96 BIOLOGICAL EFFECTS OF RADIATION 



loci, therefore, consist of some long tracks (of the order of 1 cm.) and 

 many shorter ones ranging down to a very small fraction of 1 mm. The 

 difference in the distribution of ions produced by soft and hard X-rays 

 may be visualized most readily as follows : A livhig tissue irradiated with 

 soft X-rays is interspersed with ionization loci which are very short and 

 have the appearance of dots. When the material is irradiated by hard 

 radiation, some of the ionization loci have the appearance of dots and some 

 are more like commas, with short or long tails, depending on the speed 

 of the electrons which produced them. The ion concentration in the dot 

 loci and at the head of the comma loci is substantially the same in all 

 cases. The concentration along the tails decreases rapidly at first and 

 then slowly toward the end of the tail. When the tail is very long, the ion 

 concentration is substantially constant along most of its length. The 

 fraction of the total ionization which is found in the tails of all the comma 

 loci depends on the quality of the radiation and increases with its pene- 

 trating power. In the case of ordinary X-rays produced at voltages 

 under 200 kv. peak, this fraction is always small. Recombination, 

 depeinding markedly on the ion concentration, is much more rapid in the 

 dot loci than in the tails of the comma loci. Accordingly, if the rate of 

 recombination of the ions influences their chemical and biological effec- 

 tiveness, the same total number of ion pairs liberated in a given material 

 will not produce the same chemical and biological changes when, in one 

 case, the ionization loci are dots and, in the other, they are dots and 

 commas. Such a difference, if it existed, would not be apparent in the 

 usual range of X-ray wave-lengths, but it should be noticeable when the 

 comparison is extended to the gamma-ray region. It should also be 

 particularly marked when the ionization is produced by the hard beta 

 rays of radium. 



INTENSITY OF RADIATION AND INVERSE SQUARE LAW 



The distribution of ionization in a biological medium has been dis- 

 cussed so far on the assumption that the radiation is uniformly distributed 

 throughout the material. In practice this is seldom (and strictly speak- 

 ing, never) the case. As already pointed out, the intensity of radiation 

 determines the number of ionization loci in a given volume, and not the 

 shape of the loci. Hence, the variation of the intensity of radiation 

 from point to point in the material results simply in the variation of the 

 concentration of ionization loci. We shall discuss now the factors which 

 influence the intensity of radiation. In this connection it is important to 

 have a clear idea of the meaning of this term as used here. (The same 

 expression is also used to express an entirely different quantity, as we 

 shall see later.) The fundamental concept may be presented most 

 simply on the quantum theory of radiation. It will be recalled that in 



