. The Bell System technical journal . h 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 seenupon this picture, nor in any; for neutrons make no tracks. Neutronsbear no electric charge, and hence they do not ionize the moleculesof the air or any gas as they go through, for ionization is effected onlyby electrical forces which can tear electrons out of their places inmolecules. Not making ions, they afford no footh
. The Bell System technical journal . h 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 seenupon this picture, nor in any; for neutrons make no tracks. Neutronsbear no electric charge, and hence they do not ionize the moleculesof the air or any gas as they go through, for ionization is effected onlyby electrical forces which can tear electrons out of their places inmolecules. Not making ions, they afford no footholds whereby thewater-vapor can condense and mark their passage. The expansion-chamber is frustrated; and worse yet, so are the electrical devices whichserve for detecting charged particles like fast protons or fast electrons,since they, too, depend on the ions which these can make. The neutronindeed might slip through all of our apparatus completely undetected,were it not liable to make collisions with nuclei so sharp and sudden. Fig. 9—Track of a proton recoiling from the impact of a neutron. (I. Curie andF. Joliot, Institut du Radium; Journal de Physique.) that they might be compared with impacts of one billiard-ball uponanother. Comparisons with billiard-balls are rife in physics, butseldom with so much justification. When neutrons are streamingthrough a gas, such impacts are suffered by nuclei of occasional atomsof the gas; and like a struck billiard-ball they recoil, and in recoilingthey are able to make ions and the ions then serve to reveal them. The track of such a recoiling nucleus, made visible in a cloud-chamber, is seen in Fig. 9. This was taken soon after the discoveryof the neutron, and at a time when these particles had as yet beenreleased only by using natural radioactive substances to projectalpha-particles against various targets. Such ways of producing freeneutrons are not very efficient, and accordingly one sees in the wholeexpansion-chamber one track
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