120 BIOLOGICAL EFFECTS OF RADIATION 



all subsequent effects. A complete specification of the ionization in the 

 biological material is essential for the proper interpretation of the bio- 

 logical effects produced by radiation. One must know, therefore, for 

 every point of the material, (a) the number of ions produced per second 

 per gram, or, if preferred, per cubic centimeter; (6) the characteristics 

 of the ionization loci, especially as regards the relative proportions of loci 

 of the different lengths present, and the linear distribution of ions therein. 

 Since these data cannot be determined by direct measurements, they 

 must be obtained by indirect means. The similarity of the interaction 

 between radiation and organic matter on the one hand, and air on the 

 other, furnishes the basis for the indirect method of attack which has been 

 found most practical so far. By virtue of this similarity the ionization 

 (factors a and h, above) in living tissue and air produced by radiation 

 under comparable conditions is substantially the same. Strictly speak- 

 ing, the conditions are not comparable unless the mass concentration, 

 that is, the density, of the air is at least approximately the same as that of 

 living matter. 22 Since it has not been found practical to measure ioniza- 

 tion in such a dense medium, one is forced to the use of atmospheric air. 

 The density of this medium being about ^^ooth of that of living tissues, 

 many difficulties arise. Bearing in mind that the ultimate aim of 

 ionization measurements in air is to make possible the determination or 

 estimation of the ionization in the living organism, the experimental 

 conditions must be such as will introduce the least uncertainty in the 

 final result {i.e., tissue ionization). The conditions stipulated in the 

 definition of the international roentgen fulfil this requirement in a limited 

 range of wave-lengths and for small irradiated objects, with satisfactory 

 accuracy. It can be demonstrated, however, that in the gamma-ray 

 region, the conditions imposed by this definition not only cannot be 

 fulfilled in practice, but in addition are not in general those which will 

 permit the evaluation of the ionization in the living material in the 

 simplest way.^^ In this case still greater care must be exercised. 



In view of the inherent difficulties of the subject, and the restricted 

 sense in which some technical terms are used, the biologist is urged to 

 concentrate his attention on the ionization produced in the biological 

 material under the conditions of his experiments. He will do well, for 

 instance, to think of a "dose" or quantity of radiation in terms of the 

 number of ions per unit volume which it produces in the material under 



22 This condition would be met by liquid air, if the ionization could be measured in 

 it. 



23 A detailed discussion of this problem will be found in the author's article already- 

 referred to, which will be published in Acta Radiologica. It may be stated here, 

 however, that the chief difficulty is due to the long range and tortuous path of the 

 secondary beta particles. 



