XV. ELECTRONS, NEUTRONS, AND ALPHA PARTICLES 513 



fication as particles in the size range 0.2-0.5 /i. Using Lea's analysis 

 we should conclude that, if the sensitive material is in the form of a 

 single sphere, it would have a diameter of about 0.15 yu. If the sensi- 

 tive material were distributed as a number of smaller spheres through- 

 out a kappa particle of diameter 0.5 fx, the total value of sensitive 

 material could be greater than that of a single sphere of diameter 

 0.15 /i. The disposition of the sensitive material could be decided 

 by the use of a second ionizing radiation, probably most conveniently 

 by the use of neutron radiation.- 



4. Low Velocit} Secondary Electrons, or Delta liays 



Referring again to Table 11, it will be seen that on about 12% 

 of the occasions on which the primary particle ionizes an atom the 

 ejected electron has sufficient energy to be counted as a separate 

 ionizing particle (8 ray). Although for all types of ionizing particle 

 these 5 rays account for only about 20% of the total ionization, there 

 are a variety of circumstances in which they may play an important 

 role in radiobiology. as the following examples will show. 



First, the 5 i-ays constitute spurs along an a-particle track that 

 effectively treble the length of the track with respect to any biological 

 effect that requires only the production of a single ionization. Thus, 

 the agreement between the size of the phage S 13 as estimated by 

 7 rays, X rays, and a rays, is only obtained when the 8 rays are al- 

 lowed for. In the case of the protons generated by fast neutrons, 

 e.g. (D-D) neutrons, the 8 rays only add about 25% to the effective 

 length of the tracks. In the case of jS radiation there is no appreciable 

 increase in effective track length but here other considerations arise. 

 There are many biological effects, including the breaking of chromo- 

 somes in Tradescantia, Vicia, and many other species of plant and 

 animal, that are brought about with greater efficiency by neutrons 

 and a radiation than by /3 oi- y radiation. From the trend of the 

 variation in efficiency in passing from a radiation to neutrons and X 

 rays it would have been anticipated that fast /S rays, which will 

 rarely produce more than one ion cluster in crossing a chromatid 

 0.1 M in diameter (c/. Fig. 2), would practical^ never break the chro- 

 matid thread. Kotval (33), however, found no great difference 

 between the efficiency of y and X rays, and there is little doubt that 

 this was correctly ascribed by Lea and Catcheside (S6) to the fact 

 that the 6 rays, ejected by the fast electrons, maintain a constant 



