90 BIOLOGICAL EFFECTS OF RADIATION 



presence of these ions is apt to produce chemical changes or electrical 

 disturbances which must affect biological processes. Therefore, a knowl- 

 edge of the ionization in the living organism which one is irradiating, is 

 the sine qua non of any quantitative experimental study. Accordingly, 

 it is desirable to examine this question in detail. 



It has already been mentioned that the secondary beta particle '(fast 

 moving electron) which is set in motion by the photon, removes electrons 

 from some of the atoms in its path. The beta particle is deflected some- 

 what at each encounter and follows a zigzag course. In an organic 

 medium it is apt to encounter the different complex molecules in any 

 orientation and therefore may remove an electron from any one of the 

 component atoms — hydrogen, oxygen, nitrogen, etc. Similarly, the 

 electron which has thus been removed from one atom, in attaching itself 

 to a molecule may adhere to any one of the component atoms. On this 

 basis there would be in the material molecules with unbalanced positive 

 or negative charges at different points in their configurations. The 

 resultant chemical changes would then depend, to some extent at least, 

 on the orientation of the two molecules at the time they collided. Accord- 

 ingly, recombination of ions, which, in effect, means neutralization of 

 charges, need not always result in a rearrangement of the atoms in the 

 molecules involved. When readjustments do occur, the resultant 

 molecules need not necessarily be the same, and therefore more than one 

 chemical change is apt to take place. According to this view, only part 

 of the total ionization produced in the material would be chemically, 

 and ultimately biologically, effective, first, because not all the ion recom- 

 binations would result in chemical changes, and further because not all 

 the possible chemical readjustments would necessarily influence life 

 processes. 



DISTRIBUTION OF IONS IN AN IONIZED MEDIUM 



It is obvious that the time rate of recombination of ions depends on 

 their space concentration. The more ions of opposite polarities there are 

 in a given space under given conditions, the more rapidly positives and 

 negatives will come together. Since ions are produced in pairs by radia- 

 tion, the concentration of positives and negatives is the same, and the 

 rate of recombination is proportional to the square of the number of 

 ion pairs present at any time. Other conditions being equal, the number 

 of ion pairs per cubic centimeter produced per second is proportional to 

 the intensity of the radiation. Therefore, recombination should take 

 place more rapidly the higher the intensity of the radiation. This would 

 certainly be true if the ions were distributed perfectly uniformly through- 

 out the material. In a homogeneous medium which is uniformly irradi- 

 ated the ions may be considered to be uniformly distributed, but only in 



