Neutrino Nobel Prizes — Part 2: 1995 — Frederick Reines and the detection

This is the second part of the Neutrino Nobel Prizes series. We turn from the 1962 muon-neutrino discovery to the 1956 first detection of the neutrino itself — and the 1995 Nobel Prize that recognized it nearly forty years later.

The 1995 prize

The Royal Swedish Academy of Sciences awarded the 1995 Nobel Prize in Physics to Frederick Reines for “the detection of the neutrino” and to Martin Perl for “the discovery of the tau lepton.” The combined citation was “for pioneering experimental contributions to lepton physics.”

Reines’ share of the prize specifically recognized the 1956 Cowan-Reines experiment at the Savannah River reactor, in which the first direct detection of the antineutrino was made — 26 years after Pauli’s 1930 proposal that the particle should exist.

The experiment

Cowan and Reines placed a layered scintillator-and-water detector at the Savannah River Plant nuclear reactor in Aiken, South Carolina. The reactor produced a copious flux of electron antineutrinos from fission-fragment beta decays.

The signature was inverse beta decay: $\bar\nu_e + p \to n + e^+$. The positron annihilated promptly with an electron to produce two 511-keV gammas. The neutron thermalized over about 5 microseconds and was captured by cadmium chloride dissolved in water, producing a delayed gamma cascade.

The unmistakable signature: prompt and delayed scintillation flashes separated by several microseconds. Background random coincidences couldn’t easily produce this pattern.

Cowan and Reines ran the experiment for several years through 1956. The signal-to-background ratio was about 4:1, and the rate matched theoretical expectations to within ~5%. The conclusion: the antineutrino was experimentally detected.

The telegram to Pauli

Famous as a piece of physics-historical lore: when Cowan and Reines had confirmed the detection in June 1956, they sent a telegram to Wolfgang Pauli — the man who had proposed the neutrino’s existence in 1930:

“We are pleased to inform you that we have definitely detected neutrinos from fission fragments by observing inverse beta decay of protons. Observed cross section agrees well with expected six times ten to minus forty-four square centimeters.”

Pauli, who had famously confided that he had “done a terrible thing” by predicting a particle that might never be detected, reportedly replied that he had given up waiting and gone to drink champagne. (Some versions of the story have him drinking before he received the telegram; others have him hearing about it from physicist Friedrich Reines via telephone.)

Pauli died in 1958, two years after the detection. He was alive at the time of the experimental confirmation but had not been eligible for the Nobel Prize awarded for the detection — partly because Nobel prizes are not awarded for theoretical predictions in the same way, partly because the experimental confirmation post-dated his theoretical work too closely.

Why this prize was so late

39 years from experiment to prize is unusual but not unprecedented. Several factors:

No experimental Nobel pattern for a single result. The Nobel committee tends to recognize broader experimental programs rather than single-shot results. Cowan-Reines was singular — there was no follow-up program by the same team on a similar scale.

Competing developments. The 1962 muon-neutrino discovery (1988 prize) competed for the “early neutrino physics” recognition slot. The 1969 quark discoveries, 1983 W/Z discoveries, and other major particle-physics results occupied Nobel attention through the 1970s and 1980s.

Cowan’s death (1974). Cowan was the original initiator of the project (“Project Poltergeist”) at Los Alamos. His death made the question of how to share the prize awkward — the committee could give it to Reines alone, but that involved making a judgment about credit allocation. They eventually did this in 1995.

Pauli’s death (1958). The original predictor of the neutrino couldn’t share. Awarding the prize for the detection alone, decades later, was the simpler path.

What Reines did after 1956

The Nobel citation specifies the 1956 detection. But Reines continued his neutrino-physics career for four more decades:

Atmospheric neutrinos: Reines’s group at the Kolar Gold Fields in India made the first atmospheric neutrino detection in 1965. Anomalies in the rate were noted but not at high enough significance to drive a discovery.

Solar neutrinos: Reines was involved in advocating and analyzing solar-neutrino data from various experiments throughout the 1970s-1990s.

SN 1987A: Reines was a founder of the Irvine-Michigan-Brookhaven (IMB) detector that caught 8 events from SN 1987A in February 1987. He was alive to witness this historic milestone.

Mentorship: Reines trained many of the next generation of neutrino experimentalists, who in turn led IceCube, KATRIN, SNO, and other major programs.

Reines died in 1998, three years after the Nobel and just months after the Super-K oscillation discovery that would soon win the 2015 prize.

Martin Perl and the tau lepton

Perl’s share of the 1995 prize recognized the 1975 discovery of the tau lepton at SLAC’s SPEAR collider. The tau is the third charged lepton, ~3,500 times heavier than the electron. Its discovery extended the lepton sector to three families and predicted the existence of a corresponding tau-neutrino, which was eventually directly detected in 2000.

Perl’s discovery was particularly important for completing the three-family picture of the Standard Model. Combined with the muon-neutrino discovery (Lederman et al.), it pointed to three lepton doublets — and the existence of three charged-current weak partners.

Why Pauli didn’t share

A persistent question: should Pauli have shared the Nobel for the neutrino?

By Nobel rules, no — the Nobel is awarded only to living people (with extremely rare exceptions), and Pauli died in 1958, before any Nobel was awarded for neutrino physics. Pauli did win the 1945 Nobel for the Pauli exclusion principle, so he was a Nobel laureate in his own right.

The 1995 prize citation does not mention Pauli. The 2002 prize citation mentions him in passing. In retrospect, many physicists feel that some specific recognition for the prediction itself would have been appropriate — but Nobel rules constrain this.

The next part of this series turns to the 2002 prize — recognizing both the Davis solar-neutrino program and Koshiba’s SN 1987A and Kamiokande work.

Neutrino Times Editorial Team. (2026, April 14). Neutrino Nobel Prizes — Part 2: 1995 — Frederick Reines and the detection. Neutrino Times. https://neutrino-times.com/articles/nobel-prizes-part-2-1995-reines/

Neutrino Times Editorial Team. "Neutrino Nobel Prizes — Part 2: 1995 — Frederick Reines and the detection." Neutrino Times, April 14, 2026. https://neutrino-times.com/articles/nobel-prizes-part-2-1995-reines/.

Neutrino Times Editorial Team. "Neutrino Nobel Prizes — Part 2: 1995 — Frederick Reines and the detection." Neutrino Times, 14 Apr. 2026, https://neutrino-times.com/articles/nobel-prizes-part-2-1995-reines/.

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  note         = {Accessed: 2026-04-14}
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