68 RADIATION BIOLOGY 



settles at the lower value. A proton may also capture an electron and travel on 

 as a neutral hydrogen atom when its energy falls near to 0.025 Mev or less. The 

 loss and gain of charge of ions of moderate velocity is at present (1950) under 

 experimental study by Allison and co-workers at the University of Chicago. 



The nuclear fission of uranium or other very heavy atoms (see Sect. 2-ld) 

 results in the ejection of "fragments" which are ions of medium atomic weight 

 with kinetic energies of the order of 100 Mev. Despite their high energy, these 

 ions move more slowly than their internal electrons because of their high mass 

 and high atomic number, and therefore hold on steadily to quite a number of 

 electrons during their flight. 



Even the "elastic" collisions of very "slow" heavy particles may lead to the 

 capture or loss of an electron by the particle. Therefore the particle may never 

 remain steadily neutral. This persisting exchange of electric charge at low 

 energies may be regarded as a residual ionization. Quantitative data on this 

 subject are scarce. 



3-ld. Dissipation of Energy hy Secondary Electrons. As pointed out in 

 Sect. 2-4c, the "secondary" electrons which are ejected from atoms all 

 along the track of a fast charged particle carry away an aggregate two- 

 thirds of the total energy of the particle. These electrons constitute a 

 new charged corpuscular radiation which dissipates its energy, by and 

 large, as described in the preceding sections. 



The action of the secondary electrons differs from that of the primary 

 charged particles only in so far as most of the electrons are comparatively 

 slow (see Sect. 2-4c). Therefore the inelastic collisions experienced by 

 secondary electrons affect solely the external electrons of atoms within a 

 hmited radius of action. The qualitative effect of these collisions differs 

 somewhat from the effect of collisions by very fast particles, in so far as 

 the secondaries produce comparatively more ionizations and fewer excita- 

 tions. Nevertheless, this circumstance does not affect the great simi- 

 larity of action of different ionizing radiations, since the secondaries 

 themselves are produced according to the same pattern no matter what the 

 primary radiation. 



Secondary electrons whose energy no longer suffices to disturb the 

 external electrons of other atoms undergo a large number of collisions 

 in which each atom behaves as a rigid body subject to chemical and to 

 intermolecular forces. Energy is thereby transferred directly to the 

 interatomic motion, and the electron energy is slowly dissipated in 

 packets of "thermal" size, until the electron itself becomes degraded to 

 the thermal range. Eventually, the secondary electron is captured by 

 some atom or molecule with formation of a negative ion, unless it is 

 drawn by the attraction of a positive ion (e.g., of the atom from which 

 it had been ejected) and recombines with it. Recombination with the 

 parent positive ion might be a likely event in condensed materials. The 

 final stages of the history of secondary electrons are understood very 



