8^ THE PHYSICS OF VIRUSES 



to give an area which is about that of the whole ch-ied virus. The 

 vakie of S for more s})arse ionization is definitely less. The third 

 case, a large bacterial virus, M-5 megaterium phage, shows a 

 definite flattening in cross section area, and so is similar to 

 SBMV, but also has a lower cross section for fast deuterons. The 

 total cross section at high deuteron energy loss is probably less 

 than that of the whole virus, but since it is a definite value over a 

 finite range of energy loss, there seems to be reason to believe 

 that a definite part of the virus is radiation sensitive. The fact 

 that not all of a large virus need be radiation sensitive was 

 pointed out by Lea (1947). The fourth case, of T-1, shows a 

 steady rise in effective cross section, which may perhaps be 

 showing some flattening if correction is made for the fact that 

 alpha-particle bombardment, which was used for the extreme 

 point, brings with it a larger number of energetic, secondary 

 electrons (delta rays), and these may give an excessively high 

 apparent efficiency to an alpha particle. This correction is 

 indicated by the dotted line. 



Analysis of Bombardment Results 



It has been stressed that these findings are to be expressed 

 in terms of the space relationships of energy loss by the bombard- 

 ing particle. These are most simply (but not fully) described in 

 terms of the occurrence of average primary ionizations at an 

 energy cost of 100 ev per event. Using this description, the events 

 in the three cases are shown schematically in Fig. 3.7. In this 

 representation, the two extremes of low- and high-energy loss 

 are shown in terms of the distribution of primary ionizations 

 (black dots) along the particle track. It is clear that for TMV 

 one primai-y ionization removes the infectivity. For SBMV, more 

 than this is required, but probably between one and three repre- 

 sents the number which will destroy infective function. On the 

 other hand, for T-1 there is a steady increase in effectiveness as 

 the deuteron specific ionization becomes greater. 



It is clear that all viruses do not behave in the same way as 

 regards primary ionization. The only simple case that can be 

 easily treated is TMV. The assumption can be made that one 



