14 SECONDARY ELECTRONS 



skew; for example, many more secondary electrons have an energy 

 between 10 and 20 ev than between 210 and 220 ev, and still fewer have 

 an energy between 410 and 420 ev. Therefore the location of the upper 

 limit determines only the cut-off point of the far tail of the energy 

 distribution. Compare Fig. 1. 



The shape of the energy distribution can be discussed qualitatively on 

 the basis of the classification of the collisions of the primary particle 

 into two classes, namely, "glancing" and "knock-on" collisions. This 



(electrons) 



Fig. 1. Number of secondary electrons per unit energy, N{E), receiving total energy 

 E from an incident ionizing particle, plotted against E. Note that every secondary 

 must get at least the ionization energy, /, if it is to leave the atom. The great bulk 

 of the energy transfers occur at low energy, and thus the position of the maximum 

 energy transfer, iS'inax, which may vary greatly with type of incoming particle, has 

 in spite of the great variation no large effect on the distribution of secondary energies. 

 The shaded region locates the region important for total energy loss. 



classification has already been explained to you by Morrison. Glancing 

 collisions are much (about 8-10 times) more frequent than knock-on 

 collisions in typical cases. 



One may wish to characterize the shape of the energy distribution by 

 its slope n on a logarithmic plot, that is by assuming a distribution law 

 of the type N(E) dE = dE/E'^. Now, if there were only glancing col- 

 lisions, the slope n would be roughly 4.5. If there were only knock-on 

 collisions, n would be equal to 2. Therefore the glancing collisions, even 

 though by far the most frequent, are much more unlikely to produce 

 high-energy secondaries than the knock-on collisions [(1), pp. 515^.]. 



Low-energy secondaries, say up to 50-100 ev, are due overwhelmingly 

 to glancing collisions. In this energy range the slope n of the logarithmic 

 plot of the spectrum should be of the order of 4. High-energy secondaries 

 are due to knock-on collisions. Above 100-200 ev the slope n should 

 approximate 2. (Figure 2 graphs these relations.) These qualitative 



