XV. ELECTRONS, NEUTRONS, AND ALPHA PARTICLES 507 



types of X radiation and for neutron radiation the points lie on a 

 smooth curve, the dose ratio for neutrons closely approximating that 

 for 1.5 kv. aluminum K X radiation, establishing the equivalence for 

 this particular biological reaction of proton and electron ionization. 



One has always to bear in mind, however, that the full range of a 

 1.5 kv. electron in tissue is only about 0.1 m (Fig. 11) and this may 

 impose a limitation on the effectiveness of the radiation. For ex- 

 ample, such an electron could not be expected to break each of two 

 chromatic threads 0.1 /x in diameter even if these were lying side by 

 side. This was verified by Lea and Catcheside (26) , who found indeed 

 not only that exposure of Tradcscantia pollen tubes to aluminum K 

 X radiation resulted in a negligible proportion of cells in which both 

 sister chromatids were broken at the same locus, but also a very low 

 yield of simple chromatid breaks compared with that resulting from 

 exposure to higher voltage X radiation. On the other hand, neutron 

 radiation giving protons of the same mean linear ion density as 1.5 

 kv. electrons is very effective in producing both types of chromosome 

 aberration. The cloud chamber photographs reproduced in Figures 

 2-7 illustrate the spacing of the ions along the tracks of different par- 

 ticles in relation to the size of biological structures. 



Figure 2 shows the track of a /3 particle having an energy of about 

 1 m.e.v. and traveling with approximately the speed of light {22). 

 It will be noted that ionization is in the form of clusters separated by 

 gaps in which no ions are formed. Each cluster contains an even 



Fig. 2. Wilson cloud chamber photograph of a 1 m.e.v. /3 particle {22). Length 

 of arrow indicates corresponding scale in microns of tissue in this and 

 subsequent figures. 



Fig. 3. Same /3-particle track as in Figure 2 superimposed on an electron micro- 

 graph of Escherichia coli and l^acteriophage {22). Some bacteriophage 

 may be seen at the right. 



Fig. 4. Wilson cloud chamber photograph of secondary electrons generated by 

 22 kv. X rays {22). 



Fig. 5. Wilson cloud chamber photograph of recoil protons and electrons re- 

 sulting from exposure to a mixed beam of neutrons and y rays from a 

 cyclotron {28). 



Fig. 6. Wilson cloud chamber photograph of an a-particle track at high magni- 

 fication {22). 



Fig. 7. Same a-particle track as in Figure 6 superimposed on appropriately en- 

 larged electron micrograph of tobacco necrosis virus (/). 



Fig. 8. Wilson cloud chamber photogra]ih of electron tracks sufficient in immbcr 

 to represent the conditions within a nucleus 10 ^ in diameter irradiated 

 by 2.5 r. of X rays {22). Same total ionization would Ik' i)roduced by 

 one a particle passing at random through the nucleus. 



