70 THE PHYSICS OF VIRUSES 



cases where several properties of a virus have been studied, the 

 behaviors have been different. This is of great importance be- 

 cause each function i)ro})ably occupies a different part of the 

 virus, and the radiation action is demonstrating the fact. 



To repeat in a rather different way, the use of radiation is 

 aimed at exploiting the space relationshi})s of high-energy re- 

 leases in terms of effect on the virus. It is, accordingly, vital to 

 know what these relationships are, and to conduct experiments 

 so that the original relationships are retained. 



Nature of Energy Loss by Fast Charged Particles 



The energy loss of a fast, charged particle occurs as a result 

 of its passage near an atom. If it causes an atom to become 

 excited or to lose an electron (i.e., to be ionized), the energy 

 gained by the atom is lost by the particle. This process occurs in 

 terms of a probability only. That is to say, the fast, charged 

 particle can approach 100 atoms in exactly the same way, and 

 for, say, five of these, some sort of excitation can happen whereas 

 the other 95 are entirely unchanged. This method of operation is 

 that required by modern atomic theory. The probability of 0.05 

 is determined by some quite simple considerations, but it is 

 always no more than a probability. 



The considerations are as follows. Suppose a charged particle 

 (e.g., a proton) is approaching an atom situated as at A in 

 Fig. 3.1a. As the particle moves along its track, the electric 

 field at A takes values something like the indications in Fig. 

 3.1b. Now while this field is present, the atom, or molecule, is 

 in a highly strained condition, and as a result of this strain may 

 change its configuration to one of the possible excited states, or 

 it may ionize, or, if in a molecule, dissociate. If the flying particle 

 is fast, the field, as a function of time, rises and falls rapidly. 

 If it is slow, the rise and fall is relatively slow. This is indicated 

 in Fig. 3.1c. This time plot shows two very important features of 

 radiation action. First, the slow particle produces the strain in 

 the atom or molecule for a longer time and so increases the 

 chance of response by excitation or ionization. Second, the field 

 which rises rapidly and falls rapidly has an equivalent fre- 



