108 BIOLOGICAL EFFECTS OF RADIATION 



known to permit one to make a fairly close estimate of the distribution 

 of ions in the loci under the conditions met with in practice. 



The spacial distribution of the ionization in a given material is inde- 

 pendent of the duration of the irradiation, in the sense that on the average 

 each element of volume receives the same amount of energy, compatible, 

 of course, with the variation of the intensity of radiation from point to 

 point in the material. This is analogous to saying that every square 

 foot of a plane exposed to rain receives the same number of drops, pro- 

 vided the exposure is not too short. It follows, therefore, that the num- 

 ber of ions per unit volume produced at a given point by a constant source 

 of radiation, is directly proportional to the duration of the exposure. 

 Accordingly, the main problem is to determine the number of ions liber- 

 ated in unit volume of the material per unit time under the action of 

 radiation of a given intensity and a known quality. From the definition 

 of intensity given so far it is evident that, all other conditions remaining 

 the same, this number is directly proportional to the intensity, since the 

 latter determines the rate at which energy is supplied to a given volume 

 of the material. However, when the quality of the radiation is not the 

 same, the relation between the number of ions per cubic centimeter per 

 second and intensity is much more complex. Remembering that even 

 monochromatic radiation becomes heterogeneous in its passage through a 

 biological material, it is evident that the quality of radiation is always 

 involved. This question requires very careful consideration. 



Let us examine the ionization produced by two hypothetical beams of 

 monochromatic radiation, the wave-length of which is very different. 

 For the sake of definiteness, imagine two point sources of equal ("candle") 

 power, one emitting (soft) X-rays and the other gamma rays. Per 

 unit time each source, therefore, emits the same amount of energy in all 

 directions. Remembering the derivation of intensity of radiation previ- 

 ously given, the two sources will provide the same intensity of radiation 

 at a given distance from either. A biological material placed at this 

 distance (from either source) will receive the same amount of energy per 

 unit time. The complete utilization of this amount of energy through 

 ionization of the material produces the same number of ions in both 

 cases. But, owing to the greater penetrating power of gamma rays, 

 the same number of ions is distributed through a much larger depth 

 and volume of the material than in the case of X-rays. Hence the number 

 of ions produced per second per cubic centimeter of tissue at a given 

 depth is much smaller in the case of the gamma-ray source, under the 

 conditions stipulated above. It is evident, therefore, that the rate at 

 which ions are liberated at a given point in the material depends on 

 something more than the intensity of radiation, and that this additional 

 factor is the quality of the radiation at the point in question. The 



