IONIZING RADIATION AND VIRUSES 73 



Equation 3.3 has been checked very accurately in nuclear 

 physics and cosmic-ray work. Its use, within the proper ap- 

 proximation limits, is therefore justified. 



For electrons there is the added feature of the possibility of 

 exchange between the flying electron and an orbital electron. 

 This modifies the result to 



dE ^Tre'NZ (, mv^E' 



dx mv 



r 2/^(1 - /3^) - In (1 - /3^) + 1 - /3^} (3.4) 



where E' is the kinetic energy of the electron, and j8 is the ratio of 

 electron velocity to that of light. 



One important point arises regarding loss of energy by com- 

 pounds. Chemical binding runs around 5 ev per bond, whereas 

 ionization processes are around 100 ev. It is therefore reasonable 

 that chemical binding should not affect particle energy loss, 

 to a good approximation. Preiss has verified this for C2H2, 

 C2II4, and CH4, where the loss of energy is measured to be 

 essentially that of an atomic mixture in the right chemical 

 proportions. Some useful values of I are given in Table 3.1. 



TABLE 3.1 



Values of the Effective Excitation Potential for Elements 



Element I (ev) Reference 



Returning to viruses, the energy lost in traversing the diam- 

 eter of southern bean mosaic virus by various classes of particle 

 is given in Table 3.2. 



It can be seen from the table that a 0.5-Mev deuteron, which 

 can readily traverse a virus particle, releases a very large amount 

 of energy per virus. Loss of function after receiving this much 

 energy is not surprising. On the other hand, for a 10-Mev pro- 



