TheCowan–Reines neutrino experiment was conducted by physicistsClyde Cowan andFrederick Reines in 1956. The experiment confirmed the existence ofneutrinos. Neutrinos,subatomic particles with noelectric charge and very small mass, had been conjectured to be an essential particle inbeta decay processes in the 1930s. With no charge and minuscule mass, such particles appeared to be impossible to detect. The experiment exploited a huge flux of (then hypothetical) electronantineutrinos emanating from a nearby nuclear reactor and a detector consisting of large tanks of water. Neutrino interactions with the protons of the water were observed, verifying the existence and basic properties of this particle for the first time.

During the 1910s and 1920s, the observations of electrons from the nuclearbeta decay showed that their energy had a continuous distribution. If the process involved only the atomic nucleus and the electron, the electron's energy would have a single, narrow peak, rather than a continuous energy spectrum. Only the resulting electron was observed, so its varying energy suggested that energy may not be conserved.^[1] This quandary and other factors ledWolfgang Pauli to attempt to resolve the issue by postulating the existence of the neutrino in 1930. If the fundamental principle ofenergy conservation was to be preserved, beta decay had to be a three-body, rather than a two-body, decay. Therefore, in addition to an electron, Pauli suggested that another particle was emitted from the atomic nucleus in beta decay. This particle, the neutrino, had very small mass and no electric charge; it was not observed, but it carried the missing energy.

Pauli's suggestion was developed into a proposedtheory for beta decay byEnrico Fermi in 1933.^[2][3] The theory posits that the beta decay process consists of fourfermions directly interacting with one another. By this interaction, theneutron decays directly to anelectron, the conjecturedneutrino (later determined to be anantineutrino) and aproton.^[4] The theory, which proved to be remarkably successful, relied on the existence of the hypothetical neutrino. Fermi first submitted his "tentative" theory of beta decay to the journalNature, which rejected it "because it contained speculations too remote from reality to be of interest to the reader.^[5]"

One problem with the neutrino conjecture and Fermi's theory was that the neutrino appeared to have such weak interactions with other matter that it would never be observed. In a 1934 paper,Rudolf Peierls andHans Bethe calculated that neutrinos could easily pass through the Earth without interactions with any matter.^[6][7]

Byinverse beta decay, the predicted neutrino, more correctly anelectron antineutrino (${\displaystyle {\overline{\nu }}_e}$), should interact with aproton (p) to produce aneutron (n) andpositron (${\displaystyle e^{+}}$),

${\displaystyle {\overline{\nu }}_e+p\to n+e^{+}}$

The chance of this reaction occurring was small. The probability for any given reaction to occur is in proportion to itscross section. Cowan and Reines predicted a cross section for the reaction to be about6×10⁻⁴⁴ cm². The usual unit for a cross section in nuclear physics is abarn, which is1×10⁻²⁴ cm² and 20 orders of magnitudes larger.

Despite the low probability of the neutrino interaction, the signatures of the interaction are unique, making detection of the rare interactions possible. Thepositron, theantimatter counterpart of theelectron, quickly interacts with any nearbyelectron, and theyannihilate each other. The two resulting coincidentgamma rays (γ) are detectable. The neutron can be detected by its capture by an appropriate nucleus, releasing a third gamma ray. The coincidence of the positron annihilation and neutron capture events gives a unique signature of an antineutrino interaction.

Awater molecule is composed of an oxygen and twohydrogen atoms, and most of the hydrogen atoms of water have a single proton for a nucleus. Those protons can serve as targets for antineutrinos, so that simple water can serve as a primary detecting material. The hydrogen atoms are so weakly bound in water that they can be viewed as free protons for the neutrino interaction. The interaction mechanism of neutrinos with heavier nuclei, those with several protons and neutrons, is more complicated, since the constituent protons are strongly bound within the nuclei.

Given the small chance of interaction of a single neutrino with a proton, neutrinos could only be observed using a huge neutrino flux. Beginning in 1951, Cowan and Reines, both then scientists atLos Alamos, New Mexico, initially thought that neutrino bursts from theatomic weapons tests that were then occurring could provide the required flux.^[8] For a neutrino source, they proposed using an atomic bomb. Permission for this was obtained from the laboratory director,Norris Bradbury. The plan was to detonate a "20-kiloton nuclear bomb, comparable to that dropped on Hiroshima, Japan". The detector was proposed to be dropped at the moment of explosion into a hole 40 meters from the detonation site "to catch the flux at its maximum"; it was named "El Monstro".^[9] They eventually used anuclear reactor as a source of neutrinos, as advised by Los Alamos physics division leader J.M.B. Kellogg. The reactor had a neutrino flux of5×10¹³ neutrinos per second per square centimeter,^[10] far higher than any flux attainable from otherradioactive sources. The source specifically wasbeta minus decay fromfission products, creating electron antineutrinos, for example in the fission productioniodine-131:

${\displaystyle {}_{53}{}^{131}\text{I}{⟶}_{54}^{131}{\text{Xe}}^{\ast }+\beta {}^{-}+{\overline{\nu }}_{\text{e}}^{}+606\text{keV}}$

A detector consisting of two tanks of water was employed, offering a huge number of potential targets in the protons of the water. At those rare instances when neutrinos interacted withprotons in the water,neutrons andpositrons were created:

${\displaystyle {\overline{\nu }}_e+p\to n+e^{+}}$

or rather

${\displaystyle {\overline{\nu }}_{\text{e}}^{}+{\text{H}}_2^{}\text{O}\to {\text{OH}}^{-}+\text{n}+{\text{e}}^{+}}$

Electron–positron annihilation then occurs:

${\displaystyle {\text{e}}^{+}+{\text{e}}^{-}\to 2{\gamma }_{511\text{keV}}^{}}$

The two gamma rays created by positron annihilation were detected by sandwiching the water tanks between tanks filled with liquidscintillator. The scintillator material gives off flashes of light in response to the gamma rays, and these light flashes are detected byphotomultiplier tubes. The scintillator used was thewavelength shifterPOPOP, which peaks in violet light:

${\displaystyle \gamma {}_{511\text{keV}}{}^{}+{\text{C}}_{24}^{}{\text{H}}_{16}^{}{\text{N}}_2^{}{\text{O}}_2^{}\to {\text{C}}_{24}^{}{\text{H}}_{16}^{}{\text{N}}_2^{}{\text{O}}_2^{}+\text{A}{\gamma }_{3\text{eV}}^{}}$

The additional detection of the neutron from the neutrino interaction provided a second layer of certainty. Cowan and Reines detected the neutrons by dissolvingcadmium chloride, CdCl₂, in the tank.Cadmium is a highly effective neutron absorber and gives off a gamma ray when it absorbs a neutron.

n +¹⁰⁸
Cd →^109m
Cd →¹⁰⁹
Cd +γ

The arrangement was such that after a neutrino interaction event, the two gamma rays from the positron annihilation would be detected, followed by the gamma ray from the neutron absorption by cadmium severalmicroseconds later.

The experiment that Cowan and Reines devised used two tanks with a total of about 200 liters of water with about 40 kg of dissolved CdCl₂. The water tanks were sandwiched between threescintillator layers which contained 110 five-inch (127 mm)photomultiplier tubes.

In 1953, Cowan and Reines built a detector they dubbed "Herr Auge", "Mr. Eye" in German. They called the neutrino-searching experiment "Project Poltergeist", because of "the neutrino’s ghostly nature". A preliminary experiment was performed in 1953 at theHanford Site inWashington state, but in late 1955 the experiment moved to theSavannah River Plant nearAiken, South Carolina.^[11][12][13] The Savannah River site had better shielding againstcosmic rays. This shielded location was 11 m from the reactor and 12 m underground.

After months of data collection, the accumulated data showed about three neutrino interactions per hour in the detector. To be absolutely sure that they were seeingneutrino events from the detection scheme described above, Cowan and Reines shut down the reactor to show that there was a difference in the rate of detected events.

They had predicted a cross-section for the reaction to be about6×10⁻⁴⁴ cm² and their measured cross-section was6.3×10⁻⁴⁴ cm². The results were published in the July 20, 1956 issue ofScience.^[14][15]

Clyde Cowan died in 1974 at the age of 54. In 1995,Frederick Reines was honored with theNobel Prize for his work onneutrinophysics.^[7]

The basic strategy of employing massivedetectors, often water based, for neutrino research was exploited by several subsequent experiments,^[7] including theIrvine–Michigan–Brookhaven detector,Kamiokande, theSudbury Neutrino Observatory and theHomestake Experiment. The Homestake Experiment is a contemporary experiment which detectedneutrinos from nuclear fusion in the solar core. Observatories such as these detected neutrino bursts fromsupernovaSN 1987A in 1987, the birth ofneutrino astronomy. Through observations ofsolar neutrinos, the Sudbury Neutrino Observatory was able to demonstrate the process ofneutrino oscillation. Neutrino oscillation shows that neutrinos are not massless, a profound development in particle physics.^[16]

• List of neutrino experiments
• Subatomic particles

• Cowan and Reines Neutrino Experiment
• Decay of the Neutron
• Beta Decay
• Electron Neutrinos and Antineutrinos
• Cowan & Reines Experiments: Poltergeist, Hanford, Savannah River
• The Neutrino with Dr. Clyde L. Cowan (Lecture on Nobel Prize winning experiment)

• Particle experiments

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