XV. ELECTRONS, NEUTRONS, AND ALPHA PARTICLES 



547 



neutron radiation. The problem has been discussed in some detail 

 by Zimmer (92), Aebersold and Anslow {91), and Gray (23). In es- 

 sence, the problem is the same as that of measuring the energy ab- 

 sorbed by tissue exposed to X radiation or which is uniformly per- 

 meated by a radioactive substance, and the solution depends on the 

 fact that, if into any solid medium in which ionizing particles are 

 being uniformly generated a small empty cavity is introduced, the 

 number and speed of the particles crossing any small area of the cavity 

 in any given direction will be the same as for the particles crossing an 

 equal area similarly situated in the solid. This is self-evident in the 

 case of protons for the particles moving in such a direction as to enter 

 the cavity, for it is clear that since there is no appreciable scattering 

 of protons removal of solid material to the right of area A in Figure 17 



Figure 17 



to form the cavity can have no influence on the protons crossing the 

 boundary into the cavity. It may be shown to be the case, also, for 

 particles moving in the opposite direction. It is, in fact, true gener- 

 ally for all types of particle, including particles that are easily scat- 

 tered through large angles, such as electrons. 



Suppose the cavity to be filled with gas. A group of particles 

 crossing area A in any given direction will lose a certain amount of 

 energy in the course of ionizing the gas. The same group of particles 

 passing across an equal and similarly shaped volume of solid (Fig. 

 17) will lose very much more energy. The ratio of the two energy 

 losses may be denoted by p. This ratio is known as the stopping 

 power of the solid relative to the gas for the particles in question. 

 The ratio is to a first approximation equal to the ratio of the densities 

 of the solid and the gas. The same will be true for all groups of par- 

 ticles crossing the cavity and the equivalent groups moving across an 

 identical volume of the solid. Thus, if p is taken to denote the mean 



