Biological Effects of Ionising Radiations 119 



that there is a smooth and progressive variation of effectiveness 

 with the density of the ionization along the track of the ionizing 

 particle irrespective of whether the particle is an electron, a 

 proton, or an alpha particle. 



The subject, therefore, admits of a great simplification, for 

 in general it is not necessary to contrast the numerous types 

 of radiation, but only to discuss the influence of the "linear 

 ion density" on the total amount of ionization required to bring 

 about a given biological effect. Experimentally, also, this in- 

 volves a simplification, since there are sometimes alternative 

 ways of generating particles of a given ion density, as shown in 

 table I. 



Certain points of therapeutic interest emerge from a con- 

 sideration of the data contained in this table. It will be 

 observed that strongly-filtered radium gamma rays, the beta 

 rays from radium, and both the beta rays and the X-rays from 

 a betatron operated at voltages up to 30 million volts, are 

 all bracketed at the level of 6 to 8 ions per micron. Theoretically 

 no charged particle can produce less than 6 ions per micron ; 

 moreover, the minimum is a flat one, rising particularly slowly 

 on the high-voltage side, as has been checked experimentally by 

 the study of cosmic-ray particles. While, therefore, the betatron 

 ofifers attractive possibilities from the standpoint of radiological 

 technique, there are no a priori grounds for expecting a marked 

 difference in biological effectiveness between, say, 30 million volt 

 X-rays and heavily-filtered radium gamma rays. 



A second point in the table, at w4iich large changes in the 

 conditions of generation result in little or no change in the ion 

 density of the radiation produced, occurs in the range of X-rays 

 commonly used in radiotherapy. From the biological standpoint, 

 the quality of an X-ray beam may be specified by stating the 

 average ion density of the secondary electrons to which it gives 

 rise in the irradiated tissue. Some of these electrons (photoelec- 

 trons) have the full energy of the X-ray quantum ; others (recoil 

 electrons) have only a fraction of this energy. As the kilovoltage 

 of the X-ray tube is increased, the energy of both types of elec- 

 trons increases, but those having only a small fraction of the 



