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Secondary Electrons: 

 Average Energy Loss per Ionization 



U. FANO 



Radiation Physics Laboratory 



National Bureau of Standards 



Washington, D.C. 



Following Morrison's picture of the physical action of ionizing 

 radiations on matter, I should like to elaborate a little on some details. 

 As a part of our general program, I shall deal with two particular topics, 

 namely: (1) the distribution of radiation energy by secondary electrons, 

 and (2) the factors that control the amount of ionization produced in 

 matter, but particularly in gases, per unit amount of energy distributed 

 by radiation. 



Spectrum of Secondary Electrons 



Secondary electrons are ejected from atoms under the impact of other 

 fast charged particles. They are ejected with a kinetic energy that may 

 be anywhere between zero and a certain upper limit. 



The upper limit is set by the conservation of momentum and energy 

 in the collision [(1), p. 494]. If the incident particle is a heavy one 

 (proton, alpha particle, etc.), it cannot impart to a secondary electron 

 more than twice its own speed. Thus, for example, if the incident proton 

 has an energy of 1 mev, the maximum energy of secondary electrons 

 equals 2200 ev. On the contrary, if the incident particle is itself an 

 electron, it may share any fraction of its energy with an atomic electron 

 in the course of a collision. WTien the two energies are comparable after 

 the collision, one usually calls "primary" the faster of the two electrons 

 and "secondary" the slower one; this convention amounts to fixing the 

 upper limit to the energy of the secondary electrons at one-half the 

 energy of the primary. 



Even though the upper limit to the energy of the secondary electrons 

 depends on the nature and energy of the primary particle, the energy 

 distribution of the secondaries depends but little, on the whole, on these 

 conditions. The reason is that the energy distribution is completely 



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