For a quarter of a century, the neutrino was a particle on paper. Wolfgang Pauli had proposed it in 1930 to fix beta decay. Enrico Fermi had built a quantitative theory around it in 1934. Both of them suspected the particle might never be observed. Pauli, famously, apologized for inventing it.
Then in June 1956, two American physicists — Clyde Cowan and Frederick Reines — sent Pauli a telegram from the Savannah River nuclear reactor in South Carolina: “We are pleased to inform you that we have definitely detected neutrinos from fission fragments.” Pauli replied that he had given up waiting and gone to drink champagne.
The detection of the neutrino took 26 years to move from prediction to confirmation. The story of how that happened is one of the more stubborn experimental campaigns in the history of physics.
The original plan: a nuclear bomb
In the early 1950s, Reines was working at Los Alamos on what was still active weapons research. He spent his lunchtimes thinking about a question that had been gnawing at him: was it really impossible to detect the neutrino? The conventional wisdom said yes. Neutrinos interact so feebly with matter that the average one could travel a light-year of solid lead before being stopped.
Reines’s first concrete idea — partly serious, partly born of access to extraordinary engineering — was to detonate a nuclear bomb and put a detector close enough to catch the dense burst of neutrinos. He called the proposal “Project Poltergeist.” It was approved in principle. Cowan joined him on the project shortly afterward.
But over a few months they realized something less dramatic might work just as well: a nuclear reactor produces a more or less continuous, intense stream of antineutrinos from the beta decay of fission fragments. If they were patient, a reactor experiment would catch more events than a single bomb pulse — and they would not need to detonate anything.
The trick that made it work
The experimental challenge was distinguishing a real neutrino interaction from the background noise of a working reactor. Cowan and Reines’s solution was a delayed coincidence signature — a two-part flash of light that, taken together, was almost impossible to mistake for anything else.
Their detector held about 400 liters of water, with cadmium chloride dissolved in it. When an electron antineutrino from the reactor occasionally hit a proton in the water, it would convert the proton into a neutron and emit a positron. The positron almost immediately found an electron and annihilated, producing a pair of gamma rays that the surrounding photomultiplier tubes registered as the first flash.
The neutron, meanwhile, wandered through the water for a few microseconds, bouncing off hydrogen atoms and slowing down. Eventually it would be captured by a cadmium nucleus, which in turn would emit its own gamma rays — the second flash. Two flashes, in the right order, separated by a few microseconds. Background events almost never produced that pattern. Neutrino events did.
The discovery
Cowan and Reines first ran a version of the experiment at the Hanford reactor in Washington State in 1953, with promising but inconclusive results. They redesigned and rebuilt the detector and moved it to the Savannah River Site in South Carolina, where they had better access to a high-flux reactor and shielding against cosmic rays.
The improved setup ran in 1956. They saw the signature exactly as predicted, at exactly the rate they had calculated. Crucially, the signal disappeared when the reactor was turned off. There was no ambiguity. The neutrino was real.
The 1956 detection caught antineutrinos rather than neutrinos in the strictest sense — the distinction was still being worked out. But the principle of “the neutrino exists and can be caught with patience and the right instrument” was established for good.
What it set in motion
Cowan and Reines’s experiment was the founding act of experimental neutrino physics. Without it, no Davis solar neutrino program. No SN 1987A detection. No oscillation discovery. No IceCube. No DUNE.
It also showed something more general: a particle predicted by theory could be confirmed by experiment even when intuition said it would be impossible. The pattern would repeat itself many times in 20th-century physics — for the W and Z bosons, the top quark, the Higgs, and eventually for neutrino oscillation itself.
The Nobel Prize
The Nobel Prize for the discovery did not arrive until 1995 — 39 years late. It went to Reines, who shared it with Martin Perl (for the unrelated discovery of the tau lepton). Cowan had died in 1974 and was therefore ineligible under the Nobel rules; the committee paid him explicit homage in the announcement.
Reines was 76 when the call came. In his Nobel lecture, he remembered the telegram to Pauli and Pauli’s reply, and noted with characteristic understatement that the original plan, the bomb, would probably not have worked as well as the reactor anyway.
Pauli himself had been right, of course. He had done a terrible thing. He had postulated a particle that turned out to be the most abundant matter particle in the universe — and one of the most informative.
For more on the history of neutrino physics from Pauli to the present, see our full timeline. For the contemporary picture of what these particles actually are, see What is a neutrino, anyway?.
Further reading
Primary sources
- Cowan, Reines, Harrison, Kruse & McGuire, “Detection of the Free Neutrino: A Confirmation”, Science 124:103 (1956)
- Reines & Cowan, “The Neutrino”, Nature 178:446 (1956) — the parallel announcement
- Frederick Reines’s 1995 Nobel Lecture — Reines’s own account of the experiment
Background and context
Frequently asked
Who first detected the neutrino, and when?
Clyde Cowan and Frederick Reines at the Savannah River nuclear reactor in South Carolina, USA. The decisive run was in June 1956, twenty-six years after Wolfgang Pauli proposed the particle's existence in 1930. They sent Pauli a telegram announcing the discovery; Pauli replied that he had given up waiting and gone to drink champagne.
How did the Cowan-Reines experiment work?
They used inverse beta decay: an antineutrino from the reactor hits a proton in a water tank, producing a positron and a neutron. The positron annihilates almost immediately to produce two 511 keV gamma rays. The neutron thermalizes over ~5 microseconds and is captured by cadmium, releasing a delayed gamma cascade. The coincidence of the prompt and delayed signal — separated by a few microseconds — was the unmistakable signature of an antineutrino interaction.
Why did the detection take 26 years?
Neutrinos interact extraordinarily weakly with matter. The average neutrino can travel a light-year of solid lead before being stopped. To detect even a few per day, you need both an intense source — a nuclear reactor producing about 10²⁰ antineutrinos per second — and a detector with a clean, distinctive signature that can be separated from radioactive backgrounds. The technology to do both did not exist until the mid-1950s.
Did Cowan and Reines win the Nobel Prize?
Frederick Reines won the 1995 Nobel Prize in Physics for the discovery. Clyde Cowan had died in 1974 and the Nobel Prize is not awarded posthumously. Reines shared the 1995 prize with Martin Perl, who discovered the tau lepton.
Cite this article 5 formats
APA
Neutrino Times Editorial Team. (2025, June 5). Cowan and Reines: how a reactor experiment caught the neutrino in 1956. Neutrino Times. https://neutrino-times.com/articles/cowan-reines-first-neutrino-detection-1956/
Chicago
Neutrino Times Editorial Team. "Cowan and Reines: how a reactor experiment caught the neutrino in 1956." Neutrino Times, June 5, 2025. https://neutrino-times.com/articles/cowan-reines-first-neutrino-detection-1956/.
MLA
Neutrino Times Editorial Team. "Cowan and Reines: how a reactor experiment caught the neutrino in 1956." Neutrino Times, 5 Jun. 2025, https://neutrino-times.com/articles/cowan-reines-first-neutrino-detection-1956/.
BibTeX
@misc{neutrino-times-cowan-reines-first-neutrino-detection-1956,
author = {Neutrino Times Editorial Team},
title = {Cowan and Reines: how a reactor experiment caught the neutrino in 1956},
howpublished = {Neutrino Times},
year = {2025},
month = {jun},
url = {https://neutrino-times.com/articles/cowan-reines-first-neutrino-detection-1956/},
note = {Accessed: 2025-06-05}
} RIS
TY - GEN TI - Cowan and Reines: how a reactor experiment caught the neutrino in 1956 AU - Neutrino Times Editorial Team PY - 2025 DA - 2025-06-05 PB - Neutrino Times UR - https://neutrino-times.com/articles/cowan-reines-first-neutrino-detection-1956/ ER -