504 



H. G R A Y 



tion for protons and electrons. The upper curve, the ordinates of 

 which it will be seen are at each speed (except at the lowest speeds) 

 four times as great as that of the lower curve, represents the variation 

 with speed of the much greater linear ion densitj^ of an a particle. 



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i 4000 



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 ui 

 m 



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VELOCITY (ALL PARTICLES), cm/sec. 



1 



0,5 

 ELECTRON ENERGY, e. kv. 



— T — 



1.0 



(range 0.05/i) 



— I — 

 05 



1.0 



PROTON ENERGY, m.e.v. 



— 1 — 

 1.5 



2.0 

 (ronge 70 /i) 



cr-PARTICLE ENERGY, mev (range 70^) 



Fig. \a. Curves illustrating the manner in which the total number of ions 

 formed by electrons, protons, and a particles per micron of track in tissue varies 

 with speed and energy of the particle. See also Figure 16. The curves here 

 and in Figure \h were computed from known rates of energy loss by particles 

 {1, Tables 10-12) for the hypothetical case in which the average energy expended 

 in formation of a pair of ions in tissue is 32.5 e.v. for electrons and 35 e.v. for 

 protons and a particles. 



At low speeds the a-ray ordinates are less than four times the pro- 

 ton and electron ordinates because slow a particles capture electrons, 

 which accompany them during part of their flight and are then lost 

 again, with the result that the mean charge of the a particle during 



