CONTRIBUTIONS OF PHYSICS AND CHEMISTRY 335 



tion of positive ions, negative ions, and excited molecules, at the instant 

 of production and throughout their lifetimes, is not directly observable 

 but must be deduced by the methods described by our physics panel. It 

 is noteworthy that, along the ionization tracks, the activated molecules 

 tend to be formed in small clusters, and that, when the clustering effect 

 is smoothed out, the number of activated molecules produced per unit 

 length of track varies as the square of the charge of the particle and de- 

 creases as its instantaneous speed increases. Accordingly, with different 

 types of ionizing particles and different conditions of irradiation, the dis- 

 tribution of activated molecules along the ionization tracks may vary 

 widely. This factor, we shall see, influences most radiobiological actions 

 to a significant degree. 



Once the activated molecules are produced, how can they promote a 

 radiobiological action? We have no sure information about this, but 

 radiation chemistry provides certain facts, obtained by experiments on 

 non-living aqueous systems, which indicate some possibilities.* First, 

 it has been found that both water and certain solutes can be altered by 

 irradiation. Second, there is abundant evidence that, in many cases, 

 the solute does not need to be directly activated by the ionizing particles 

 but can be altered indirectly by reaction with activated water. In such 

 a case, if two or more reactive solutes are present, they compete for the 

 activated water molecules. This is usually called the protection effect. 

 Moreover, if only one solute is present, the number of molecules trans- 

 formed per unit of irradiation is independent of solute concentration. 

 This is known as the dilution effect. Third, there is evidence that H 

 atoms and OH radicals are rapidly formed from the original activated 

 molecules and enter actively into the mechanisms of radiochemical 

 action. 



The foregoing facts from radiation chemistry are confirmed by radia- 

 tion biochemistry. The protection effect and the dilution effect, as well 

 as evidence for the role of H and OH, have all been observed in irradia- 

 tion studies of the physiological inactivation of important biological sub- 

 stances, particularly enzymes, in aqueous solution. Moreover, there is 

 evidence that, in addition to the "indirect" action on these important 

 solutes through activation of water molecules (possibly followed by the 

 intervention of H and/or OH), there is sometimes a detectable "direct" 

 action initiated by activation of the solute molecules by the original 

 ionizing particles. These biochemical observations greatly encourage us 



* For reasons given elsewhere (3), it is assumed that radiation chemistry ahnost 

 certainly is a basic portion of a radiobiological action. However, Read (4) has re- 

 cently suggested a mechanism of chromosome breaks which includes no hypothetical 

 chemistry. 



