Biological Effects of Ionizing Radiations 121 



quantum energy become relatively more numerous, with the 

 result that the mean energy of all the electrons of both types 

 changes only very slowly. Detailed calculation ^^ shows that the 

 average energy, and, therefore, the average ion density of the 

 secondary electrons, is almost constant for all X-ray-quantum 

 energies between 15 and 90 kilovolts — i.e., roughly for the radia- 

 tions from X-ray tubes operated at all voltages between 30 and 

 180 kilovolts. In consequence, it is not to be expected that a 

 change in X-ray quality within this range will be accompanied 

 by any appreciable change in the biological effect of a given 

 total amount of ionization per unit volume of tissue.* The 

 number of experimental investigations dealing with this point 

 is legion, because the range of X-ray qualities in question 

 happens to be at the same time the most accessible and the most 

 interesting in current radiotherapy As might be expected, these 

 investigations do not all lead to the same conclusion. It may be 

 said, however, that there are no solid grounds for doubting the 

 accuracy of the inference from ion-density considerations, and 

 it would be possible to point to a number of very careful investi- 

 gations, outstanding among which are probably those of 

 Packard,^'^' ^^ who studied the percentage mortality among ir- 

 radiated Drosophila eggs, which show particular biological effects 

 to be independent of X-ray quality over this range to a high 

 degree of accuracy. It appears, indeed, almost in the light of a 

 freakish prank of Nature that she should have tempted so many 

 to investigate a region destined to bear so little fruit. 



The Influence of Ion Density on Radiochemical Yield 



Many substances are decomposed when exposed to any of 

 the ionizing radiations. When the decomposition takes place in 

 the gaseous phase, the number of molecules decomposed is 

 usually of the same order as the number of ions found by the 



* This does not necessarily imply, of course, that the biological effect of a given 



dose, measured in rontgens, will be independent of X-ray quality. It is just in this 



region that the ratio of the ionization produced in tissue to the doge in rontgens 

 ma,y show ^ marked dependence on X-ray quality. 



