106 BIOLOGICAL EFFECTS OF RADIATION 



practical purposes, it is not advisable to push the filtration much beyond 

 this point. The secondary radiation emitted by the filter in the case of 

 gamma rays is very troublesome. It includes electrons which may travel 

 at a speed approaching that of the primary beta rays. They are pro- 

 duced, of course, by photoelectric or Compton collisions as in the case of 

 X-rays, but they travel much faster because the energy of the gamma 

 photons is much higher. Since the ionization produced in a given length 

 of path is very much greater in the case of a beta particle than in that of a 

 photon of equal energy, the secondary electrons emitted by the filter 

 may produce marked spurious effects. In biological experiments it is 

 very desirable to have an organic material several millimeters thick as a 

 secondary filter. 



ESSENTIAL FACTORS FOR THE CORRELATION OF IONIZATION AND 



BIOLOGICAL EFFECTS 



The subject matter presented so far enables the biologist to form a 

 mental picture of the primary reactions which occur in a biological 

 material while it is being irradiated. Particular emphasis has been laid 

 on ionization — its origin, spacial and linear distributions, and the influence 

 of the quality of radiation thereon. Ionization is the only thing we know 

 of to which, rightly or wrongly, we may attribute all other effects of 

 radiation. Hence, any attempt at a correlation of the biological effects of 

 radiation requires of necessity a quantitative knowledge of the ionization 

 in the living materials studied. But this essential information is very 

 diflacult to obtain. In the first place, there is no way of measuring 

 directly the ionization produced by radiation in a liAing, or even a dead, 

 organism. The usual method employed in determining the degree of 

 ionization in gases by measuring the largest electric current — saturation 

 current^which the ions are capable of conveying, cannot be used in the 

 case of tissues. For, any electric field applied to the material to obtain 

 the saturation current (were this possible) sets up simultaneously a much 

 larger current which has nothing to do with the ionization produced by the 

 radiation. In other words, the material is a fairly good conductor to 

 start with, and any increase in conductivity due to the radiation is 

 utterly imperceptible. Therefore the best one can do is to attempt to 

 determine the ionization in the living material by indirect means. When- 

 ever indirect means are employed to get quantitative data, assumptions 

 are likely to be made, explicitly or implicitly. It is then extremely 

 important in making use of the data, to bear in mind the assumptions 

 made and the restrictions which they impose. While the necessity of 

 such caution is obvious, radiological literature nevertheless shows that it 

 is rarely exercised. In the remaining paragraphs an attempt will be 

 made to point out the principal limitations of the indirect method 



