, given at the same dose rate, are 

 compared, it is frequently found that the doses required to bring about the 



7 



