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.
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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.
In beta decay the predicted particle, the electron antineutrino (ν
e) , should interact with a proton (p) to produce a neutron (n) and positron (e+
) – the antimatter counterpart of the electron.
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 was not 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.
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 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]
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