Fermi Proposes the Neutrino Theory of Beta Decay Summary

  • Last updated on November 10, 2022

Enrico Fermi used quantum mechanics to derive a theory of radioactive beta decay involving the neutrino and a new weak force of nuclear interactions, which led to many new elementary particle discoveries.

Summary of Event

Enrico Fermi’s theory of beta decay solved one of the most puzzling problems of radioactivity and provided strong evidence for the existence of the neutrino and of a new type of weak force in nuclear interactions. Radioactivity was discovered by Antoine-Henri Becquerel in 1896 in the form of spontaneous and continuous radiation from various compounds of uranium that could penetrate layers of black paper to expose a photographic plate. Additional radioactive elements were discovered by Marie Curie and others in the next few years, and Ernest Rutherford Rutherford, Ernest demonstrated more than one type of radiation from these materials in 1898 at the Cavendish Laboratory in Cambridge. He showed that one component could be absorbed by a single piece of paper, whereas another component was about one hundred times more penetrating. He called the short-range radiation alpha rays and the penetrating radiation beta rays. [kw]Fermi Proposes the Neutrino Theory of Beta Decay (Nov.-Dec., 1933) [kw]Neutrino Theory of Beta Decay, Fermi Proposes the (Nov.-Dec., 1933) [kw]Beta Decay, Fermi Proposes the Neutrino Theory of (Nov.-Dec., 1933) Beta decay Neutrinos;beta decay Radioactive beta decay [g]Italy;Nov.-Dec., 1933: Fermi Proposes the Neutrino Theory of Beta Decay[08440] [c]Science and technology;Nov.-Dec., 1933: Fermi Proposes the Neutrino Theory of Beta Decay[08440] [c]Physics;Nov.-Dec., 1933: Fermi Proposes the Neutrino Theory of Beta Decay[08440] Fermi, Enrico Pauli, Wolfgang Dirac, Paul Adrien Maurice Becquerel, Antoine-Henri Yukawa, Hideki

Enrico Fermi.

(The Nobel Foundation)

By the end of 1899, Becquerel and several other researchers had shown that beta rays are deflected by a magnetic field in the same direction as cathode rays (electrons) and thus consist of negatively charged particles. In 1900, Paul Villard Villard, Paul in France discovered an even more penetrating radiation that was not deflected by a magnetic field. These rays were eventually called gamma rays and were shown to be electromagnetic waves like X rays but with higher frequencies. Meanwhile, Becquerel showed that the deflection of beta particles was consistent with the behavior of electrons, with some of them ejected at speeds of up to about half the speed of light.

By 1903, Rutherford succeeded in observing a much smaller magnetic deflection of alpha rays in the direction of positively charged particles, and by 1909 he had identified them as positive helium ions (helium atoms that have lost their two electrons). Other differences between alpha and beta particles soon became evident. The deflection of alpha particles was found to be well collimated, whereas beta particle deflections formed a diffuse and extended image on a photographic plate. Thus alpha decay seemed to produce particles of a single velocity compared with beta particles with a variety of energies.

By the end of 1903 at McGill University in Montreal, Rutherford and Frederick Soddy Soddy, Frederick proposed that alpha and beta decay result from a transmutation of the atoms of one element into another. Eventually, it was shown that alpha decay produces a new element with atomic number (of nuclear protons) reduced by two and atomic mass number (protons and neutrons) reduced by four, corresponding to the charge and mass of the helium nucleus. Beta decay increases the atomic number by one with no change in atomic mass number.

The spontaneous energy associated with radioactive decay was at first a mystery that seemed to contradict the law of conservation of energy. This mystery was partially resolved with Albert Einstein’s theory of relativity in 1905, which showed the equivalence of mass and energy and could account for radioactive energy from the decrease in mass between the parent and product nuclei. The energy of alpha particles was found to agree closely with this change in mass, but beta particles did not fit as well. By 1914, James Chadwick Chadwick, James confirmed the continuous distribution of beta particle energies, in addition to some definite energies associated with so-called conversion electrons ejected from their orbits around the nucleus by gamma rays. The continuous beta energy spectrum was found to vary from small energies up to a maximum value that corresponded to the decrease in mass in the nuclear transmutation.

For several years, the problem of the continuous distribution of energies among beta particles perplexed physicists. Calorimeter measurements by Lise Meitner Meitner, Lise and others showed that the average energy of beta decay was only about one-third the value expected from the difference in masses of the parent and product nuclei. In his Faraday lecture of 1931, Niels Bohr Bohr, Niels suggested that energy conservation might not apply to beta decay. Additional problems arose with the quantum concept of particle spin, indicating that angular momentum might not be conserved in the beta decay of an electron with spin.

One way to save the conservation principles was suggested by Wolfgang Pauli at a meeting of the American Physical Society in Pasadena, California, in June, 1931. He suggested that the beta decay electrons might be accompanied by light neutral particles too penetrating to be observed or have any effect on a calorimeter experiment. The beta particle and the neutral particle would share the available energy so that their sum would be equal to the mass-energy difference between parent and product nuclei. Such a particle would have zero or near-zero rest mass, given that some beta electrons were at or near the maximum possible energy. It would be neutral to conserve charge and have spin equal to the electron spin but possibly in the opposite direction to conserve angular momentum.

In Rome, Pauli’s light neutral particle was called the neutrino (Italian diminutive form of the word “neutron”) to distinguish it from the neutron, which was discovered by Chadwick in 1932 and shown to have a mass slightly greater than the proton. Pauli’s neutrino hypothesis was first published in a report on discussions at the Solvay Conference on physics held in Brussels in October of 1933. After returning from the conference, Fermi began to develop a quantitative theory for the role of the neutrino in beta decay from the equations of quantum mechanics. He followed the same approach that Paul Adrien Maurice Dirac used in his 1928 relativistic quantum theory of photon emission and pair creation (particle and antiparticle). He postulated the simultaneous creation of an electron and an antineutrino (same as a neutrino except for spin) when a nuclear neutron converts into a proton during beta decay. In doing this, Fermi introduced a new weak force of much shorter range than the familiar gravitational and electromagnetic forces. The equation he derived for the probability of beta emission contained a coupling constant for the weak interaction that was determined from beta decay data to be 100 billion times smaller than the corresponding coupling constant in Dirac’s theory of electromagnetic interactions. The weakness of this interaction is one factor in the relatively long half-life for beta decay.

Fermi sent his findings to the British journal Nature at the end of 1933, but it was promptly rejected. It was then accepted for publication in the German journal Zeitschrift für Physik in January, 1934. Fermi’s theory successfully explained the exact form of the beta decay energy spectrum, the decay half-life, and its relation to beta particle energies, and other characteristics of beta decay. After the 1934 discovery of artificial radioactivity with positron (positive electrons) emission, physicists realized that Fermi’s theory also fit this positive beta decay in which nuclear protons convert to neutrons by the emission of a positron and neutrino. The discovery of electron capture in 1937, in which a nuclear proton converts to a neutron by capturing one of its atomic electrons, could also be explained with Fermi’s theory by assuming the emission of a neutrino. The beta decay theory was Fermi’s theoretical masterpiece and the foundation for many new discoveries.


Fermi’s beta decay theory led to the development of new concepts of nuclear interactions and the discovery of new elementary particles. In 1935, Hideki Yukawa used the theories of Fermi and Dirac (for weak and electromagnetic interactions) as models for his theory of the strong nuclear force that holds protons and neutrons together in the nucleus. His theory included the prediction of the meson as the field quantum to transmit the strong interaction, and he also proposed a field quantum to mediate weak interactions in beta decay instead of the direct coupling used by Fermi. Yukawa showed that the meson would have mass equal to about two hundred electron masses; his theory implied a much larger mass for the field quantum to transmit weak interactions, now called the W-particle because of its much shorter range.

In 1937, a 207-electron-mass particle was discovered in cosmic rays, but Fermi and his associates showed that this so-called muon interacted with matter by a weak force. A 273-electron-mass particle, now called the pion, was discovered in 1947. It interacted by a strong force and matched the properties of Yukawa’s meson. The neutrino was more difficult to detect because it passed readily through matter. In 1956, C. L. Cowan Cowan, C. L. and Frederick Reinries Reinries, Frederick confirmed the effects of neutrinos near a nuclear reactor, where enough were produced for a few to interact with protons to yield nearly simultaneous neutrons and positrons, easily detected in this so-called reversed beta decay. By 1961, experiments revealed a difference between the electron neutrino of ordinary beta decay and the muon neutrino produced when pions decay into muons. Theories predicting a large production of neutrinos when stars explode were confirmed by the supernova of February, 1987, when about ten neutrino events were recorded in a Japanese detector.

The most important result of Fermi’s beta decay theory was the development of a unified electroweak theory Electroweak theory in 1967 by Steven Weinberg Weinberg, Steven and independently by Abdus Salam. Salam, Abdus These researchers showed that the electromagnetic and weak interactions are different aspects of a single electroweak force requiring three new massive field quanta. The electroweak theory was dramatically confirmed in 1983 when a 135-member team led by Carlo Rubbia Rubbia, Carlo used proton collisions in a giant accelerator in Geneva to produce positive and negative W-particles with mass about eighty-five times the proton mass and neutral Z-particles with about ninety-seven proton masses, in exact agreement with the Weinberg-Salam theory. Beta decay Neutrinos;beta decay Radioactive beta decay

Further Reading
  • citation-type="booksimple"

    xlink:type="simple">Beyer, Robert T., ed. Foundations of Nuclear Physics. New York: Dover, 1949. This volume contains facsimile copies of a dozen foundational articles, including Fermi’s original 1934 beta decay article in German, the 1932 article by Chadwick on the existence and mass of the neutron, and Yukawa’s 1935 article on the meson theory of the strong nuclear force and beta decay. The second half of the book is a 120-page bibliography of the most important early articles on nuclear physics.
  • citation-type="booksimple"

    xlink:type="simple">Cropper, William H. Great Physicists: The Life and Times of Leading Physicists from Galileo to Hawking. New York: Oxford University Press, 2001. Presents portraits of the lives and accomplishments of important physicists and shows how they influenced one another with their work, including Fermi, Pauli, and others. Includes glossary and index.
  • citation-type="booksimple"

    xlink:type="simple">Eisberg, Robert, and Robert Resnick, ed. Quantum Physics of Atoms, Molecules, Solids, Nuclei, and Particles. 2d ed. New York: John Wiley & Sons, 1985. An intermediate-level college physics textbook with good sections on beta decay, Fermi’s theory of weak interactions, and the electroweak theory. Theoretical results are discussed with equations, graphs, and diagrams, but without lengthy mathematical derivations.
  • citation-type="booksimple"

    xlink:type="simple">Evans, Robley D. The Atomic Nucleus. New York: McGraw-Hill, 1955. A classic graduate-level textbook on nuclear physics with excellent historical documentation, with more than thirty pages of bibliography. A chapter on beta-ray spectra gives a good outline of experimental data and a twenty-page sketch of the mathematical details of Fermi’s beta decay theory.
  • citation-type="booksimple"

    xlink:type="simple">Glasstone, Samuel. Sourcebook on Atomic Energy. 3d ed. Princeton, N.J.: D. Van Nostrand, 1967. A well-organized introduction to atomic and nuclear physics with good historical detail. Chapters on radioactivity, nuclear radiations, and elementary particles provide good background on the discovery of beta decay, Fermi’s theory, and the neutrino.
  • citation-type="booksimple"

    xlink:type="simple">Piel, Gerard. The Age of Science: What Scientists Learned in the Twentieth Century. New York: Basic Books, 2001. An overview of the scientific achievements of the twentieth century. Includes many illustrations and index.
  • citation-type="booksimple"

    xlink:type="simple">Polkinghorne, John. Quantum Theory: A Very Short Introduction. New York: Oxford University Press, 2002. Aims to make quantum theory accessible to the general reader. Among the concepts discussed are uncertainty, probabilistic physics, and the exclusion principle. Includes mathematical appendix and index.
  • citation-type="booksimple"

    xlink:type="simple">Segrè, Emilio. From X-Rays to Quarks: Modern Physicists and Their Discoveries. San Francisco: W. H. Freeman, 1980. A very readable historical account of modern physics and of the personalities involved in it. A chapter on radioactivity and a brief treatment of Fermi’s beta decay theory provide personal details by an early associate of Fermi. Many interesting historical photographs and diagrams are included.
  • citation-type="booksimple"

    xlink:type="simple">Strachan, Charles. The Theory of Beta-Decay. Elmsford, New York: Pergamon Press, 1969. An introduction to the theory of beta decay, the neutrino, and weak interactions is given in part 1. In part 2, several original articles are reprinted, including an English translation of Fermi’s 1934 article on beta decay.

Elster and Geitel Study Radioactivity

Becquerel Wins the Nobel Prize for Discovering Natural Radioactivity

Boltwood Uses Radioactivity to Determine Ages of Rocks

Geiger and Rutherford Develop a Radiation Counter

Rutherford Discovers the Proton

First Artificial Radioactive Element Is Developed

Segrè Identifies the First Artificial Element

Hahn Splits the Uranium Atom

Categories: History