86 



ISOTOPIC TRACERS AND NUCLEAR RADIATIONS 



[Chap. 4 



divided by the number of atoms per cubic centimeter. Using an approximate 

 form of the energy-loss formula as an example, a is given by 



4ire A z~Z . 2mv 2 

 a ■- -r- log 



mv 



1 



ev/cm 2 



4.7. Mass Stopping Power. The mass stopping power is defined as the 

 energy loss per centimeter divided by the density of the absorber or as the 

 stopping power per unit density. From Bragg's empirical rule as well as 



2 4 6 8 10 12 14 



ENERGY OF PROTONS (MEV) 



Fig. 17. Atomic stopping power of various elements relative to air. 



from the stopping formula, it is found to vary approximately as the inverse 

 square root of the atomic mass of the absorber. 



4.8. Range of Heavy Charged Particles. The range of a charged particle 

 in an absorber is the total path length traversed before the particle is brought 

 to rest by complete loss of its kinetic energy. With an initial energy E , in 

 units of Mc 2 , the range is calculated with the aid of the energy-loss formula by 

 the integral 



CEo 



R = / dE 



J —dE/dx 

 where —dE/dx = energy loss per unit length of path 



cm 



