8 PHYSICAL PROCESSES IN LIVING MATTER 



though they may still excite complex molecules and transfer their 

 kinetic energy in elastic collision into simple heat motion. We are 

 almost at the point of chemistry. Loss by elastic collision would require 

 perhaps 10~^° sec, but much molecular excitation might take place 

 earlier. Now the free electrons will become captured by a molecule of 

 the medium. In water one will most frequently find this capture leading 

 to a free negative hydroxy 1 radical, setting free a neutral H atom as well 



Density 

 (arbitrary units) 



Initial effect 



Angstroms from track 

 Lateral distributions 



Fig. 5. Qualitative representation of the lateral distribution of disturbed atoms and 

 molecules across an ionizing track. To the left is shown the distribution immediately- 

 after passage of a fast electron; to the right, the distribution of diffusing positive and 

 negative ions after an alpha particle has traversed water, about 10"^** sec earlier. 

 Note the charge separation due to the faster motion of the secondary electrons which 

 are captured some distance from the track. [After Lea (6).] 



in the reaction. What the spatial distribution of these initial products 

 will be is a little obscure. The feathery track will be smeared out by 

 the diffusion processes, but the caging effect of the surrounding medium 

 and the frequently high density of ions along the track itself will combat 

 this free diffusion by making recombination easy, and even by the 

 overall electric field which attracts negative ions toward the generally 

 more concentrated positive-ion core. (Figure 5 suggests the situation.) 

 It is most doubtful that any of the quantitative treatments yet given 

 [see (6), p. 59, and (7)] are adequate for the complex problem involved 

 here. It can be said that the diffusion from a lightly ionizing track may 

 spread the radicals over sizable distances, even a tenth micron or two, 

 whereas in the alpha case it seems likely that most radicals will recom- 

 bine before some 10~^ sec, have passed, in a distance of only a couple of 



