62 



MUTATION AND PLANT BREEDING 



Table 2. — Ion Density Produced by Different Ionizing Radiations or Particles. ' 



micron and going up to 1 30,000 for very dense tracks made by urani- 

 um fission fragments. Since each ionization is estimated to require 

 32.5 ev, it is apparent that the denser the ionizations along the track 

 the higher the rate of energy transfer from the ionizing particles to 

 the atoms or molecules of the tissue. This concept known as linear 

 energy transfer (LET) is defined as the energy released by the radi- 

 ation per unit length of track in the absorbing material. It is usually 

 expressed in units of kev/ji and the LET varies directly with the 

 square of the charge and inversely as the square of the velocity (or 

 energy) of the particles. The charge on an ionizing particle does not 

 change along its path, but as velocity (energy) remaining is gradually 

 reduced, the frequency of ionization increases up to a maximum. The 

 rate of energy loss for electrons in water is shown in Figure 3. The 

 denser parts of the tracks are known as tails and are the most effective 



