PRINCIPLES OF RADIOLOGICAL PHYSICS 119 



exactly, except in the very direction of the incident radiation. The disturbances 

 radiated by the atoms in the direction of incidence add up and combine with the 

 incident radiation to form a single, smoothly propagating, beam. However, the 

 characteristics of propagation of this beam de])end on the intensity of the currents 

 induced in the atoms, which tliereby determine tlie macroscopic optical i^roperties 

 of the material. 



When the atoms are not distributed uniformly, the disturbances scattered by 

 the atoms in any odd direction no longer neutralize each other exactly. A 

 macroscopic net scattering arises in this way. This scattering can be observed 

 directly, for example, from a colloidal suspension, if it is moderately weak (Tyn- 

 dall effect) . If the scattering is too intense the medium appears "turbid." 



Light which enters a turbid medium diffuses all about, by repeated scattering, 

 until it escapes from a surface of the medium or is eventually absorbed. 



5. KINETICS OF RADIATION ACTION 



5-1. INTRODUCTION 



As mentioned earlier, our understanding of the action of radiation on 

 matter is fairly satisfactory only up to the production of activations in 

 single atoms or molecules. Most of the observable effects of radiation 

 follow the initial molecular actions, probably through a complex sequence 

 of events which involve a large number of atoms. These effects may 

 appear to us as chemical or biological phenomena. We refer to them 

 loosely as "macroscopic effects" in contrast with the initial molecular 

 effects. 



Some general inferences regarding the unknown mechanisms leading to 

 the eventual macroscopic effects may be drawn from the dependence of 

 these effects upon the physical characteristics — quantity and quality — of 

 a radiation treatment. Analogy with the indirect study of the reaction 

 mechanisms in physical chemistry suggests the name of "kinetics" for 

 the study of the manner of variation of macroscopic effects as a function 

 of the characteristics of a radiation treatment. The principles of the 

 kinetics of radiation action are reviewed in the following sections. 



Much emphasis has been laid on studies of this kind and therefore it is 

 of some importance to assess their significance correctly. The general 

 inferences which can be drawn from the kinetics of radiation effects are 

 often straightforward and well grounded. If, however, an attempt is 

 made to draw detailed conclusions, elaborate hypothetical working 

 models of uncertain significance must be relied upon. 



The examples that illustrate the following discussion are drawai mostly 

 from radiobiology, but chemical examples would be equally pertinent. 

 The chemical systems, such as a solution of ferrous sulfate, on which the 

 action of radiations has been tested have a far simpler and better known 

 constitution than biological systems. Nevertheless, there is limited posi- 

 tive knowledge regarding the mechanisms of radiochemical reactions 



