84 RADIATION BIOLOGY 



action consists in heating and merely increases the ever-present chance of 

 chemical rearrangements activated by thermal fluctuations. 



All ionizing radiations produce the great majority of chemical activa- 

 tions through a single mechanism, namely, through glancing collisions of 

 charged particles against the external electrons of atoms. Therefore all 

 ionizing radiations are expected to have essentially similar effects, in par- 

 ticular the following: 



(a) There is no way of affecting the relative average amounts of energy 

 absorbed by specific groups of atoms or by larger portions of matter except 

 by modifying their crude constitutions (see Sect. 3-4). 



(h) The average chemical potency of activations (as well as the energy 

 involved in each individual activation) does not depend on the physical 

 characteristics of the ionizing radiations inducing them. It exceeds con- 

 siderably the potency of activations induced by ultraviolet light or by 

 other radiations of even less potency. 



(c) Activations induced in different atoms, or groups of atoms, are cor- 

 related in space and time because they occur along the paths of fast 

 charged particles. 



(d) Physical control of the speed of charged particles traversing matter 

 affects the average distance between successive activations, or clusters of 

 activations, induced by ionizing radiations. Any resulting variation of 

 the over-all effectiveness of the radiations should be attributed to inter- 

 action effects between the phenomena developing from neighboring 

 activations. 



The action of ultraviolet light (as well as of visible or infrared light in so 

 far as they are chemically effective) is exerted directly on the "chromo- 

 phoric" groups of atoms which absorb individual photons. The main 

 characteristics of the effects of optical radiations are: 



(a) Limited chemical potency, in proportion to the amount of energy 

 carried by each photon (see the chart in Fig. 1-5). 



(6) Absence of any correlation in time and space between the activation 

 of different atoms or groups of atoms. 



(c) Selective activation of those atoms, or groups of atoms, which are 

 best fitted to absorb the radiation used. This selective activation affords 

 good potentialities for specific action, which find applications in nature 

 (as, for example, in the mechanism of color vision), but they have not 

 proved very effective as tools of experimental research. 



4. PENETRATION OF RADIATIONS IN MATTER 



The following sections deal with the macroscopic distribution of radia- 

 tion effects in various portions of a material. By and large, deep-lying 

 portions of a material are shielded from the radiation by the more super- 



