reines cowan neutrino experiment

1956 – First discovery of the neutrino by an experiment

In this experiment, for which they were awarded a Nobel Prize in Physics in 1995, Clyde L. Cowan and Frederick Reines used a nuclear reactor, expecting to produce neutrino fluxes on the order of 10 12 to 10 13 neutrinos per second per cm 2 , far higher than any attainable flux from other radioactive sources. The neutrinos would then interact with protons in a tank of water, creating neutrons and positrons. Each positron would create a pair of gamma rays when it annihilated with an electron. The gamma rays were detected by placing a scintillator material in a tank of water. The scintillator material gives off flashes of light in response to the gamma rays and the light flashes are detected by photomultiplier tubes.

However, this experiment wasn’t conclusive enough, so they came up with a second layer of certainty. They would detect the neutrons by placing cadmium chloride into the tank. Cadmium is a highly effective neutron absorber (and so finds use in nuclear control rods) and gives off a gamma ray when it absorbs a neutron. The arrangement was such that the gamma ray from the cadmium would be detected 5 microseconds after the gamma ray from the positron, if it were truly produced by a neutrino.

They performed the experiment preliminarily at Hanford, but later moved the experiment to the Savannah River Plant near Augusta, Georgia where they had better shielding against cosmic rays. This shielded location was 11m from the reactor and 12m underground. They 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 three scintillator layers which contained 110 five-inch (127 mm) photomultiplier tubes.

After months of data collection, they had accumulated data on about three neutrinos per hour in their detector. To be absolutely sure that they were seeing neutrino events from the detection scheme described above, they shut down the reactor to show that there was a difference in the number of detected events. They had predicted a cross-section for the reaction to be about 6×10 -44 cm 2 and their measured cross-section was 6.3×10 -44 cm 2 . Their results were published in 1956.

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The neutrino lived for a quarter of a century as a theoretical suggestion before its discovery in the 1950s, as reported in two Physical Review articles. Identifying the elusive particle required a detection system of unprecedented size, allied with ingenious data analysis, to pull a tiny signal out of a noisy background. The discovery of the neutrino, with roots in the wartime Manhattan project, marked the growing importance of “big science” projects that would increasingly dominate particle physics.

In 1930 Wolfgang Pauli proposed that an undetected particle–he called it the “neutron”–is emitted along with a positron in radioactive beta decay. This new particle would explain some puzzling aspects of the nuclear decay data. Enrico Fermi later called Pauli’s particle the “neutrino” to distinguish it from the now-familiar neutron, discovered in 1932. The neutrino was thought to rarely interact with other particles and so would be very difficult to detect.

In the early 1950s, Frederick Reines and Clyde Cowan of the Los Alamos National Laboratory in New Mexico designed a detector to identify neutrinos in the intense outburst of particles and radiation given off in a nuclear bomb test. But they abandoned that plan when they came up with a more sensitive scheme that could work with the smaller but steadier neutrino emission from a reactor.

Reines and Cowan focused on the reaction in which a neutrino hits a proton, creating a neutron and a positron. (In fact, the incoming particle is an antineutrino, but theorists didn’t yet know whether the neutrino and the antineutrino were distinct particles). A tank containing 300 liters of water supplied an abundance of protons. Any positron generated in the water would rapidly slow down and annihilate with an electron, producing a pair of detectable gamma rays.

But those detections would be enormously outnumbered by positrons created by other reactor emissions, as well as cosmic rays. In order to also detect the neutrons from neutrino-proton reactions, Reines and Cowan dissolved cadmium chloride in the water–at up to 40 kilograms per hundred liters. The water slowed down fast neutrons, allowing them to be captured by cadmium nuclei into an excited state that decayed by emission of an MeV photon. Only when this nuclear gamma ray followed an annihilation pair by a few microseconds would the experiment register a neutrino-induced event.

Using this “delayed coincidence” method at the Hanford Site in Washington state, Reines and Cowan announced a “probable” detection of the neutrino in 1953 in the Physical Review . They then moved to a more powerful reactor at Savannah River, South Carolina, where they used two 200-liter detection tanks surrounded by an even larger detection apparatus, and with better shielding of other reactor emissions. After amassing 100 days of running time, they announced in Science neutrino detections at a rate of 3 per hour [1] following up with a detailed account in the Physical Review in 1960.

Accepting a share of the 1995 Nobel Prize in physics (Cowan died in 1974), Reines remarked that before his and Cowan’s work, a “big” physics experiment might use a one-liter detector. It was their background in weapons research, suggests Francis Halzen of the University of Wisconsin, Madison, that emboldened Reines and Cowan to tackle such a large increase in experimental scale. Halzen, principal investigator of the planned IceCube neutrino observatory at the South Pole, recalls a conversation in which Reines emphasized how important it was that he approached neutrino detection with a theorist’s frame of mind–because a seasoned experimenter would have said there was no chance of success.

–David Lindley

David Lindley is a freelance science writer in Alexandria, Virginia.

• C. L. Cowan, Jr., F. Reines, F. B. Harrison, H. W. Kruse, and A. D. McGuire, “Detection of the Free Neutrino: A Confirmation,” Science 124, 103 (1956)

More Information

1995 Nobel Prize in physics information

IceCube experiment

Detection of the Free Antineutrino

F. Reines, C. L. Cowan, Jr., F. B. Harrison, A. D. McGuire, and H. W. Kruse

Phys. Rev. 117 , 159 (1960)

Published January 1, 1960

Detection of the Free Neutrino

F. Reines and C. L. Cowan, Jr.

Phys. Rev. 92 , 830 (1953)

Published November 1, 1953

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The neutrino experiment, also called the Cowan and Reines neutrino experiment, was performed by Clyde L. Cowan and Frederick Reines in 1956. This experiment confirmed the existence of the antineutrino—a neutrally charged subatomic particle with very low mass.

In the 1930s, through the study of beta decay, it was apparent that a third particle, one of nearly no mass and with neutral charge existed and was not observed.

This was due to a continuous spread of kinetic energy and momentum values for electrons emitted in beta decay. The only way this was possible was if there was a particle of neutral charge and almost no mass (or possibly no mass) produced in the decay.

Potential for experiment

In beta decay the predicted particle, the electron antineutrino (anti- ν e ) - should interact with a proton to produce a neutron and positron - the antimatter counterpart of the electron.

anti- ν e + p + -> n + e +

The positron quickly finds an electron, and they annihilate each other. The two resulting gamma rays (γ) are detectable. The neutron can be detected by its capture on an appropriate nucleus, releasing a gamma ray. The coincidence of both events - positron annihilation and neutron capture - gives a unique signature of an antineutrino interaction.

Most hydrogen atoms bound in water molecules have a single proton for a nucleus. Those protons serve as a target for the antineutrinos from a reactor. For heavier nuclei, with several protons and neutrons, the interaction mechanism is more complicated and is not always well described by considering the constituent protons as free.

Cowan and Reines used a nuclear reactor, as advised by Los Alamos physics division leader J.M.B. Kellogg,[1] as a source of a neutrino flux of 5×1013 neutrinos per second per square centimeter;[2] far higher than any attainable flux from other radioactive sources.

The neutrinos then interacted (as shown above) with protons in a tank of water, creating neutrons and positrons. Each positron created a pair of gamma rays when it annihilated with an electron.

The gamma rays were detected by placing a scintillator material in a tank of water. The scintillator material gives off flashes of light in response to the gamma rays and the light flashes are detected by photomultiplier tubes.

However, this experiment wasn't conclusive enough, so they came up with a second layer of certainty. They detected the neutrons by placing cadmium chloride into the tank. Cadmium is a highly effective neutron absorber and gives off a gamma ray when it absorbs a neutron.

n + 108 Cd -> 109 Cd* -> 109 Cd + γ

The arrangement was such that the gamma ray from the cadmium would be detected 5 microseconds after the gamma ray from the positron, if it were truly produced by a neutrino.

The results

They performed the experiment preliminarily at Hanford Site, but later moved the experiment to the Savannah River Plant in South Carolina near Aiken where they had better shielding against cosmic rays. This shielded location was 11 m from the reactor and 12 m underground.

They 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 three scintillator layers which contained 110 five-inch (127 mm) photomultiplier tubes.

After months of data collection, they had accumulated data on about three neutrinos per hour in their detector. To be absolutely sure that they were seeing neutrino events from the detection scheme described above, they shut down the reactor to show that there was a difference in the number of detected events.

They had predicted a cross-section for the reaction to be about 6×10−44 cm2 and their measured cross-section was 6.3×10−44 cm2. Their results were published in the July 20, 1956 issue of Science.[3][4]

Clyde Cowan died in 1974; Frederick Reines was honored with the Nobel Prize in 1995 for his work on neutrino physics.[5]

Homestake Experiment (a contemporary experiment which detected neutrinos from beta decays in the sun)

^ "The Reines-Cowan Experiments: Detecting the Poltergeist". Los Alamos Science 25: 3. 1997. ^ Griffiths, David J. (1987). Introduction to Elementary Particles. John Wiley & Sons. ISBN 0-471-60386-4. ^ C. L Cowan Jr., F. Reines, F. B. Harrison, H. W. Kruse, A. D McGuire (July 20, 1956). "Detection of the Free Neutrino: a Confirmation". Science 124 (3212): 103–4. Bibcode 1956Sci...124..103C. doi:10.1126/science.124.3212.103. PMID 17796274. ^ Winter, Klaus (2000). Neutrino physics. Cambridge University Press. p. 38ff. ISBN 9780521650038. This source reproduces the 1956 paper. ^ "The Nobel Prize in Physics 1995". The Nobel Foundation. Retrieved 201-06-29.

Homestake experiment

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