XV. ELECTRONS, NEUTRONS, AND ALPHA PARTICLES 509 



object even as large as a bacillus, and, further, that a bacillus will 

 frequently be traversed by a /3 particle (either primary, or produced 

 by high voltage X or 7 radiation) without being ionized at all. It is 

 also evident that an enormous number of /3 particles, that is, a very 

 high dose, would be needed in order to be sure of leaving a single ion 

 pair in, say, half the bacteriophage particles. 



Figure 4 shows tracks produced by 22 kv. electrons resulting from 

 photoelectric absorption of silver K radiation. It will be seen that, 

 although the magnification is five times less than in Figure 3, the 

 ions are much closer together, and hnally, as the electron is slowing 

 down, cease to be resolved. This illustrates the general rule that the 

 slower the particle the closer together the clusters and the greater the 

 specific ionization or linear ion density. 



Figure 5, reproduced from a paper by Lawrence (28) illustrates 

 the ionization produced by the radiation from a cyclotron. The con- 

 trast between the ion density along the recoil proton tracks produced 

 by the neutrons and along the electron tracks produced by 7 rays, 

 which accompany the neutrons, is very striking. 



Figure 6 shows a-particle tracks at effectively very much higher 

 magnification than in the previous photographs. The bulges on the 

 tracks are some of the larger clusters of ions; a number of 5-ray tracks 

 are visible (see below) (39). 



Figure 7 shows one of the a-ray tracks of Figure 6 superimposed 

 upon an appropriately enlarged electron micrograph of tobacco necro- 

 sis virus, reproduced by Lea {1). It will be seen that the a particle 

 leaves many clusters inside even such a small particle, by extreme 

 contrast with the faster )3 particle, which frequently fails to leave a 

 single cluster inside the very much larger Escherichia coli (Fig. 3). 

 The difference between X-ray and a-ray ion distribution is also 

 rather strikingly shown in Figure 8. This figure shows actual cloud 

 chamber tracks of 20 kv. electrons sufficient in number to amount 

 roughly to an X-ray dose of 25 r. in a nucleus 10 /jl in diameter. The 

 same total number of ions would be produced by a single a particle 

 passing at random through this nucleus. 



3. Irrefjularities in Ion Spacing and Their Biological 



Significance 



The tendency of ions to be formed in clusters was cA'ident in the 

 j8-ray photograph (Fig. 2). It is less evident in the other photo- 



