ENERGY DISSIPATION BY SECONDARY ELECTRONS 



15 



theoretical predictions are confirmed by the available experimental 

 evidence. However, this evidence is not very abundant. 



log/ 



log£ 



Fig. 2. The energy distribution among secondary electrons, plotted now on a log-log 

 scale. Note that the shaded region, which again represents the bulk of the second- 

 aries, comes mainly from the glancing collisions. The knock-on collisions become 

 dominant only for the infrequent but relatively energetic encounters. The slope of 

 the skew distribution is much steeper for the glancing than for the knock-on col- 

 lisions, as explained in the text. 



It would be helpful to have reliable and detailed tables of the energy 

 distribution of secondary electrons, but such tables do not seem to be 

 available at this time. 



Energy Dissipation by Secondary Electrons 



As we have seen, the great majority of the secondary electrons have 

 a rather low energy, even though their aggregate energy amounts to 

 about two-thirds of the energy lost by a fast particle. Electrons whose 

 energy amounts to no more than 100 or 200 ev can transfer energy only 

 to the external electrons of atoms, and this only when passing right 

 through or very close to an atom. On the other hand, every passage in 

 the proximity of an atom has a fair chance of leading to a collision with 

 energy transfer. Also, low-energy electrons experience frequent, re- 

 peated, large-angle deflections. 



Low-energy secondaries dissipate most of their energy within a short 

 distance from their point of origin. This distance is of the order of 10 A 

 in solid or liquid materials and about 1000 times as large in gases at 

 atmospheric pressure. This energy is dealt out in the form of activations 

 (excitations or ionizations) at points irregularly scattered in the proximity 



