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CHAPTER Vj Radiobiology 



INTRODUCTION 



Radiation is used in biology in several ways. First, light of various 

 visibLe, wavelengths is used as a source of energy for biological functions. 

 Second, as detailed in the section on light absorption effects, light may 

 be used to analyze such diverse subjects as the pigments responsible for 

 the reception of biologically important light and for analysis of the con- 

 centration and shape of molecules. The two kinds of light which have 

 not yet been discussed here are the ionizing and the infrared radiations. 

 The latter, since they have thus far given few if any biologically impor- 

 tant results, will not be discussed. The former include short wavelength 

 ultraviolet radiation (which has been briefly treated) and the radiation 

 composed of the physical particles: electrons, neutrons, protons, alphas, 

 and photons in the x-ray and gamma-ray energies. These particles have 

 the distinguishing property of being so energetic that they are easily 

 able to ionize one or more of the atoms in the organisms, thereby creating 

 a disturbance great enough to break one or more chemical bonds. A 

 drastic effect might then be expected to ensue, in terms of the alteration 

 of the biological integrity of the irradiated material. Indeed, most 

 studies with these ionizing particles have concerned themselves with the 

 killing of the target organisms. We will see, however, that the particles 

 can do many other things. 



The fundamental mechanism of action of these ionizing radiations 

 rests on the ejection of an electron, leaving an ionized atom or molecule 

 behind. Since the outermost electron is most weakly bound to its atom 

 or molecule, it is this electron which is usually ejected. And, since it is 

 this same outer electron which is involved in chemical bonds, the bond 

 is readily broken by the ionization. 



The ejection of the electrons takes place by two mechanisms, depend- 

 ing on the type of particle involved. Electrically charged particles 

 (electrons, protons, and alphas), when flying past an atom or molecule, 

 exert an electric force on the electrons, and thereby effect their ejection. 

 The uncharged particles (x-rays, gamma-rays, and neutrons) then must 

 have a somewhat less direct mode of action. For those who have studied 

 physics, it is only necessary to say that the photoelectric effect and the 



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