Neutrino History — Part 1: From Pauli's proposal to the eve of detection (1930–1955)

This is Part 1 of the six-part Neutrino History series. We start where everything started — in 1930 in Zürich, with a Swiss-Austrian theorist who had a problem with beta decay and a ball to attend.

1930: The desperate remedy

By the late 1920s, physicists had a problem with radioactivity. When certain nuclei decayed by beta-emission, the electron came out with a continuous spectrum of energies, sometimes much less than the maximum allowed. The energy difference simply vanished. Either energy conservation broke down at the nuclear level — an idea Niels Bohr was prepared to entertain — or something invisible was carrying away the missing energy.

On December 4, 1930, Wolfgang Pauli wrote what is now the most famous letter in the history of neutrino physics. He had been invited to a small radioactivity conference in Tübingen, Germany. He did not want to go — he was attending a ball in Zürich that weekend. So he wrote a letter to the conference instead.

The letter opened “Dear radioactive ladies and gentlemen” and proposed what Pauli called “a desperate remedy”: a new, electrically neutral, nearly massless particle that must be emitted along with the electron in beta decay, carrying away the missing energy. The proposal saved energy conservation at the cost of postulating a particle that nobody had ever seen. Pauli apologized for the suggestion. He thought it would never be directly detected.

For the full story, see Pauli’s 1930 letter and the Wolfgang Pauli profile.

1932: A naming problem

Pauli had called his particle the “neutron” in his original letter. The name became inconvenient in 1932, when James Chadwick discovered the actual neutron — the heavier neutral particle inside the nucleus. The community needed a different name for Pauli’s particle.

Enrico Fermi, then leading the Via Panisperna group in Rome, suggested “neutrino” — Italian for “little neutral one.” The diminutive distinguished it from the heavier Chadwick neutron and captured Pauli’s intent that the particle was very light. The name stuck. So did the underlying physics.

1933: Fermi’s theory of beta decay

In late 1933, Fermi wrote what he called his “tentative theory of beta rays.” The paper, submitted to Nature, was rejected as too speculative. Fermi published it in Italian and German journals instead. It quickly became one of the most influential theoretical papers of the twentieth century.

Fermi’s theory introduced the weak interaction as a distinct fundamental force (separate from electromagnetism, the strong nuclear force, and gravity). The interaction caused a neutron in a nucleus to transform into a proton plus an electron plus a neutrino. The theory predicted in detail how beta-decay spectra should look — and matched the data quantitatively.

The theory also predicted that a neutrino, if it ever interacted directly with anything, would interact via the weak force. This was the basis for everything later in neutrino physics: from Reines and Cowan’s 1956 detection through Super-Kamiokande’s 1998 oscillation discovery, every neutrino interaction we have ever caught has been a weak-interaction event predicted by Fermi’s framework.

1934: Slow neutrons

Through the mid-1930s, the Via Panisperna group systematically bombarded every available element with neutrons from a radium-beryllium source. One day, in 1934, they discovered that putting paraffin or water between the neutron source and the target dramatically increased the resulting radioactivity rather than reducing it as you might expect from absorption.

Fermi quickly understood: the hydrogen in the paraffin was slowing down the neutrons, and slow neutrons had a much larger cross-section for absorption by the target nuclei. This was the discovery of slow neutrons — a finding that opened the practical possibility of nuclear fission and won Fermi the 1938 Nobel Prize. The same neutron capture work would later enable the reactor antineutrino fluxes that made Cowan and Reines’s 1956 detection possible.

1937: Majorana’s last paper

Ettore Majorana, the most original mind in the Via Panisperna group (Fermi reportedly called him “a genius of the rank of Galileo and Newton”), published his last paper in 1937. The paper, “Symmetric theory of electrons and positrons,” showed that the Dirac equation has a special real-valued solution describing a neutral fermion that is identical to its own antiparticle.

The mathematical structure has since been called a Majorana fermion. Whether neutrinos are Majorana fermions — whether neutrinos are their own antiparticles — is still an open question, central to the modern hunt for neutrinoless double-beta decay. Majorana himself disappeared in 1938 under mysterious circumstances at age 31.

1938 onward: The diaspora

The Italian situation deteriorated rapidly in the late 1930s. The 1938 racial laws made positions impossible for Jewish scientists. Mussolini’s regime became hostile to intellectual independence. International funding dried up.

The Via Panisperna group scattered. Fermi used his 1938 Nobel ceremony in Stockholm as cover to emigrate to the United States. Segrè went to Berkeley. Pontecorvo went to Paris and Canada (and eventually to the Soviet Union in 1950). Rasetti went to Canada. Amaldi alone stayed in Italy.

By 1939, the most productive physics laboratory in the world was gone. The neutrino’s name was Italian, but its future would be written in many other countries.

1946: Pontecorvo’s chlorine idea

While working at Chalk River in Canada, Bruno Pontecorvo proposed in 1946 the technique that would eventually catch solar neutrinos directly. The proposal was to use a tank of carbon tetrachloride (later perchloroethylene) and look for the rare conversion of chlorine-37 to argon-37 by an incoming electron neutrino.

Pontecorvo had identified the reaction:

ν_e + ³⁷Cl → ³⁷Ar + e⁻

The argon-37 is unstable; counting the few atoms produced would constitute a neutrino measurement. The idea sat in the literature for two decades until Raymond Davis built the experiment at the Homestake gold mine starting in the late 1960s. Pontecorvo’s chlorine idea is the foundation of half a century of solar neutrino physics.

1955: The eve of detection

By 1955, the neutrino had a name, a theoretical framework (Fermi’s beta-decay theory), a proposed detection technique (Pontecorvo’s chlorine), and a serious effort underway to actually catch one. Two American physicists at Los Alamos — Frederick Reines and Clyde Cowan — had been working since 1951 on a different detection scheme using reactor antineutrinos, intense fluxes that had become available only after the Manhattan Project’s reactor program.

The neutrino had not yet been confirmed in any laboratory. Twenty-five years after Pauli’s “desperate remedy,” the particle was still hypothetical in the strict sense. But the experimental confirmation was about to arrive.

In Part 2 we’ll pick up the story in 1956 with the Savannah River detection, then watch the field discover that there are multiple neutrino flavors and that the Sun seems to be sending fewer neutrinos than theory predicts.

Neutrino Times Editorial Team. (2026, January 29). Neutrino History — Part 1: From Pauli's proposal to the eve of detection (1930–1955). Neutrino Times. https://neutrino-times.com/articles/neutrino-history-part-1-1930-1955/

Neutrino Times Editorial Team. "Neutrino History — Part 1: From Pauli's proposal to the eve of detection (1930–1955)." Neutrino Times, January 29, 2026. https://neutrino-times.com/articles/neutrino-history-part-1-1930-1955/.

Neutrino Times Editorial Team. "Neutrino History — Part 1: From Pauli's proposal to the eve of detection (1930–1955)." Neutrino Times, 29 Jan. 2026, https://neutrino-times.com/articles/neutrino-history-part-1-1930-1955/.

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