IONIZATION AND BIOLOGICAL EFFECTS 91 



the macroscopic sense of the same number per unit volume. If the ion 

 distribution could be viewed with a microscope, it would be found 

 that the ions are really concentrated along certain tortuous lines which 

 mark the paths of the secondary beta particles. Accordingly, there are 

 islets in the material in which at any one time no ions are produced. 

 However, since the point at which a secondary beta particle is liberated is 

 merely a matter of chance, the probabihty of ionization of every molecule 

 in a homogeneous medium is the same. Therefore, all the molecules may 

 be ionized at one time or another in the course of an indefinitely long 

 exposure, or during a finite exposure to an extremely high radiation 

 intensity. 



In addition to the special concentration of the ions along certain lines 

 in the material, we must consider also the distribution along these lines. 

 It is well known that the number of ions produced per centimeter of path 

 of particles traveling at high speed depends on the speed of the particles. 

 In fact, it has been determined experimentally that the greatest concen- 

 tration of ions occurs near the end of the path, when the speed of the 

 particle is relatively low. It follows from the foregoing considerations 

 that the ions in a homogeneous medium, uniformly irradiated, are con- 

 centrated largely within very small regions interspersed throughout the 

 material. 



Living tissue, considered on the submicroscopic scale, with which we 

 are dealing, is not homogeneous. Its atomic components are largely 

 hydrogen, oxygen, nitrogen, and carbon, but there are also present in 

 small or minute quantities elements of higher atomic numbers, such as 

 iron or calcium. The transformation of electromagnetic radiation 

 (photons, cf. K. K. Darrow, Paper I) into high-speed electrons by which 

 ionization is produced, depends very largely on the atomic number of the 

 element in the path of the radiation. The higher the atomic number, 

 the larger by far the amount of energy abstracted from the radiation and 

 the larger the resultant ionization. It follows, therefore, that in a living 

 tissue, the ionization will be most intense in the neighborhood of the 

 molecules containing elements of higher atomic numbers. This results 

 in further localization of the ionization. 



Recombination, which is influenced markedly by the number of ion 

 pairs per unit volume, is much more rapid than it would be if all the ions 

 were evenly distributed throughout the material. When the intensity 

 of radiation is increased, the total number of ions produced per second in 

 a given material is increased in the same proportion. This increase is 

 brought about by a proportional augmentation of the number of the beta- 

 particle tracks, which we may call the ionization loci. The distribution of 

 ions at each locus remains the same as before, unless perchance two of the 

 loci coincide wholly or in part. In liquids or solids, where the mean free 



