PRIMARY EFFECTS (ON CHEMICAL COMPOSITION) 241 



As more is learned about effects of ionizing radiation on metabolic proc- 

 esses, physiological measurements of effects on rates of specific processes 

 within cells and tissues will probably add much-needed refinements to the 

 useful LD 50 number. 



Incidentally, one should realize that only a small amount of energy need be 

 absorbed to cause damage. It is the form in which this energy enters the tis- 

 sue that is critical. Thus the LD 50 for man, 400 rads, is only 400 x 100 

 ergs/g. This is 0.001 cal/g, roughly enough energy, if in the form of heat, to 

 raise the body temperature only 0.001 deg! Because the energy is concen- 

 trated in packets, so that when it is absorbed it tears apart the molecules 

 of important biological structures, localized damage occurs at sensitive sites, 

 enabling a small quantity of energy to promote death. Table 9-3 gives some 

 useful irradiation data and conversion factors. 



TABLE 9-3. Some Useful Numbers. 



1 rad = 100 ergs/ gram = 6.24 x 10 13 electron volts/gram. 



1 roentgen of hard X or 7 delivers 0.98 rad to water. 



1 curie of radioactive substance delivers 3.7 x 10'° disintegrations/second. 



1 curie of Co 60 gives a dose rate of 1 .35 roentgen/hour at 1 meter from the source. 



1 curie of radium gives a dose rate of 0.83 roentgen/hour at 1 meter. 



1 curie of cesium 137 gives a dose rate of 0.33 roentgen/hour at 1 meter. 



PRIMARY EFFECTS (on Chemical Composition) 



Direct and Indirect Action 



Two schools of thought have arisen on the question of how the primary 

 effects occur. However, there are so many variables involved that it is un- 

 likely that either will ever be proved to be unequivocally wrong. 



The fact is that the solution after irradiation contains molecules (chemi- 

 cals) which were not there before irradiation. One school maintains that 

 this is because the solute dissolved in the water acted as a target, was blown 

 apart by the incoming "bullets," and the fragments rearranged into a new 

 molecule. The other school remembers that the whole target (tissue, for 

 example) can be at least 80 per cent water, that eight out of ten potential 

 targets are water molecules, and maintains that the primary act is the ex- 

 citation of water, followed by its decomposition into the active chemicals 

 hydrogen atom and hydroxyl radical. Enough energy is left over so that 

 these are thrown violently apart. Hydrogen is a reducing agent, which can 

 donate an electron to become H + in solution; OH is an oxidizing agent 

 which can accept an electron to become OH in solution. From this view, 

 then, these molecular fragments, H and OH, cause the formation of new 



