. The Bell System technical journal . be sure that these tracks are due to thenuclei which I have named? This question is far too deep to be an-swered in this place, and I can only assure the listener that while suchpictures by themselves cannot suffice for the proof, an unassailable ^ In the language employed in chemistry, these are exothermic reactions. 306 BELL SYSTEM JTECHNICALl^JOURNAL proof can be and has been given by other and electrical methods ofobserving these newborn particles. But how about the lower reaction of Fig. 6—the one which reallyconcerns us, since all this digression is
. The Bell System technical journal . be sure that these tracks are due to thenuclei which I have named? This question is far too deep to be an-swered in this place, and I can only assure the listener that while suchpictures by themselves cannot suffice for the proof, an unassailable ^ In the language employed in chemistry, these are exothermic reactions. 306 BELL SYSTEM JTECHNICALl^JOURNAL proof can be and has been given by other and electrical methods ofobserving these newborn particles. But how about the lower reaction of Fig. 6—the one which reallyconcerns us, since all this digression is designed chiefly to exhibit theorigin of free neutrons? In Fig. 7 no track appears which can beattributed to either a He^ nucleus or a neutron; and no such tracksappear in other similar pictures. The absence of He^ is, however,due to a simple cause; these nuclei are born with insufficient energyto traverse the wall of the tube. To observe their tracks it is necessaryto suppress the tube-end, to fill the expansion-chamber with heavy. Fig. 8—Tracks of protons, H^ nuclei and He^ nuclei resulting from the twodeuteron-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 directlyinto it. When this is done, all of the region of the gas which theimpinging deuterons can reach becomes completely filled with theions formed along their many tracks, and appears as a flare on thephotograph (Fig. 8). Out of the flare project the tracks of the new-born nuclei. Those which stretch clear across the picture are in partthose of protons, in part those of H* nuclei born from the first in addition one sees a number of short tracks which terminatenot far from the edge of the flare itself. These are the tracks of He^nuclei—not merely guessed, but proved, to be such. RADIOACTIVITY—ARTIFICIAL AND NATURAL 307 Where, however, are the tracks of the neutrons? They are not seenu
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