INTRODUCTION 107 



Compton effect play the major role for the photons. For those who 

 have not studied physics, it may suffice to say that the photons, acting 

 like billiard balls, strike the electrons and knock them out of their orbits. 

 Even if an inner electron is ejected, the result will normally be an 

 ionization and breaking of bonds because the electrons outside the orbit 

 of the ejected electron will now cascade inward, and finally the outer- 

 most (bonding) electron will fall into an inner orbit and the bond will 

 be broken. Neutrons act primarily also in a billiard-ball fashion, hitting 

 protons (hydrogen nuclei) which have an almost identical mass, and 

 knocking them out of their places. These protons now serve as agents 

 of charged-particle irradiation of other atoms and produce the effects 

 observed. 



In all these instances, the ejected electrons (and the occasionally 

 occurring photons) will also produce effects, termed secondary effects, 

 which must be taken into account in any complete reckoning of the 

 results of irradiation. 



Up to this point, we have treated only direct action of radiation on 

 the biological materials of interest. Radiation experiments are usually 

 done on organisms suspended in water. Also, even single cells are pri- 

 marily water in makeup, and complicated organisms contain internal 

 fluids within vessels. Therefore, if radiations can produce effects on 

 water, there is another source of biological effects if the products of 

 water irradiation can react with the biological materials. 



There has been extensive study of the effects of radiations on water. 

 Water has been split into two kinds of products. The ions H + and OH~ 

 are the well-known kind. The kind less known to students in their intro- 

 ductory studies of science is known as the free radical. An atom of 

 hydrogen is not normally found alone in nature, because it is too reactive 

 a substance to endure as such very long. Indeed, the stable elementary 

 form of hydrogen is the molecule with two atoms of hydrogen. The 

 hydrogen atom or radical is not normally free, so that when it is found 

 uncombined, it is called a free radical. This particular radical, H, differs 

 from H + in that it has an electron in orbit about it, and is therefore 

 electrically neutral. A more general definition of free radicals is based 

 on the electrically neutral form of a normally electrically charged atom 

 or group of atoms. Thus any neutralized ion would be termed as free 

 radical. And, if this is not its normal state, it is very reactive chemically. 



Water can be split into its component free radicals: H and OH. The 

 latter is a powerful oxidizing agent, pulling electrons strongly to itself 

 in order to make itself into the stable OH - ion. Thereby it will break 

 chemical bonds and produce consequent biological effects. Experiments 

 have shown that these free radicals are indeed produced by ionizing 

 radiation, so that indirect effects of radiation may be expected, due to 



