258 



RADIATION BIOLOGY 



solute. Thus the original concept of the action of ionizing radiations — 

 direct dissipation of energy within a molecule (caused by collision of an 

 ionizing particle with the molecule), i.e., the ''target theory" — was sup- 

 plemented by the "indirect action" theory. 



THE INDIRECT ACTION THEORY 



The indirect action of radiation on aqueous solutions will be discussed 

 in some greater detail, starting with two typical examples. 



If a solution of ferrous sulfate is irradiated (Fricke and Morse, 1929) an 

 oxidation of the ferrous sulfate to ferric sulfate occurs. From Fig. 4-1 it 



10 



40 



20 30 



DOSE, orbitrory units 



Fig. 4-1. The action of different doses of X rays on 0.00878 M ferrous sulfate in 

 0.8 N sulfuric acid. {Fricke and Morse, 1929. Reproduced by pertnission of the 

 authors.) 







0,000 12,000 



2000 4000 6000 8000 

 DOSE, r 

 Fig. 4-2. Exponential relation between degree of inactivation and X-ray dose for 

 carboxypeptidase (C.P.) solutions. {Dale, 1940. Reproduced by permission of the 

 editors of the Biochemical Journal.) 



can be seen that the unchanged fraction of the initial concentration of 

 ferrous sulfate is a linear function of the X-ray dose. This is a case of the 

 simple oxidation of a solution of an inorganic substance. If instead of an 

 inorganic solute the chemical change of an organic compound is investi- 

 gated, the unchanged fraction of the initial concentration is, in general, 

 an exponential function of the X-ray dose. This can be seen from Fig. 

 4-2 in which the activity of the crystalline enzyme carboxypeptidase is 



