Cowan–Reines neutrino experiment

The experiment exploited a huge flux of (then hypothetical) electron antineutrinos 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.

[1] This quandary and other factors led Wolfgang Pauli to attempt to resolve the issue by postulating the existence of the neutrino in 1930.

This particle, the neutrino, had very small mass and no electric charge; it was not observed, but it carried the missing energy.

By this interaction, the neutron decays directly to an electron, the conjectured neutrino (later determined to be an antineutrino) and a proton.

Fermi first submitted his "tentative" theory of beta decay to the journal Nature, which rejected it "because it contained speculations too remote from reality to be of interest to the reader.

In a 1934 paper, Rudolf Peierls and Hans Bethe calculated that neutrinos could easily pass through the Earth without interactions with any matter.

The usual unit for a cross section in nuclear physics is a barn, which is 1×10−24 cm2 and 20 orders of magnitudes larger.

The coincidence of the positron annihilation and neutron capture events gives a unique signature of an antineutrino interaction.

The hydrogen atoms are so weakly bound in water that they can be viewed as free protons for the neutrino interaction.

Beginning in 1951, Cowan and Reines, both then scientists at Los Alamos, New Mexico, initially thought that neutrino bursts from the atomic weapons tests that were then occurring could provide the required flux.

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 a nuclear reactor as a source of neutrinos, as advised by Los Alamos physics division leader J.M.B.

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