Neutrinos were postulated by Pauli in 1930 to explain the missing energy in β-decay. Reines and Cowan used the Savannah River reactor to observe inverse β-decay, where an antineutrino interacts with a proton to produce a positron and a neutron. The positron annihilates immediately, emitting two 0.511 MeV γ-rays, while the neutron is captured by Gd after a short delay, releasing several MeV γ-rays. Detecting this prompt-delayed γ coincidence effectively suppressed backgrounds and provided unequivocal evidence of neutrinos. The technique became the foundation of later solar, atmospheric and oscillation neutrino experiments.

Inverse β-decay ν̄_e + p → e⁺ + n requires antineutrinos above a 1.8 MeV threshold; commercial reactors produce about 2×10²⁰ ν̄_e per second. Reines and colleagues deployed roughly 200 L of liquid scintillator in four tanks, reading γ signals with photomultiplier tubes. A pair of 511 keV γ-rays within 0.3 µs constituted the “prompt” signal, while multi-MeV γ-rays 5–200 µs later from neutron capture formed the “delayed” signal. The coincidence yielded a few events per hour, matching the theoretical cross-section (~10⁻⁴³ cm²). These measurements directly supported lepton-number conservation and the emerging V–A theory.

The detector employed organic liquid scintillator doped with Cd-Cl and 0.1% Gd to raise the neutron-capture efficiency to ≈88%. Total background for the triple-coincidence signature was suppressed below 12 h⁻¹, while neutrino candidates were observed at 24 h⁻¹, giving 5.7σ statistical significance. The measured average cross-section (3.1 ± 0.6)×10⁻⁴³ cm² agreed with Fermi-theory predictions folded with the β-spectrum. Successor experiments—Kamiokande, Daya Bay, JUNO—combine Reines-style coincidence with water Cherenkov or large-volume scintillators to measure θ₁₃ and probe the neutrino mass hierarchy.