108 RADIOBIOLOGY 



the migration of these radicals to the biological materials. If the purpose 

 of the irradiation is to produce a biological effect, it makes no difference 

 that there are both direct and indirect actions. If, however, we wish to 

 study some property of the biological substance itself, it is useful to 

 restrict the situation to one of direct actions only. 



This can be done in several ways, the most important of which is to 

 add biologically inert materials to the solution of organisms in such 

 concentration that the diffusing radicals and ions are much more likely 

 to encounter the added materials than the organisms in question. For 

 example, if we add glycerol to a solution of viruses, a not atypical 

 situation would find 0.01 M glycerol and 10 13 viruses per liter. The 

 glycerol would then be present in a concentration of 10 21 molecules per 

 liter, or 10 8 more concentrated than the viruses. So, neglecting the size 

 difference between virus and glycerol, any diffusing radical or ion will 

 encounter (and give up its energy to) 10 8 glycerol molecules before a 

 single virus will be affected, on the average. Thus the direct hits on the 

 viruses will far outnumber the indirect hits, and so we may neglect 

 these indirect effects. 



In practice, broth, gelatin, and many small sulfur-containing molecules 

 have been found to be highly efficient protective agents. The protective 

 action of these agents is still incompletely understood, and forms an 

 important part of current research in radiobiology. 



THE TARGET THEORY 



In this section, we assume that only direct effects of radiations occur. 

 If ionizing radiation impinges on a solution of organisms, the probability 

 of producing an effect will depend, first, on the probability that the in- 

 cident particle goes through the organism. Clearly, the bigger the or- 

 ganism, the greater the probability that the line of flight of the incident 

 particle will pass through it. Indeed, the probability is proportional to 

 the projected cross section of the organism. Second, given that the line 

 of flight is through the organism, there will be an effect only if an ioniza- 

 tion is produced within it. If the incident particle produces an average of 

 one ionization every micron of its path, the probability of producing 

 an ionization within the organism will depend strongly on its thickness. 

 If, for instance, the organism is 0.1 micron thick, then only one organism 

 in ten will be ionized. If, on the other hand, the organism is 10 microns 

 thick, then each organism will be ionized 10 times, on the average. Thus 

 we are led to an important parameter of radiations: the number of 

 ionizations per unit of path length. Currently, this aspect of ionizing 

 particles is described in terms of the LET (linear energy transfer), 

 which is the amount of energy transferred per unit of path length. This 



