74 THE PHYSICS OF VIRUSES 



TABLE 3.2 

 Rate of Energy Loss by Various Kinds of Particle 



Energy loss in ev per 

 Particle 100 A in average protein Loss in SBMV 



. 5-Mev deuteron 



1 . 0-Mev deuteron 



3 . 0-Mev deuteron 

 10. 0-Mev deuteron 

 10. 0-Mev proton 

 10. 0-Mev alpha particle 



2. 0-Mev electron 



ton, the loss of energy is less by a factor of 10, and for a 2-Mev 

 electron it is quite small. 



Space Distribution of Primary Ionizations 



The actual energy loss is in no sense a continuous process 

 but occurs in discrete events, primary ionizations and primary 

 excitations. The number of the former along an electron track 

 can be measured in a Wilson cloud chamber, and in this way the 

 average energy release per primary ionization can be estimated. 

 This can be done, and the process is described by the author 

 (Pollard, 1953). The figure found is 110 ev, on the average, for a 

 primary ionization. Associated with each primary ionization are 

 about three primary excitations with an average energy of 10 

 ev each. Each primary ionization has secondary electron tracks 

 associated with it, but the ionization produced by these, averag- 

 ing two ion pairs, is mostly within 7 A of the primary event. 



The picture to carry in mind is represented in Fig. 3.2. The 

 kind of energy loss for four representative particles along the 

 length of the track is shown in Fig. 3. '2a. The ionization and 

 excitation events are confined to about 5 A distance from the 

 track for a deuteron, and about 25 A for an electron. In Fig. 

 3.2b, the appearance looking down the track is shown. The 

 virus traversed is supposed to have a thickness of 500 A. 



Exploitation of these space relationships can be made in three 

 ways. (a). Heavy particles, which are unavoidably randomly 



