PHYSIOLOGICAL EFFECTS 



249 



into a modern version of the Law of Bergonie and Tribondeau, and write, as 

 a memory aid: 



\dN/dt] e ,<W > oc, dD/dt, [0 2 ], [NO], rbe' 



/ 



m, [P] 



Survival studies have been pursued vigorously in the past few years. The 

 exponential decay law N = N e~ aD is followed rigorously by irradiated 

 haploid (simple-chromosome) yeast cells — linear portions on Figure 9-7. In 

 this case a has a value (Table 9-4) of 17.2 x 10 -5 rads -1 at a dose rate of 

 425 rads/min, with the oxygen concentration equilibrated with air. The 

 value of a drops rapidly as the water of the medium (and hence in the cell) 

 is partially replaced by such materials as glycerol. Furthermore, the sen- 

 sitivity does not change down to -10°C, but drops to 4.9 x 10~ 5 when 

 the solution freezes. By way of contrast, bacterial cells are about 100 times 

 less sensitive than human cells to irradiation (Table 9-4), but eventually 

 show the membrane rupture and internal reorganizations of all others 

 (Figure 9-8). 



TABLE 9-4. Some Measured Fractions Killed per 

 and the Corresponding LD 50 's. 



Rad (i.e., the Radiation Sensitivity, a) 



Note: a ■ LD, n = 0.693; a = -2.303 



'50 



dD 



In contrast to this simple, first-order law, it has been found that if chromo- 

 somal material is present in quantities which are multiples of some basic 

 unit (diploids, tetraploids, etc.), the rate of destruction of cells by irradia- 

 tion is proportional to some power (of the number of cells, N) different from 



