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Abstract
Reactor antineutrinos are detected through inverse beta decay on protons, ν̄e + p → e⁺ + n, using 200 liters of cadmium-loaded water as target surrounded by liquid-scintillator tanks. The delayed coincidence between the prompt positron annihilation and the neutron capture on cadmium provides an unambiguous signature; the measured cross-section agrees with Fermi-theory prediction within the quoted experimental uncertainty.
Significance in the evidence base
Moved the neutrino from theoretical postulate to observed reality. Established the delayed-coincidence technique in liquid scintillator as the foundational method of reactor antineutrino detection — still in use today at KamLAND, JUNO, and their successors.
External references
The experimental setup
The Reines-Cowan detector at the Savannah River Plant consisted of two tanks of organic liquid scintillator instrumented with banks of photomultiplier tubes, sandwiching a target volume of ~200 liters of water dissolved with cadmium chloride. The assembly sat roughly 11 meters from the reactor core and about 12 meters underground, providing shielding against cosmic-ray muons.
A reactor antineutrino incident on a proton in the water undergoes inverse beta decay: The positron thermalizes in a few picoseconds and annihilates with an atomic electron, producing back-to-back 511 keV gamma rays that deposit energy in the adjacent scintillator tanks — the prompt signal. The neutron thermalizes over ~5 μs and is captured on Cd (Cd(,)Cd*), which releases a cascade of gamma rays totaling ~9 MeV — the delayed signal.
The time-delayed coincidence between prompt and delayed signals, with the expected energy windows and spatial correlation, was the unambiguous experimental signature.
The result
The authors report a detection rate of events per hour above background, with the reactor on. The corresponding cross-section cm² matched Fermi-theory predictions within uncertainties.
The experiment took more than three years of development: an earlier attempt at the Hanford reactor in 1953 had been dominated by cosmic-ray backgrounds, and the Savannah River move was a substantial engineering upgrade. The final published result was the culmination of four years of systematic work.
The telegram to Pauli
On 14 June 1956, Reines and Cowan sent Pauli a telegram at ETH Zurich informing him of the confirmed detection. The exchange — Pauli’s reply “Everything comes to him who knows how to wait” — has become part of the folklore of twentieth-century physics.
Impact
The detection closed the 26-year gap between Pauli’s 1930 postulate and experimental confirmation. It validated the Fermi theory, established the delayed-coincidence-in-scintillator technique that subsequent reactor experiments (Savannah River follow-ups, Goesgen, Bugey, KamLAND, JUNO) have refined rather than replaced, and opened an era of neutrino-as-experimental-particle that continues to the present.
Reines received the 1995 Nobel Prize in Physics for this work. Cowan had died in 1974 and was not eligible.