IONIZING RADIATION AND VIRUSES 85 



fore, something more about the action of ionizing radiation on 

 SBMV which must be invoked to explain the data. Further 

 studies will be needed to bring this out. 



The case of M-5 phage is somewhat similar to SBMV. The 

 deuteron cross section for very high ion density is 2.05 X 10~^^ 

 cm^, which is not far from that of the whole virus as seen in 

 electron micrographs. The electron inactivation volume is not in 

 agreement with the above figure for a spherical sensitive region. 

 The value found is 4.0 X 10~^^ cm^ and applying the same con- 

 siderations as for TMV, without correction for random orienta- 

 tion, we find irrH = 4.0 X lO"!^ and 2r/ = 2.05 X lO'^S so that 

 r = 12.5 A and / = 8,200 A. The sensitive part is, therefore, 

 long and thin and must be coiled up in the virus. Unfortunately, 

 this does not fit the ion-density curve, for this indicates that 

 there is an average of one primary ionization per target thickness 

 at an energy loss of 250 ev/100 A, giving a target thickness of 

 100 X 110/250 = 44 A. The diameter calculated above is 25 A, 

 about half of this. It is once again likely that a critical energy 

 must be expended in M-5 phage before inactivation can occur. 



There seems, however, to be no doubt that the sensitive 

 volume is long and thin, perhaps of the order of 50 X 3,000 A. 



Turning to the fourth virus, T-1 phage, there is a clear neces- 

 sity for some added feature beyond the sensitivity of a certain 

 part of the virus to one primary ionization. At all events, this 

 part can not be concentrated in one volume. In the first place, 

 no definite maximum area can be assigned. In Fig. 3.6 an attempt 

 to correct the cross section for fast secondaries (delta rays) has 

 been made and is shown as the dotted line. Making a guess at 

 the maximum value we get 15 X 10~^^ cm^. The inactivation 

 volume found by Slater (1951) for electron bombardment is 

 2.4 X 10~^^ cm^. Treating the sensitive volume as spherical, 

 the radius from the first result is 218 A (rather less than the 

 electron-micrograph radius of the head, which is 250 A), but 

 from the second figure it is 83 A, which doesn't agree at all. 

 The two figures can be reconciled by assuming a cylindrical 

 sensitive volume, which then needs to have a radius of 10 A and 

 a length of 7,400 A. This fits nothing that is known about T-1 



