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Radiation in Living Matter:\^^ — ^^ 



The Physical Processes 



p. MORRISON 



Cornell University 

 Ithaca, New York 



Radiation as a Localized Reagent 



The beam of radiation, whatever its type, is a source of energy in 

 highly available form. One can estimate roughly the free energy of such 

 a beam, taking into account both the energy it contains and its entropy, 

 or high-degree order. For a typical x-ray beam, say 50 r per min at 

 1 mev, it is easy to show that one is dealing with a sample of radiation 

 at a temperature of about 10* degrees Kelvin. A few minutes of exposure 

 to such a beam corresponds to the introduction into the irradiated 

 volume of free energy fully comparable to that made available by the 

 injection of a strong reagent like nitric acid, up to a concentration of, 

 say, 10~^ molar. It is no wonder that rather small total energies have 

 widespread biological effects. It is likewise evident that the history of 

 this free energy, between its introduction in such potency and its final 

 expression in the reaction of an organism, is bound to be a long and 

 complex story, which the 5 days of our symposium will by no means be 

 able to detail. 



Examination of a typical cell, say an individual of E. coli, on the 

 atomic scale will help fix the space-time picture of the radiation inter- 

 action in recognizable terms. Such a cell is a wonderfully organized 

 collection of some 10^^ atoms, mostly in the molecules H2O, of course, 

 with many others. About 10"^ ion pairs within the cell produced by x- 

 radiation are enough to lower by a good factor its chances of indefinitely 

 multiplying. (See Fig. 1.) Those ions are formed not at a constant 

 rate, but in short bursts of a hundred at a time, bunched in 10~^" sec 

 or less, and spread out along a tortuous and branching path crossing the 

 cell volume. With alpha particles the same effect on multiplication 

 requires a few dozen alphas crossing the cell one by one, leaving in their 

 roughly straight wakes similar short bursts of ionic produce, arranged 

 in columns with nearly every molecule on the path seriously disturbed. 



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