IONIZING RADIATION AND VIRUSES 93 



with the virus dimensions given for comparisons. It can be 

 seen that the cross sections of the smaller viruses, as determined 

 from heavy-particle bombardment at high rates of energy loss, 

 are in fair agreement with the accepted area. These do not 

 depend on the concept that one primary ionization inactivates 

 the whole virus, but often correspond to 10 or 20 such ioniza- 

 tions for the effect to be produced. The volume figures, based on 

 this assumption, do not (in general) agree with the whole virus, 

 which can be taken to mean that there is an internal structure 

 of different sensitivity. Radiation measurements should be able 

 to shed some light on this. However, more must be known of 

 radiation action before this can be determined; and to show what 

 other features are present we consider two other experiments 

 which bear on this, though they do not as yet clear it up. 



Combined Thermal and Ionizing-Radiation Action on a 



Virus 



Heat, or perhaps more properly, exposure to temperature, 

 inactivates viruses. It does so by a persistent low-energy agita- 

 tion of some sensitive molecular structure which after a while, 

 aided by fluctuations, gives way to produce inactivation. The 

 threshold energy at the sensitive point turns out to be of the 

 order of 30,000 calories/mole or thereabouts, which is about 

 1.3 ev. This is far below the 110 ev of a primary ionization, and 

 so one would think that it could confidently be asserted that 

 changing the temperature of a virus while it is being irradiated 

 should have no effect. This turns out to be partly true and 

 partly not true. If the radiation is volume random, so that no 

 more than one average primary ionization can occur in a sensi- 

 tive volume at a given time, the measured inactivation volume 

 is (for one bacterial virus — T-1) constant from —80° C to 45° C. 

 This was found by Adams and the author (1952). Above 45° C, 

 the inactivation volume rises rather sharply. These results are 

 shown in Fig. 3.8a. Moreover, if the radiation is densely ionizing, 

 there is a thermal effect even as low as liquid-air temperatures, 

 as can be seen from Fig. 3.8b. The thermal effect becomes more 

 marked at higher temperatures, indicating that at about 45° C an 



