PRINCIPLES OF RADIOLOGICAL PHYSICS 77 



tions of activations in a portion of matter might either enhance or depress 

 the macroscopic effect of radiation which is observed eventually. 

 Instances of enhancement and of depression of radiochemical or radio- 

 biological actions have actually been found to result from crowding of 

 activations (see Sects. 5-5b and c). 



The following pages deal with the factors which govern the proximity 

 of the activations produced by radiations. Ionizing and nonionizing 

 radiations behave quite differently in this respect because the action of 

 ionizing radiations is concentrated along the tracks of charged particles 

 but the action of nonionizing radiation does not follow any such system- 

 atic pattern. 



Since light produces activations by the direct absorption of different 

 photons, these activations take place in different molecular groups inde- 

 pendently, in a wholly uncorrelated random fashion. If an atom has been 

 excited by light, adjacent atoms have a normal average chance of being 

 excited as well, just like other similar atoms in any portion of the material 

 equally exposed to the radiation beam. On the other hand, if an atom 

 has been excited by an ionizing radiation, a fast charged particle must 

 have passed in the proximity of that atom and therefore the chance of 

 excitation of adjacent atoms by the same particle is particularly high. 



In general, the only systematic influence upon the proximity of activa- 

 tions arises from their arrangement along and about the tracks of fast 

 charged particles. The essential data required to describe this arrange- 

 ment may be derived from the discussion of the collisions of charged 

 particles in Sect. 2-4. A picture of the distribution of activations results 

 by considering a number of separate questions such as the average 

 number and kind of collisions along the track of a particle, the statistical 

 fluctuations in the distribution of these collisions, and the distribution of 

 activations produced by secondary electrons. 



3-6a. Collision Distance along the Tracks of Fast Particles. The mean 

 number of inelastic collisions experienced by a charged particle traversing 

 a thickness of matter has been discussed in Sect. 2-4c. Formula (22) 

 indicates the mean number of colHsions leading to a particular kind of 

 excitation. Formula (21) indicates the mean number of all inelastic col- 

 lisions which affect electrons bound in a particular way within a particular 

 kind of atom. The mean total number of all inelastic collisions is 

 obtained by adding the colhsions which affect all the differently bound 

 electrons in all kinds of atoms within the material. 



The average distance I of two successive collisions of any specified kind 

 along the track of a particle is the reciprocal of the mean number of those 

 collisions per unit length of the track. This average distance varies 

 greatly depending on the charge and speed of the particle, as shown by 

 the formulas. For example, collisions are spaced thousands of times 

 more closely along the track of a 1-Mev a particle than along the track of 



