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proof can be and has been given by other and electrical methods of 

 observing these newborn particles. 



But how about the lower reaction of Fig. 6 — ^the one which really 

 concerns us, since all this digression is designed chiefly to exhibit the 

 origin of free neutrons? In Fig. 7 no track appears which can be 

 attributed to either a He^ nucleus or a neutron; and no such tracks 

 appear in other similar pictures. The absence of He^ is, however, 

 due to a simple cause; these nuclei are born with insufficient energy 

 to traverse the wall of the tube. To observe their tracks it is necessary 

 to suppress the tube-end, to fill the expansion-chamber with heavy 



Fig. 8 — Tracks of protons, H^ nuclei and He^ nuclei resulting from the two 

 deuteron-deuteron reactions. (P. I. Dee and C. W. Gilbert, Cavendish Laboratory; 

 Proceedings of the Royal Society.) 



hydrogen in the gaseous form, and to project the deuterons directly 

 into it. When this is done, all of the region of the gas which the 

 impinging deuterons can reach becomes completely filled with the 

 ions formed along their many tracks, and appears as a flare on the 

 photograph (Fig. 8). Out of the flare project the tracks of the new- 

 born nuclei. Those which stretch clear across the picture are in part 

 those of protons, in part those of H^ nuclei born from the first reaction. 

 But in addition one sees a number of short tracks which terminate 

 not far from the edge of the flare itself. These are the tracks of He^ 

 nuclei — not merely guessed, but proved, to be such. 



