They eventually used a nuclear reactor as a source of neutrinos, as advised by Los Alamos physics division leader J.M.B. 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. Given the small chance of interaction of a single neutrino with a proton, neutrinos could only be observed using a huge neutrino flux. 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. The hydrogen atoms are so weakly bound in water that they can be viewed as free protons for the neutrino interaction. Those protons can serve as targets for antineutrinos, so that simple water can serve as a primary detecting material. The coincidence of the positron annihilation and neutron capture events gives a unique signature of an antineutrino interaction.Ī water molecule is composed of an oxygen and two hydrogen atoms, and most of the hydrogen atoms of water have a single proton for a nucleus. The neutron can be detected by its capture by an appropriate nucleus, releasing a third gamma ray. The two resulting coincident gamma rays ( The positron, the antimatter counterpart of the electron, quickly interacts with any nearby electron, and they annihilate each other. The usual unit for a cross section in nuclear physics is a barn, which is 1 ×10 −24 cm 2 and 20 orders of magnitudes larger.ĭespite the low probability of the neutrino interaction, the signatures of the interaction are unique, making detection of the rare interactions possible. Cowan and Reines predicted a cross section for the reaction to be about 6 ×10 −44 cm 2. The probability for any given reaction to occur is in proportion to its cross section. The chance of this reaction occurring was small. Potential for experiment īy inverse beta decay, the predicted neutrino, more correctly an electron antineutrino ( ν ¯ e In a 1934 paper, Rudolf Peierls and Hans Bethe calculated that neutrinos could easily pass through the Earth without interactions with any matter. 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. 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. The theory, which proved to be remarkably successful, relied on the existence of the hypothetical neutrino. Neutrino (later determined to be an antineutrino) and a proton. By this interaction, the neutron decays directly to an electron, the conjectured The theory posits that the beta decay process consists of four fermions directly interacting with one another. Pauli's suggestion was developed into a proposed theory for beta decay by Enrico Fermi in 1933. This particle, the neutrino, had very small mass and no electric charge it was not observed, but it carried the missing energy. Therefore, in addition to an electron, Pauli suggested that another particle was emitted from the atomic nucleus in beta decay. If the fundamental principle of energy conservation was to be preserved, beta decay had to be a three-body, rather than a two-body, decay. This quandary and other factors led Wolfgang Pauli to attempt to resolve the issue by postulating the existence of the neutrino in 1930. Only the resulting electron was observed, so its varying energy suggested that energy may not be conserved. 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. Neutrino interactions with the protons of the water were observed, verifying the existence and basic properties of this particle for the first time.ĭuring the 1910s and 1920s, the observations of electrons from the nuclear beta decay showed that their energy had a continuous distribution. 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. With neither mass nor charge, such particles appeared to be impossible to detect. Neutrinos, subatomic particles with no electric charge and very small mass, had been conjectured to be an essential particle in beta decay processes in the 1930s. The experiment confirmed the existence of neutrinos. Cowan and Stevens Institute of Technology and New York University alumnus Frederick Reines in 1956. The Cowan–Reines neutrino experiment was conducted by Washington University in St. Institute of Technology Experimental confirmation of neutrinos
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