158 Applied Biophysics 



of the macromoleciilar viruses, every virus particle ionized is 

 inactivated, we are able to use radiation experiments to estimate 

 the size of the virus particle. 



Suppose that D rontgens is the dose which produces an aver- 

 age of one ionization per virus particle. Since 1 rontgen corre- 

 sponds to the production of approximately 2 X 10^" ionizations 

 per gram, D rontgens corresponds to the production of 1 ioniza- 

 tion per e^rams. This, then, is the mass of the virus 



^ 2 X W~D ^ 



particle. 



This calculation, while satisfactorily illustrating the principle, 

 is somewhat simplified. The ionizations produced in an irradi- 

 ated material are not distributed spatially at random, as the 

 above calculation has tacitly assumed, but are localized along 

 the paths of ionizing particles, as described by Gray. If an 

 ionizing particle passes through a virus particle, usually more 

 than one ionization will be i)r()duced in it, the actual number 

 depending on the diameter of the virus and the ion-density, i.e., 

 the number of ionizations produced per micron path, of the 

 ionizing particle. The ion-density is greater in alpha-ray ex- 

 periments than in X-ray experiments, and is greater with X-rays 

 than with gamma rays. We shall, therefore, expect that the 

 inactivation doses will increase in the order gamma rays, X-rays, 

 alpha rays, since a radiation which produces several ionizations 

 in one virus particle, when one would suffice to inactivate it, 

 is inefficient. 



Table I shows that the experimental results ^^ confirm this 

 expectation for a bacteriophage. Similar results with plant 

 viruses have been obtained by Lea and Smith. ^- 



TABLE I. 

 Inactivation of Phage S-13 



(Phage diameter 16 m\i) 



Gamma X- Alpha 



rays rays rays 



Inactivation dose in millions of rontgens 0.58 0.99 3.5 



Inferred "target" diameter in mu 15.5 15.9 16.0 



