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BELL SYSTEM TECHNICAL JOURNAL 



made the test with a pair of Geiger counters set on opposite sides of 

 the bombarded lithium, and got a positive result; but it is the ex- 

 pansion-chamber which is suited by its nature for supplying the most 

 magnificent of proofs. To achieve this, one must put the bombarded 

 target of lithium in the middle of the chamber, and photograph the 

 tracks from above; and since the bombarding stream must come 

 through vacuum while the chamber must be filled with moistened air, 

 the target must be separated from the air by walls of mica thick 



FAST I 

 PROTONS 



-SHUTTER 



KWWWWWW^ kWWNWWWSM 



MERCURY 

 VAPOR 

 LAMP 



a 



MICA WINDOWS 

 ' SUPPORTED 

 ON GRID 



Fig. 12 — Diagram of arrangement for observing tracks of fragments by the expansion- 

 method. (After Dee and Walton) 



enough to withstand the pressure and thin enough to let the fragments 

 pass. The scheme is clearly depicted in Fig. 12. One notices that the 

 design is such that the pairs which are observed are those of w^hich the 

 directions are nearly at right angles to the proton-beam — the "sym- 

 metrical case" aforesaid. 



This experiment was first performed by Kirchner of Munich, who 

 got several pictures of paired fragments from lithium bombarded by 

 protons. Fig. 13 shows an example. (The third track is rather 



