--- title: "Cowan and Reines: finding the neutrino in two flashes | Sandbox Physics" description: "Cowan, Reines and their colleagues recognized a neutrino by a pair of related signals: a flash followed shortly by a second one, checked against backgrounds." canonical_url: "https://sandboxphysics.com/stories/neutrinos/reactor-first-detection/" language: "en" page_type: "story" alternate_languages: zh-CN: "https://sandboxphysics.com/zh/stories/neutrinos/reactor-first-detection/" last_modified: "2026-10-09" --- [2026 Nobel Prize in Physics](https://sandboxphysics.com/nobel/2026/physics/) # Cowan and Reines: finding the neutrino in two flashes A flash on its own could be ordinary radioactivity. A second, related signal arriving shortly afterwards gave researchers something much more useful to look for. Sandbox Physics · 2026-10-09 · First ideas ## Look for what the neutrino leaves behind A nuclear reactor produces a great many electron antineutrinos, the antimatter counterparts of electron neutrinos. Almost all pass through nearby matter. Occasionally, one interacts with a proton and produces two other particles: a positron and a neutron. Those two particles gave Clyde Cowan, Frederick Reines and their colleagues a way to recognize the interaction. A positron is the electron’s positively charged counterpart. It slows down and annihilates with an electron, producing gamma rays. The neutron takes longer: it slows down and is eventually captured by a nucleus, producing another signal. The experiment reported in 1956 used water targets containing cadmium, surrounded by scintillation detectors that produced light when radiation deposited energy in them. The apparatus at Savannah River was built to catch the products of the reaction. These were the visible clues to a particle that had arrived unseen. $$ \bar\nu_e+p\rightarrow e^++n $$ Inverse beta decay: an electron antineutrino and a proton produce a positron and a neutron. The detector looks for the signals those outgoing particles leave behind. Sources: [1] · [2] ## The useful pause between the signals The first signal arrives promptly. The second follows when the neutron is captured, in this apparatus by cadmium, and releases gamma radiation. A pair arriving in the right order and close together in time is more distinctive than either signal alone. Researchers still have to decide how long to wait for the second signal. Wait too briefly and some genuine neutron captures will arrive after the window has closed. Wait much longer and unrelated pulses have more chances to be mistaken for the partner. The equation below shows this tradeoff in a simple model. Neutron-capture delays follow an exponential distribution with a chosen mean waiting time. A wider window accepts more genuine pairs, but the gain gradually levels off. $$ \epsilon(\Delta t)=1-e^{-\Delta t/\tau} $$ In this ideal model, epsilon is the fraction of genuine captures accepted before the window closes; tau is the mean capture delay. The window starts at zero. Real experiments can also reject signals that arrive too soon. Sources: [2] 1. 01 **First signal** A positron starts the clock 2. 02 **Second signal** A neutron is captured later 3. 03 **Check the pairs** Shift times to estimate chance overlaps A guide to how the measurement works. The diagram is schematic. ## Sometimes two unrelated signals line up Detectors also record pulses from ordinary radiation and other sources. Suppose one pulse happens to look like the first signal and an unrelated pulse soon follows it. The pair can satisfy the timing rule even though no neutrino interaction produced it. These are called accidental coincidences. They become more common when either kind of single pulse becomes more frequent, or when the waiting window gets longer. The estimate below expresses that simple idea. One way to estimate this background is to shift one set of recorded times far enough to break the true short-delay pairs, then count the new pairs. The accidental overlaps remain. In our Lab, this shifted record gives a control sample that you can compare with the original selection. $$ R_{\mathrm{acc}}\simeq R_pR_d\Delta t $$ For independent, steady streams of pulses and a short time window, the accidental pair rate is approximately the two single-pulse rates multiplied by the window width. A background process that produces genuinely related pulses needs a different check. Sources: [1] · [2] ## How the team checked the explanation The historical experiment did more than find close pairs. The researchers examined the signal energies, the waiting times, and what changed when the reactor was operating. They also investigated other radiation that could imitate the expected signal. Each check answered a different doubt. Were the energies appropriate for the reaction? Were the delays consistent with neutron capture? Did the signal depend on the reactor? Could reactor-related neutrons or gamma rays account for it instead? Together, these comparisons made the antineutrino interpretation convincing. The 1956 paper is worth reading with those questions in mind: much of the discovery lies in how carefully the team tested the other explanations. Sources: [1] · [2] ## Try making the window wider Open the reactor Lab and acquire a record. Keep that record fixed while you widen the coincidence window. Watch how many genuine pairs are accepted, and how many accidental pairs come along with them. The total count can rise even when the extra events make the sample less useful. Next switch off the simulated reactor and acquire a new record. Unrelated background pulses still produce coincidences. Finally, raise the background rate and compare the recorded pairs with the shifted control. The control is an estimate, so its count will fluctuate too. This exercise uses illustrative rates and a simple capture-time model. It assumes the pulse categories are known and leaves out effects such as missed pulses and detector dead time. It teaches one part of the historical method with simulated data; the original 1956 records are not used. Sources: [2] ## What we learned The pair of signals gave Cowan, Reines and their colleagues a recognizable clue. Timing, energy measurements and checks on the backgrounds helped establish what had caused it. ## Try it in the Lab [Reactor coincidence](https://sandboxphysics.com/labs/reactor-coincidence/) ## Primary sources & revision 1. [Cowan, Reines, Harrison, Kruse & McGuire · Detection of the Free Neutrino (1956)](https://wwwusers.ts.infn.it/~candelis/IFNS/CowRein.pdf) 2. [Reines et al. · Detection of the Free Antineutrino (1960)](https://doi.org/10.1103/PhysRev.117.159) First published 2026-10-09; last revised 2026-10-09. Original explanatory text and diagrams by Sandbox Physics. Illustrations are schematic; no experimental event records are reproduced here. This is an independent educational publication, not an official Nobel or experiment collaboration publication. [← Previous: Fermi’s next question: how often should the unseen particle appear?](https://sandboxphysics.com/stories/neutrinos/fermi-beta-decay-theory/) [Next: Why did physicists need a second kind of neutrino? →](https://sandboxphysics.com/stories/neutrinos/brookhaven-muon-neutrino/) ## Keep reading Neutrinos · Reading path ### From missing energy to a new particle 1. [Article **Where did the energy go? The puzzle that led to the neutrino**](https://sandboxphysics.com/stories/neutrinos/beta-decay-missing-energy/) 2. [Article **Pauli’s letter: proposing a particle no one had seen**](https://sandboxphysics.com/stories/neutrinos/pauli-neutrino-letter/) 3. [Article **Fermi’s next question: how often should the unseen particle appear?**](https://sandboxphysics.com/stories/neutrinos/fermi-beta-decay-theory/) 4. [You are here **Cowan and Reines: finding the neutrino in two flashes**](https://sandboxphysics.com/stories/neutrinos/reactor-first-detection/) Neutrinos · Topic ### Neutrinos and the Nobel Prize Which neutrino results won the Nobel Prize, and what evidence stood behind each one? 1. [Article **Why do neutrinos keep appearing in the Nobel Prize?**](https://sandboxphysics.com/stories/neutrinos/neutrinos-and-nobel-prizes/) 2. [Article **How does a faint signal become a neutrino discovery?**](https://sandboxphysics.com/stories/neutrinos/neutrino-discovery-evidence-chain/) 3. 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[Article **SN 1987A: a supernova seen from underground**](https://sandboxphysics.com/stories/neutrinos/sn1987a-burst/) 10. [Article **1930–2026: a timeline of neutrino evidence**](https://sandboxphysics.com/stories/neutrinos/neutrino-evidence-timeline-1930-2026/) Neutrinos · Topic ### What beta decay reveals about neutrinos Beta decay first pointed to the neutrino. How do beta decays now limit its mass, and how do double-beta decays test whether it is its own antiparticle? 1. [Article **Where did the energy go? The puzzle that led to the neutrino**](https://sandboxphysics.com/stories/neutrinos/beta-decay-missing-energy/) 2. [Article **Fermi’s next question: how often should the unseen particle appear?**](https://sandboxphysics.com/stories/neutrinos/fermi-beta-decay-theory/) 3. [You are here **Cowan and Reines: finding the neutrino in two flashes**](https://sandboxphysics.com/stories/neutrinos/reactor-first-detection/) 4. [Article **Where does a reactor’s antineutrino spectrum come from?**](https://sandboxphysics.com/stories/neutrinos/reactor-antineutrino-spectrum-origins/) 5. [Article **KATRIN: limiting neutrino mass at the end of the beta spectrum**](https://sandboxphysics.com/stories/neutrinos/katrin-beta-endpoint-neutrino-mass/) 6. [Article **Dirac or Majorana: is the neutrino its own antiparticle?**](https://sandboxphysics.com/stories/neutrinos/dirac-majorana-neutrino-nature/) 7. [Article **Double-beta searches: why use several nuclei?**](https://sandboxphysics.com/stories/neutrinos/global-neutrinoless-double-beta-searches/) 1 more group Neutrinos · Topic ### Neutrinos from reactors and accelerators Reactors and accelerators are neutrino sources that people control. What have detectors placed near them and far from them shown? 1. [You are here **Cowan and Reines: finding the neutrino in two flashes**](https://sandboxphysics.com/stories/neutrinos/reactor-first-detection/) 2. [Article **KamLAND: testing a solar mystery with reactors on Earth**](https://sandboxphysics.com/stories/neutrinos/kamland-reactor-oscillations/) 3. [Article **Daya Bay: why compare a near detector with a far one?**](https://sandboxphysics.com/stories/neutrinos/daya-bay-near-far/) 4. [Article **RENO and Double Chooz: why repeat the mixing-angle measurement?**](https://sandboxphysics.com/stories/neutrinos/reno-double-chooz-independent-mixing-tests/) 5. [Article **Short reactor baselines: look for a change with distance**](https://sandboxphysics.com/stories/neutrinos/short-baseline-reactor-sterile-tests/) 6. [Article **K2K and MINOS: test flavor change with a distance you choose**](https://sandboxphysics.com/stories/neutrinos/k2k-minos-controlled-baselines/) 7. [Article **T2K: why look slightly away from the beam axis?**](https://sandboxphysics.com/stories/neutrinos/t2k-off-axis-beam-and-appearance/)