IONIZING RADIATION AND VIRUSES 



81 



cubic centimeter. Again, the interpretation of V in terms of virus 

 structure has to be made. 



The third type of experiment is illustrated by Fig. 3,6 where 

 the effective cross section, S, taken from deuteron bombard- 

 ment experiments, is plotted against the energy loss on the part 

 of the deuteron for four viruses: TMV, SBMV, T-1 coli, and 

 M-5 inegaterium phage. The deuteron energy loss is varied by 



1 r 



"T r 



_•.. 



-0- — (VI-5 Phage 



T-l Phage 

 SBMV 



100 200 300 400 500 600 700 800 900 



Energy loss in ev per 100 A 



Fig. 3.6. Inactivation cross sections of TMV, M-5, SBMV, and T.l for 

 bombardment by deuterons of various energies and, hence, rates of energy loss. 

 The cross section approaches a maximum for TMV, M-5, and SBMV, but 

 not for T-l. 



interposing foils in the path of the beam, thus reducing the 

 velocity of the particle and increasing the rate of energy loss. 

 Three types of curve are seen which illustrate three classes of 

 virus and probably express three different internal morphologies. 

 The first, for TMV (a purified preparation assayed by lesion 

 count on Nicoiiana glutinosa), shows no change in ^S as a func- 

 tion of ion density, or rate of energy loss. Within reason, 8 

 corresponds to the whole area (actually 80%) of the infectious 

 unit of TMV. The second, for southern bean mosaic virus, shows 

 an increasing cross section which flattens at high ion density 



