DISCOVERY OF THE NEUTRINO — COWAN 413 



small regions of the nucleus or, at best, only held on the average there. 

 Wolfgang Pauli, however, suggested in 1931 that the rules held fast, 

 but that there was a new, small, electrically neutral particle which was 

 emitted simultaneously with the beta particle and which carried away 

 the missing energy and momentum. 



Unorthodox proposals such as this seldom find a friendly audience — 

 nor did this one. In the early 1930's, few took Pauli seriously, but one 

 who did was Enrico Fermi. Building a theory analogous to the theory 

 of gamma decay (which describes the creation of a photon by a 

 nucleus) but in which an electron and Pauli's little particle were pro- 

 duced simultaneously, Fermi succeeded in 1934 in devising an equation 

 which described the phenomena of beta decay with uncanny accuracy. 

 It correctly predicted the shapes of the energy spectra for various 

 kinds of beta decay and correctly predicted the half-lives of these 

 various radioactive nuclei. With such impressive success with Pauli's 

 little neutral particle, Fermi suggested that it be named "neutrino." 



In constructing his theory, Fermi had used the results obtained by 

 P. A. M. Dirac in 1928 in which Dirac had succeeded in finding an 

 equation for the electron which satisfi.ed the theory of relativity. An 

 unexpected result of Dirac's work was the prediction of the existence 

 of positive electrons in nature — a prediction confirmed by the observa- 

 tion of "positrons" by Carl D. Anderson in 1932. Fermi applied 

 this theory not only to the beta particle (the fast electron ejected by 

 a decaying nucleus) but also to the neutrino. Thus, the neutrino would 

 not only be coupled with an antineutrino in nature (as the electron 

 is to an antielectron ; the positron), but also would have an intrinsic 

 spin angular momentum of % unit, the same as does the electron. 

 In using these theoretical predictions of the Dirac equation, Fermi 

 was building a complete conservation into his own theory: That of 

 energy, of linear momentum, of angular momentum, of electric charge, 

 and of "light particles" (now called "leptons"). 



INTERACTIONS AND THE PENETRATION OF MATTER 



Natural phenomena are treated by modern physics in terms of "in- 

 teractions," or basic forces which can be looked upon as causing 

 things to happen. The "constant of gravitation," the G in Newton's 

 equation for the gravitational attraction between two masses, is the 

 most venerable of the "interactions" we know of in nature. Electrical 

 phenomena are described in terms of the Coulomb interaction, and 

 nuclear reactions in terms of a "strong" nuclear force. For his theory 

 of beta decay, Fermi postulated yet another interaction — that which 

 causes the decay. The strength of the interaction affects the rapidity 

 with which a given event will occur. In radioactive decay it de- 

 termines the half-life of any given radioactive species. Conversely, 

 if the half -life is measured for a given species, and if the theoretical 



