20 SECONDARY ELECTRONS 



two effects. The ion species of lower ionization potential predominates, and the 

 difference in energy becomes available for excitation. 



Fano: 



I should like to ask Burton whether he would expect the cross section to change 

 markedly. 



Burton: 



There is competition between the positive and negative ions after they are 

 thermaUzed. The probability of capture increases as the electron gets nearer and 

 nearer thermal energies, but we must consider also the probability of positive-ion 

 capture against the probabiUty of negative-ion capture. 



Platzman: 



I think that at the present time the effect of the efficiency of ionization by 

 slowly moving positive ions cannot be predicted adequately by existing theory. 

 It may be that such ions are ionized with greater efficiency than heretofore 

 reaUzed. Our knowledge of the efficiency of ionization by ions whose velocity 

 is in the region of orbital velocity or Ko of that value is most inadequate. 



Fano : 

 I have discussed this matter with London, who doesn't see how this could be. 



Platzman : 



The efficiency of ionization of a gas by a heavy particle penetrating with 

 velocity comparable to or lower than the velocities of valence electrons in atoms 

 of the gas is a question of very considerable importance which is often dismissed 

 inadequately or erroneously and which merits brief mention here. 



To say that a heavy particle of velocity Vq {vq = c/137 = velocity of the 

 electron in the lowest orbit of the hydrogen atom), which has a most appreciable 

 energy (25 kev for a proton, 99 kev for an alpha particle), does not ionize with 

 good efficiency is not justified by any established knowledge and is probably also 

 incorrect. The basis for this frequently encountered statement lies in the fact 

 that, as the velocity of the particle decreases, for values below vq, the particle 

 spends an increasing proportion of its time as a neutral atom; effects on atoms 

 of the gas therefore tend to be Umited to direct encounters, in which colliding 

 and struck atoms interpenetrate, and such collisions take on an increasingly 

 adiabatic character as the coUision velocity falls. (Some ionization, but with 

 extremely small yield, is known to occur even at the lowest velocities.) Thus, 

 at very low particle velocities, the probability of excitation or ionization of the 

 gas atom will be small, whereas the probabiUty of scattering— deflection of the 

 particle, with resultant energy less as direct momentum transfer to the atom as 

 a whole— will be great. At sufficiently low particle velocity, therefore, the simple 

 "nuclear coUision" is the dominant mode of energy loss. Just how low this 

 velocity must be is an important question. Unfortunately, the theory of pene- 

 tration phenomena has thus far been unable to cope with the problem, at least 



