Biological Effects of Ionizing Radiations 125 



3 

 clusters will be given by — micra where A is the ion density of 



A 



3 

 the radiation. For gamma rays, — is about 300 m^, for hard 



A 



X-rays 60 m\x, for soft X-rays 20 m^, and for alpha rays 



0.85 mfi.* 



The diameters of the smaller viruses range from 15 to 50 mji. 

 The relation between the size of the virus and the spacing of 

 the ions is thus roughly that shown in figure 1 for the four 

 radiations mentioned. Even allowing for unevenness in the 

 spacing of the ion clusters, it is evident that only rarely will 

 a single ionizing particle give rise to more than one ion cluster 

 within a virus particle irradiated by gamma rays. As long as 

 this obtains, the chance that a cluster is formed within any 

 given virus particle is just equal to the total number of clusters 

 formed per unit volume of the medium multiplied by the volume 

 of the virus, and, therefore, the inactivation dose should be 

 independent of ion density. 



On the other hand, an alpha particle will produce many ion 

 clusters within even the smallest virus particle or gene, so that, 

 if one cluster suffices for inactivation, this radiation must neces- 

 sarily be inefficient, and a large dose will be needed to produce 

 a given degree of inactivation. 



In figure 2, the experimentally determined efficiencies of a 

 number of radiations in inactivating virus preparations are 

 plotted against the mean distance between ion clusters for six 

 virus particles, ranging in size from 16 to 64 mji.** The 

 theoretical variations for spheres of 15 and 50 m^ diameter 

 are drawn in full. It will be seen, that, in accordance with ex- 

 pectation, the experimental values of the efficiency begin to show 

 a dependence on ion density just at the point where the distance 

 between clusters is comparable with the size of the particle. 



The relation between inactivation dose and ion density thus 



* The millimicron (m|Ll) "= 1/1,000 micron = lO-" millimeter. 



** The term "efficiency of a radiation" will be used throughout this article to 

 mean a quantity inversely proportional to the total amount of ionization per unit 

 volume of tissue required to produce a given biological effect. 



