156 Applied Biophysics 



which is not unexpected in view of the fact that the energy 

 involved in ionization exceeds the binding energy of an atom 

 in a molecule. This (approximate) result has been established 

 for substances in the solid, liquid, and gaseous states, and for 

 substances ranging in molecular weight from about 20 to about 

 20,000. There are some notable exceptions, but these are prob- 

 ably to be explained on the basis, on the one hand, of recom- 

 bination of the products of decomposition giving low yields, or, 

 on the other hand, of chain reactions giving enhanced yields. 



Many substances undergo chemical change when irradiated 

 in dilute aqueous solution. Among inorganic solutes, reducing 

 agents are oxidized, and oxidizing agents are reduced, while 

 organic solutes are usually eventually converted to CO2 and 

 hydrogen. These reactions in dilute aqueous solution take place 

 with doses of radiation much smaller than would be necessary 

 to produce the same percentage chemical change in the solute 

 if irradiated dry, and the number of solute molecules reacting 

 greatly exceeds the number of solute molecules directly ionized 

 by the radiation. Evidently, the ionization of the water is able 

 to lead to chemical change in the solute, and it is believed ^^ 

 that the explanation lies in the production of free H atoms and 

 OH radicals following the ionization of the water. 



Inactivation of Viruses 



Both the direct action of radiation, i.e., chemical change due 

 to ionization in the molecule concerned, and the indirect action, 

 i.e., chemical change in the solute molecules due to ionization in 

 the solvent, have been demonstrated in studies of the inactivation 

 of viruses by X-rays. Thus, in figure 1,^^ it is shown that in 

 sufficiently concentrated solution, the dose required to inactivate 

 a given percentage of a virus is independent of the concentration 

 of the solution, indicating that in such solutions the direct action 

 is predominant, but that in sufficiently dilute solutions, the dose 

 required to inactivate a given percentage of virus diminishes, 

 showing that in dilute solution, the indirect action predom- 

 inates. 



