76 RADIATION BIOLOGY 



vanish instantaneously if the field is suddenly switched out. The time in 

 which the polarization effect decays to 1/e, that is, to 37 per cent of its 

 initial value, serves as a measure of the lag with which the polarization 

 adjusts itself to variations of the field and is called the "relaxation time" 

 of the system. Table 1-6 shows values of the relaxation time for a few 

 typical systems. 



If the electric force oscillates with a period much longer than the relaxa- 

 tion time of either system in Fig. 1-45, that system stays steadily polar- 

 ized according to the instantaneous value and direction of the force. If, 

 on the contrary, the period of oscillation is much shorter than the relaxa- 

 tion time, the polarization cannot possibly follow the variations of the 

 force and necessarily remains very small. Finally, if the period of oscilla- 

 tion has the same order of magnitude as the relaxation time, the polariza- 

 tion barely succeeds in following the variations of the force, and the 

 system then absorbs a maximum amount of energy by dissipation into 

 heat due to electrical resistance or internal friction. 



Table 1-6. Sample Data on the Relaxation Time of Polar Systems 



Relaxation Time, Sec 



Liquid H2O at 20°C 0.4 X lO"!" 



Solid H2O at -5°C 2.7 X IQ-s 



Solid H2O at -22°C 18 X IQ-" 



Liquid C3H7OH at 20°C 0.9 X 10"" 



Liquid C3H7OH at 0°C 1.6 X 10-i« 



Liquid C3H7OH at -60°C 26 X lO"" 



Energy thus absorbed is not capable of producing chemical transforma- 

 tions in the irradiated matter directly, owing to the low potency of the 

 radiation, but may do so indirectly through heating effects. These 

 effects may be localized if a certain portion of the material exposed to the 

 radiation has an electric relaxation time which just matches the period 

 of oscillation of the radiation. 



3-6. SPATIAL DISTRIBUTION OF ACTIVATIONS 



We have been dealing, thus far, with, the mechanisms through which 

 radiations produce chemical activations within a material. Information 

 on the spatial distribution of the activations within a material, i.e., on 

 the proximity of adjacent activations, is also important for the interpreta- 

 tion of their ultimate biological effects. 



It seems clear a priori that the proximity of different activations may 

 affect the chance of biological effects. Suppose, for example, that activa- 

 tions result in the formation of diffusible active radicals. These radicals 

 have a greater chance of colliding if they arise close to one another than 

 otherwise. The collision may conceivably yield a still more active 

 product, and also an inactive one. Thus an increase in the concentra- 



