CHEMICAL SPECIES INDUCED BY X-RAYS IN CELLS 



103 



that hydrogen sulpliide can donate hydrogen atoms to a radical, or 

 some species, that has a very short lifetime, and that can very rapidly 

 become toxic to the cell in the absence of oxygen. This constitutes part 

 of the oxygen-independent portion of the general response. 



THE RADIATION SENSITIVITY PROFILE 



The studies described above enable us to construct a diagram inter- 

 relating the various kinds of damage we can infer from some of the 

 experiments to date. We call this the "radiation sensitivity profile" 

 (Fig. 6; Powers and Kaleta, 1960). The one we demonstrate in this 



O2 -independent-*' 

 3S7o 



la 



Oj- dependent 

 62% 



lb 

 NO 'after 



in 



H2S during Heat after 



n 



N2 during O2 during 



9-0 14 



-24%— l47o- 



Very short- 

 lived or 



'immediate" 

 species 



Short- 

 lived 

 radicals 



29-0 



38-0 



38% 



Long-lived 

 radicals 



24% 



Very short 

 lived or 



'immediate" 

 species 



— I-3IX^ 



2-60 X- 



4-22 X 



Fig. 6. — The radiation sensitivity profile of bacterial spores irradiated with soft X-rays 

 (18 keV mean) at about 16 kr/min at room temperature. On the heavy horizontal line 

 are indicated the inactivation constants observed under the described circumstances. 

 The Roman numerals are the designations for the various components of radiation 



injury. (From Powers and Kaleta, 1960). 



discussion is for X-rays of mean energy about 1 8 kV delivered at a dose 

 rate of approximately 16,000 r/min at room temperature. It is necessary 

 to specify these three items, since the relative response of each of the 

 classes diagrammed may change with changes in these variables. 



COMMENTS 



These and other experiments already performed or planned should 

 result in a partial understanding of some of the early events in this cell 

 caused by high-energy radiation. The experiments under way include 

 the effect of dose-rate, linear energy transfer, and the inteiTelation of 

 temperature with these. The very important problem of moisture is 



