122 RADIATION BIOLOGY 



radiation seems to afford an attractive opportunity for studies of the 

 kinetics of radiation action. Possibly, biological phenomena which dis- 

 play, at least on the surface, an "all or none" character appear to be of a 

 more fundamental nature or more readily accessible to a mechanistic 

 interpretation. Be that as it may, the following sections deal almost 

 entirely with macroscopic effects whose magnitude is indicated by the 

 frequency of some "event." 



Chemical effects of radiation also belong to this class of macroscopic 

 effects, since the yield of a radiochemical reaction represents the number 

 of molecules which have experienced a certain well-defined transforma- 

 tion. In other words, a chemical substance exposed to radiation may 

 be properly regarded as a population of molecules. 



In most instances, when a test population, whether of organisms or of 

 molecules, is exposed to radiation, only a minute fraction of the physical 

 action of the radiation serves to affect the elements of the population. 

 For example, when fruit flies are irradiated, their genetic material absorbs 

 a negligible portion of the radiation. Under these conditions the fre- 

 quency of "events" resulting from the treatment is determined only by 

 the radiation dose and not by the magnitude of the genetic population 

 treated and tested. The number of mutants detected in the offspring of 

 treated organisms is proportional to the number of offspring tested, to 

 within the unavoidable statistical fluctuations. 



However, in some instances, when an excess of material is exposed, some 

 elements of the population treated shield other elements of the population. 

 For example, if a suspension of bacteria exposed to ultraviolet light is 

 very thick, a further increase of the number of bacteria exposed may 

 result in a reduction of the hght absorbed by each bacterium. Allow- 

 ance must, of course, be made for this effect when it occurs. In extreme 

 cases, the radiation dose determines the total number of "events" pro- 

 duced in a population rather than the per cent of the population affected. 



This phenomenon occurs particularly in a number of radiochemical 

 reactions produced in aqueous solutions. Even though most of the radia- 

 tion energy is dissipated in the water, the resulting chemical action 

 appears to be channeled automatically toward the solute molecules. 

 For example, a dose of 20,000 r of X rays liberates 0.00005 mole of hydro- 

 gen per Hter of formic acid solution, whether the concentration of formic 

 acid is 0.1 or 0.0001 M (Fricke, Hart, and Smith, 1938). 



A similar phenomenon of channeling of radiation action occurs in cer- 

 tain fluorescent materials. A large fraction of the energy dissipated may 

 reappear in the form of hght radiated by special atoms or molecules which 

 constitute only a minute proportion of the material. 



These examples indicate that the dependence of a macroscopic effect 

 upon the concentration of the reacting material may provide important 

 information on the mechanism of action. 



