The idea of the neutrino Laurie M. Brown Citation: Physics Today 31, 9, 23 (1978); doi: 10.1063/1.2995181 View online: https://doi.org/10.1063/1.2995181 View Table of Contents: http://physicstoday.scitation.org/toc/pto/31/9 Published by the American Institute of Physics Articles you may be interested in Fermi's Theory of Beta Decay American Journal of Physics 36, 1150 (1968); 10.1119/1.1974382 Corrections Physics Today 32, 66 (1979); 10.1063/1.3043360 Neutrino physics Physics Today 10, 12 (1957); 10.1063/1.3060455 The Multiple-Mirror Telescope Physics Today 31, 30 (1978); 10.1063/1.2995182 Proton or prouton?: Rutherford and the depths of the atom American Journal of Physics 65, 707 (1997); 10.1119/1.18640 The image of scientists in The Big Bang Theory Physics Today 70, 40 (2017); 10.1063/PT.3.3427 The idea of the neutrino To avoid anomalies of spin and statistics Pauli suggested in 1930 that a neutral particle of small mass might accompany the electron in nuclear beta decay, calling it (until Chadwick's discovery) the neutron. Laurie M. Brown During the 1920's physicists came to ac- tron, and that both are created at their which conflict with data on the hyperfine cept the view that matter is built of only moment of emission. Perhaps because of structure of atomic spectra. two kinds of elementary particles, elec- the rapid acceptance of Fermi's theory • Although both alpha and gamma decay trons and protons, which they often and the tendency to rethink history "as it show the existence of narrow nuclear en- called1 "negative and positive electrons." should have happened," the true nature ergy levels, the electrons from a given A neutral atom of mass number A and of Pauli's proposal has been partly over- beta-decay transition emerge with a broad atomic number Z was supposed to contain looked and its radical character insuffi- continuous spectrum of energy. A protons, all in the nucleus, and A neg- ciently emphasized. Contrary to the The strong contrast between the suc- ative electrons, A — Z in the nucleus and impression given by most accounts, Pau- cesses and the failures of quantum me- the rest making up the external electron li's "neutron" has some properties in chanics applied to the nucleus are no- shells of the atom. Their belief that both common with the neutron James Chad- where more evident than in a book by protons and negative electrons were to be wick discovered in 1932 as well as with George Gamow.3 In it, all the passages found in the nucleus arose from the ob- Fermi's neutrino. concerning electrons in the nucleus are set servations that protons could be knocked off in warning symbols (skull and cross- out of light elements by alpha-particle Flaws in the model bones in the original manuscript). bombardment, while electrons emerged By the end of 1930, when our story be- Some physicists (among them Niels spontaneously (mostly from very heavy gins, quantum mechanics had triumphed Bohr and Werner Heisenberg4) took these nuclei) in radioactive beta decay. Any not only in atomic, molecular and crystal difficulties to indicate that a new dy- other elementary constituent of the atom physics, but also in its treatment of some namics, possibly even a new type of would have been considered superfluous, nuclear processes, such as alpha-particle space-time description, might be appro- and to imagine that another might exist radioactivity and scattering of alpha priate on the scale of nuclear distances was abhorrent to the prevailing natural particles from nuclei (including the case and energies, just as quantum mechanics philosophy. of helium, in which quantum-mechanical begins to be important on the atomic Nevertheless, in December 1930 interference effects are so important). scale. These physicists were impressed However, the situation regarding elec- by the similarity of the nuclear radius to Wolfgang Pauli suggested a new elemen- z 2 tary particle that he called a neutron, trons in the nucleus was felt to be critical. the value e' /mc , the classical electron with characteristics partly like that of the The main difficulties of the electron- radius of H. A. Lorentz. At this distance nucleon we now call by that name, and proton model of the nucleus were: it had been anticipated that electrody- partly those of the lepton that we now call • The symmetry character of the nuclear namics would probably fail (and maybe, neutrino (more precisely the electron wave function depends upon A, not Z as with it, the special theory of relativity). antineutrino, but this distinction is not predicted by the model; when A — Z is Bohr was willing to relinquish the con- needed here). Pauli's neutron-neutrino odd the spin and statistics of the nucleus servation of energy, except as a statistical idea became well-known to physicists are given incorrectly. For example, ni- law, in parallel with the second law of even before his first publication of it, trogen (Z = 7, A = 14) was known from thermodynamics. At the same time Heisenberg was considering the intro- which is in the discussion section fol- the molecular band spectrum of N2 to lowing Heisenberg's report on nuclear have spin 1 and Bose-Einstein statis- duction of a new fundamental length into structure at the Seventh Solvay Confer- tics. the theory. It seemed that anything 2 ence, held in Brussels in October 1933. • No potential well is deep enough and might be considered acceptable as a way narrow enough to confine a particle as out of the dilemma—or perhaps anything Shortly after attending this conference, except a new elementary particle. Enrico Fermi published his theory of beta light as an electron to a region the size of decay, which assumes that a neutrino al- the nucleus (the argument for this is ways accompanies the beta-decay elec- based on the uncertainty principle and Pauli's proposal relativistic electron theory). It was in this context of ideas that Pauli Laurie M. Brown is a professor in the Department • It is hard to see how to "suppress" the dared to suggest the existence of a new of Physics and Astronomy, Northwestern Uni- very large (on the nuclear scale) magnetic neutral particle. His proposal, intended versity, Evanston, Illinois. moments of the electrons in the nucleus, to rescue the quantum theory of the nu- 0031-9228/78/3109-023$0.50 © 1978 American Institute of Physics PHYSICS TODAY / SEPTEMBER 1978 23 GOUDSMIT COLLECTION, AIP NIELS BOHR LIBRARY 1957 Zurich lecture. After pointing out one of the major difficulties with the nu- clear model containing only protons and electrons (the symmetry argument men- tioned above), Pauli says: "I tried to connect this problem of the spin and statistics of the nucleus with the other of the continuous beta spectrum, without giving up the energy theorem, through the idea of a new neutral parti- cle." Neutrinos—ejected or created? It is often overlooked in discussing the history of the neutrino idea that Pauli suggested his particle as a constituent of the nucleus, with a small but not zero mass, together with the protons and the electrons. (Chien-Shiung Wu, for ex- ample, emphasizes the non-conservation of statistics that would occur in beta decay without the neutrino.67'9 However, Pauli refers rather to the spin and statistics of stable nuclei such as lithium 6 and nitro- gen 14.) This point is of some signifi- cance; had Pauli proposed in 1930 that neutrinos were created (like photons) in transitions between nuclear states, and that they were otherwise not present in the nucleus, he would have anticipated by PAULI ON THE WAY TO PASADENA, 1931 three years an important feature of Fer- mi's theory of beta decay. Pauli did not cleus from its contradictions, was pre- discrete quantum states of the nucleus (as claim to have had this idea when he wrote sented in good humor as a "desperate indicated by alpha and gamma emission), the Tubingen letter, but he did say (in his remedy," although it was a serious one. some workers, including Meitner, thought Zurich lecture) that by the time he was (The Viennese version would have been, that the beta rays were radiating some of ready to speak openly of his new particle, according to the old joke: desperate, but their energy as they emerged through the at a meeting of The American Physical not serious.) During the next three years strong electric field of the nucleus.5'67 Society in Pasadena, held in June of 1931, he lectured on what he called the "neu- This led C. D. Ellis and William he no longer considered his neutrons to be tron" at several physics meetings and he Wooster at the Cavendish Laboratory in nuclear constituents. It is for this reason, discussed it privately with colleagues. Cambridge, England, who did not believe he says, that he no longer referred to them Pauli's first proposal was put forward in the radiation theory, to perform a ca- as "neutrons"; indeed, that he made use only tentatively, as he recalled in a lecture lorimetric experiment with radium E of no special name for them. However, he delivered in Zurich in 1957, after re- (bismuth) as a source. Their result, later there is evidence, as we shall see, that ceiving news of the experiments con- confirmed in an improved experiment by Pauli's recollections are incorrect; that at firming parity violation in beta decay.5 Meitner and W. Orthmann,8 was that the Pasadena the particles were called neu- Invited to a physics meeting in Tubingen, energy per beta decay absorbed in a trons and were regarded as constituents Germany, which he was unable to attend thick-walled calorimeter was equal to the of the nucleus.