IONIZING RADIATION AND VIRUSES 



71 



quency .si)ectriim with many high-frequency components, 

 whereas the other has mainly lower frequency components. 

 Since we have the familiar equivalence E = hv, where E is 

 energy, h is Planck's constant, and v is frequency, the very-high- 

 energy transitions are less likely for a slow particle than for a 

 fast particle. 



(a) 



Position of Particle 



Field 

 at 

 A 



Time 



Fig. 3.1. (a) represents the flight of a fast particle past an atom at A. (b) 

 shows schematically the electric field at A as the particle passes, (c) shows the 

 same referred to time, allowing for the velocity of the particle. 



Notice that, because all that is important is the field, the mass 

 of the flying particle is unimportant. 



This whole process has been subjected to rigorous theoretical 

 analysis, for the case where A is a hydrogen atom, by Bethe 

 (1930) and Bloch (1931). A simple and clear account of the 

 theory is given by Fermi in his book Nuclear Physics (1949). 

 The extension to more complex atoms can be made by a mixture 



