NUCLEUS OF THE ATOM — ANDERSON 237 



the regions of space involved are of the order of, say, one-billionth of 

 an inch; and become meaningless when applied to phenomena con- 

 fined to regions of space a thousand times smaller in extent, such as 

 those required for a nucleus. This difficulty will be resolved only by 

 the extension or revision of the present theory and by the development 

 of new fundamental theoretical concepts. 



But, as we stated, the 1929 model of the nucleus, when uncritically 

 judged, had many successes. It could account, for instance, for the 

 charge and mass of all nuclei. The existence of isotopes (nuclei of 

 the same charge but different mass) and of isobars (nuclei of the 

 same mass but different charge) could be explained simply by choos- 

 ing the proper numbers of protons and electrons out of which to 

 build the nucleus. 



The three well-known types of rays emitted by radioactive sub- 

 stances seemed also to have a place in this picture. Gamma rays, a 

 form of radiant energy consisting of streams of high-energy photons, 

 were attributed to the release of potential energy stored in the 

 nucleus. Alpha rays, helium nuclei consisting each of 4 protons 

 and 2 electrons, represented an alternative mode of release of poten- 

 tial energy in which the 4 jDrotons and 2 electrons removed from the 

 nucleus were bundled together in a stable unit which carried off the 

 energy. Beta rays, comprising a third form of the release of energy, 

 consisted simply of streams of fast-moving electrons. 



In a word, then, all the positive electricity in the world was locked 

 up in the form of protons, and appeared only in nuclei. All the 

 negative electricity existed as electrons, some of which were locked in 

 nuclei while the remainder encircled the nuclei as extranuclear elec- 

 trons. The proton and the electron constituted the two fundamental 

 building stones out of which all matter was to be constructed. 



THE NEUTRON 



Destiny, however, had arranged for a sudden change of this whole 

 point of view. The first inkling of the new knowledge came in 

 1930 ; though the full import of its significance was not realized until 

 nearly 2 years later. W. Bothe and H. Becker in Germany, in 1930, 

 allowed alpha particles to strike, not nitrogen as Rutherford had 

 done, but instead the very light metal beryllium. With their ap- 

 paratus they did not observe the emission of protons, but instead a 

 very j^enetrating ray capable of passing through several inches of 

 lead, even more lead than the most penetrating gamma rays which 

 arise in the natural radioactive substances. From its great pene- 

 trating power they concluded that their new ray must be a very 

 powerful gamma ray. This, as we shall see later, was not entirely 

 correct. 



