RADIOACTIVITY— ARTIFICIAL AND NATURAL 307 



Where, however, are the tracks of the neutrons? They are not seen 

 upon this picture, nor in any; for neutrons make no tracks. Neutrons 

 bear no electric charge, and hence they do not ionize the molecules 

 of the air or any gas as they go through, for ionization is effected only 

 by electrical forces which can tear electrons out of their places in 

 molecules. Not making ions, they afford no footholds whereby the 

 water-vapor can condense and mark their passage. The expansion- 

 chamber is frustrated ; and worse yet, so are the electrical devices which 

 serve for detecting charged particles like fast protons or fast electrons, 

 since they, too, depend on the ions which these can make. The neutron 

 indeed 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 and 

 F. Joliot, Institut du Radium; Journal de Physique.) 



that they might be compared with impacts of one billiard-ball upon 

 another. Comparisons with billiard-balls are rife in physics, but 

 seldom with so much justification. When neutrons are streaming 

 through a gas, such impacts are suffered by nuclei of occasional atoms 

 of the gas; and like a struck billiard-ball they recoil, and in recoiling 

 they 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 discovery 

 of the neutron, and at a time when these particles had as yet been 

 released only by using natural radioactive substances to project 

 alpha-particles against various targets. Such ways of producing free 

 neutrons are not very efficient, and accordingly one sees in the whole 

 expansion-chamber one track and one only (though I must interpolate 

 that according to our present knowledge, many thousands of neutrons 



