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BELL SYSTEM TECHNICAL JOURNAL 



The next important question is, how the fragments are identified as 

 protons, or as alpha-particles, or otherwise, from the data. Few as 

 yet are the cases in which the identification is full and undeniable. 

 In the earlier paper I described Stetter's measurements of charge- 

 to-mass ratio for the fragments produced by impacts of alpha-particles 

 against boron, carbon, fluorine and aluminium, which gave values 

 identical with that for protons within the observational uncertainty 

 of five per cent. As for the fragments produced by impacts of protons, 

 the best direct evidence is that which appears in Fig. 7. Cockcroft 



? 5 



z 

 o 

 \- 



111 ^ 



o 



9 2 



o I 



Hill 



5.0 5.6 6.0 6.5 7.0 



AIR EQUIVALENT IN CENTIMETERS 



Fig. 7 — Ionization produced in a shallow chamber by fragments (of the trans- 

 mutation of lithium by protons) which have passed through screens of various 

 thicknesses. (Cockcroft and Walton) 



and Walton had an ionization-chamber only 3 mm. across, and the 

 fragments from bombarded lithium traversed it completely, producing 

 a few thousand ion-pairs apiece which were detected and measured 

 with the aid of an amplifying circuit of Wynn-Williams according to 

 the method C4. When mica sheets were interposed in the path of the 

 fragments from the lithium, they were slowed down but still kept 

 energy enough (so long as the sheets were not too thick altogether) 

 to travel across the chamber; and the curve of Fig. 7 represents the 

 number of ion-pairs produced per fragment, as function of a quantity 

 X proportional to the thickness of mica which the fragments have 



