Reines and Cowan: The 1956 Detection at Savannah River

Twenty-six years after Pauli postulated the neutrino as a bookkeeping device to save energy conservation in beta decay, the particle was detected. The experiment that accomplished it — Frederick Reines and Clyde Cowan’s measurement at the Savannah River nuclear reactor in 1956 — remains a model of how to design an experiment for a signal thought to be undetectable.

The problem

Bethe and Peierls had estimated in 1934 that the neutrino cross-section at MeV energies was of order $10^{-43}$ cm² — corresponding to a mean free path through lead of roughly a light-year. The obvious conclusion was that no conceivable terrestrial experiment could ever observe one.

The obvious conclusion was wrong, but only because nobody in 1934 had foreseen the arrival of nuclear reactors. A commercial reactor with 1 GW thermal power produces roughly $10^{20}$ antineutrinos per second. A detector within a few metres of the core sees a flux large enough that, despite the tiny cross-section, a kilogram-scale target would produce several events per day. The question becomes not whether the neutrino can be detected but whether the event signature can be distinguished from background.

The signature

Reines and Cowan proposed to detect electron antineutrinos via inverse beta decay on free protons in water: ${\bar{\nu }}_e+p\to e^{+}+n$ Both final-state particles give distinct observable signatures:

• The positron thermalises and annihilates with an atomic electron within picoseconds, producing two 511 keV gamma rays.
• The neutron thermalises by elastic scattering on protons over several microseconds, then is captured on a nucleus, releasing further gamma rays.

A detector capable of recording both signals with appropriate energy and timing resolution can demand their delayed coincidence — a prompt flash followed a few microseconds later by a delayed flash, both at the same location. Accidental backgrounds that mimic a single flash are common; backgrounds that mimic the full time-correlated pair are vanishingly rare.

The detector

The final Savannah River apparatus consisted of a cubic tank holding about 200 litres of water, into which cadmium chloride had been dissolved. Cadmium has an unusually large cross-section for thermal neutron capture — ${}^{113}$Cd absorbs a neutron within about 5 μs of thermalization and immediately releases a cascade of gamma rays totalling ~9 MeV.

The water target was sandwiched between two tanks of organic liquid scintillator, each viewed by a bank of photomultiplier tubes. Inverse beta decay events inside the water produced:

1. The prompt signal: 1–2 MeV of energy deposited in the scintillator from the positron kinetic energy and the 511 keV annihilation gammas
2. The delayed signal: 9 MeV of gamma energy from the neutron capture, 5 μs later

The coincidence of these two signals, within the expected energy windows and the expected time window, was the experimental signature of a genuine inverse-beta-decay event.

The detector was placed about 11 metres from the reactor core, at a depth of 12 metres below ground to suppress cosmic-ray muon backgrounds.

The earlier attempt at Hanford

Before Savannah River, Reines and Cowan had attempted a similar measurement at the Hanford reactor in 1953. The detector there was smaller, the background higher, and the result was inconclusive. The move to Savannah River was a substantial upgrade — larger target mass, better shielding, cleaner scintillator — and the lessons learned from Hanford were essential in turning the concept into a working experiment.

The Hanford and Savannah River efforts together consumed four years of development. Reines later described the process as “learning by doing, and learning the hard way.”

The result

Publishable data accumulated through late 1955 and the first half of 1956. The final paper in Science, submitted in June 1956, reports a rate of $2.88\pm 0.22$ reactor-correlated events per hour, above a background that could be independently measured with the reactor off. The cross-section extracted from the rate, $\sigma \approx (6.3\pm 1.5)\times 10^{-44}{\text{ cm}}^2$ matched Fermi-theory prediction within uncertainties.

The paper is concise — two pages — and ends without fanfare. It documents a detection. Bethe and Peierls’s 1934 “no practically possible way” had been overtaken by reactor technology.

The telegram

On 14 June 1956 — more than a year before the formal publication — Reines and Cowan cabled Pauli at ETH Zurich:

We are happy 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 the minus forty-four square centimeters.

Pauli replied the following day by cable:

Everything comes to him who knows how to wait.

He is said to have opened a case of champagne and to have acknowledged the bet he had long ago lost against his own hypothesis being confirmed in his lifetime.

Legacy: the technique

The delayed-coincidence-in-scintillator technique Reines and Cowan developed at Savannah River is still the workhorse of reactor antineutrino detection seven decades later. KamLAND, Double Chooz, Daya Bay, RENO, JUNO, and TAO all use variations of the same approach — though with gadolinium loading (which gives a sharper, higher-energy neutron-capture signal than cadmium) and with much larger target volumes (1 to 20 kilotons rather than 200 kilograms).

Legacy: the institution

Reines went on to found the Department of Physics at UC Irvine in 1966 and led an expansive programme including deep-underground atmospheric-neutrino detection, the IMB proton-decay detector, and supernova-neutrino observation. IMB’s 8-event SN 1987A detection was part of his continuing involvement in the field thirty years after Savannah River.

Cowan left Los Alamos for the Catholic University of America in 1957 and continued experimental work on cosmic rays and atmospheric radioactivity. He died in 1974 at age 54, twenty-one years before the Nobel committee honoured the 1956 detection. Reines received the 1995 Nobel Prize alone — a citation for “pioneering experimental contributions to lepton physics” — and explicitly credited Cowan in his Nobel lecture.

A template

The Savannah River experiment established a template now common across particle physics: identify a unique signature composed of multiple time-correlated signals, design the detector to tag all of them, and rely on the coincidence itself — not on absolute rates — to reject backgrounds. The approach is used in dark-matter detection, in neutrinoless double-beta-decay searches, and in the search for exotic long-lived particles at colliders. Whenever a signal is too rare to detect directly, the answer is to measure something correlated to it, and to demand everything line up.

Seventy years later, it still works.