]^74 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



however, there is a small but finite probability that the a-particle 

 may escape through the barrier, carrying with it the energy which 

 it possesses within the nucleus. Such a view gives a rational ex- 

 planation of the spontaneous radioactivity shown by uranium and 

 thorium and their products, and also accounts in a general way for 

 the well-known Geiger-Nuttall empirical relation which shows a 

 close connection between the speed of the expelled a-particle from 

 an element and the period of its transformation. In addition, Ga- 

 mow has shown that on the wave-mechanics a charged particle like 

 a proton has a small probability of entering a nucleus even if its 

 energy is much too small on classical views to approach close to the 

 nucleus. This theory accounts for the observation that compar- 

 atively slow particles can cause transformations, and also for the 

 increase of efficiency of transformation with rise of energy of the 

 bombarding particle. 



It is difficult to obtain convincing evidence of the relation, if any, 

 between the two units of structure of the atom, the neutron, and 

 the proton. It is difiicult to measure the mass of the neutron with 

 accuracy, but the evidence indicates that it is slightly heavier than 

 the mass of the proton. It appears likely that the proton and neu- 

 tron are closely related and under some conditions are mutually 

 interchangeable within the nucleus. 



The expulsion of a negative electron from the nucleus may be 

 connected with the change of a neutron into a proton, while the 

 escape of a positive electron may be connected with the reverse opera- 

 tion. The peculiarities of the emission of electrons, both positive 

 and negative, from radioactive atoms may possibly be traced to this 

 interchange. 



Before any detailed theory of nuclear structure can be attempted, 

 it is necessary to find the nature and magnitude of the forces at small 

 distances between the various components of the nuclear structure. 

 Important information on these points ought to be obtained from a 

 close study of the scattering of protons and neutrons in hydrogen. 

 In default of any complete theory, some of the outstanding features 

 of the relation between nuclei — for example, the differences between 

 even- and odd-numbered elements — can be explained in a general 

 way by assuming special types of forces between the elementary 

 particles. 



The spontaneous transformation of the radioactive bodies gave us 

 the negative electron and a-particle as constituents of nuclei; the 

 study of artificial transmutation has given us in addition three new 

 particles, the proton, neutron, and positive electron, as well as a host 

 of new radioactive bodies. In addition, as we have seen, we owe our 

 conception of isotopes to the study of the chemistry of radioactive 



