PRINCIPLES OF RADIOLOGICAL PHYSICS 



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E 



on biological materials. Different atomic groups respond to light of 

 different wave lengths. In principle this makes it possible to channel the 

 action of radiation by introducing suitable molecular groups which 

 absorb specifically light of certain wave lengths. Conversely the action 

 of certain wave lengths is understood to be channeled through groups of 

 proper specificity. In practice, however, the absorption bands of 

 molecular groups in the liquid or solid state are not too sharp. Therefore 

 this specificity of action does not actually serve as a tool of investigation 

 as well as might be wished. 



3-5. ACTION OF FAR-INFRARED AND RADIOFREQUENCY RADIATION 



Electromagnetic radiation can dissipate energy by inducing electric 

 oscillations in larger amounts of matter when its frecjuency is too low to 

 match the characteristic frequencies of oscillations of atoms or of atomic 

 aggregates. This absorption, like the absorption of atoms, becomes par- 

 ticularly intense if the period of oscillation of the radiation matches a 

 characteristic time of response of the material exposed to radiation. 



Consider the two examples shown 

 in Fig. 1-45, namely, a cylindrical 

 conductor and a suspension of par- 

 ticles which are positively charged on 

 one side and negatively charged on 

 the other. (These particles may be 

 simple molecules or larger colloidal 

 particles; water and ammonia mole- 

 cules, among many others, are " polar " 

 molecules, i.e., they have an uneven 

 internal distribution of charges as 

 shown in the figure.) 



Figure 1-45 shows how either of 

 these systems becomes " polarized " by 

 electric induction under the action 

 of an electric force. Electrons that 

 move freely through the cylinder 

 accumulate at its bottom surface and 

 leave a net positive charge on the top 

 surface. A majority of the particles in suspension take an orientation 

 with their positively charged side pointing upward. 



Neither of these effects is achieved instantaneously. The motion of 

 electrons through the conducting cylinder is opposed by the electric 

 resistance of the material and by self-induction and capacity elTects. 

 The reorientation of suspended particles is opposed by the impacts 

 between the suspended particles and the molecules of the medium or, as it 

 may be said, by internal friction. Vice versa, the polarization does not 



Fig. 1-45. Diagram of polarization 

 effects. Above: An electric field in- 

 duces positive and negative charges 

 on the top and bottom surfaces of a 

 conductor. Below: An electric field 

 changes the initial random orienta- 

 tion of polar molecules in a suspension. 



