CHEMICAL SPECIES INDUCED BY X-RAYS IN CELLS 



95 



point, radiation sensitivity increases slowly to a maximum at 30°C 

 (303°K) (Fig. 2). It tluMi decreases markedly, and reaches a minimum 

 at 80°C at a level that is appreciably below that ol)served even at the 

 very lowest temperatures. While it is not ])ossible to interpret these 

 results in terms of free radicals from this evidence alone, we shall see 

 that this is consistent with and contributes to, the free-radical hypothe- 

 sis. The marked inversion of radiation sensitivity at the higher tempera- 

 tures is the consequence of annealment of free radicals. The effect of 

 post-irradiation exposure of the spores to heat is described in the next 

 paragraph. 



n 

 O 





o 



u 



c: 



g 

 +-> 

 o 

 > 



u 

 o 



50 100 



150 200 250 

 Temperature 



Fig. 2. — The relationships among radiation sensitivity of spores (the ordinate), tem- 

 perature during irradiation (the abscissa), and post-irradiation thermal and NO treat- 

 ments. (From Powers et al., 1960a). 



Nitric oxide 



In another series of experiments, w^e used the gas nitric oxide as a 

 modifying agent (Powers et al, 1960b). This gas is most effective when 



