Neutrino detector

A neutrino detector is a physics apparatus which is designed to study neutrinos. Because neutrinos only weakly interact with other particles of matter, neutrino detectors must be very large to detect a significant number of neutrinos. Neutrino detectors are often built underground, to isolate the detector from cosmic rays and other background radiation. The field of neutrino astronomy is still very much in its infancy – the only confirmed extraterrestrial sources so far as of 2018 are the Sun and the supernova 1987A in the nearby Large Magellenic Cloud. Another likely source (3 standard deviations ) is the blazar TXS 0506+056 about 3.7 billion light years away. Neutrino observatories will 'give astronomers fresh eyes with which to study the universe.' A neutrino detector is a physics apparatus which is designed to study neutrinos. Because neutrinos only weakly interact with other particles of matter, neutrino detectors must be very large to detect a significant number of neutrinos. Neutrino detectors are often built underground, to isolate the detector from cosmic rays and other background radiation. The field of neutrino astronomy is still very much in its infancy – the only confirmed extraterrestrial sources so far as of 2018 are the Sun and the supernova 1987A in the nearby Large Magellenic Cloud. Another likely source (3 standard deviations ) is the blazar TXS 0506+056 about 3.7 billion light years away. Neutrino observatories will 'give astronomers fresh eyes with which to study the universe.' Various detection methods have been used. Super Kamiokande is a large volume of water surrounded by phototubes that watch for the Cherenkov radiation emitted when an incoming neutrino creates an electron or muon in the water. The Sudbury Neutrino Observatory is similar, but uses heavy water as the detecting medium. Other detectors have consisted of large volumes of chlorine or gallium which are periodically checked for excesses of argon or germanium, respectively, which are created by neutrinos interacting with the original substance. MINOS uses a solid plastic scintillator watched by phototubes; Borexino uses a liquid pseudocumene scintillator also watched by phototubes; and the NOνA detector uses a liquid scintillator watched by avalanche photodiodes. The proposed acoustic detection of neutrinos via the thermoacoustic effect is the subject of dedicated studies done by the ANTARES, IceCube, and KM3NeT collaborations. Neutrinos are omnipresent in nature such that every second, tens of billions of them 'pass through every square centimetre of our bodies without us ever noticing.' Many were created during the big bang and others are generated by nuclear reactions inside stars, planets, and other interstellar processes. Some may also originate from events in the universe such as 'colliding black holes, gamma ray bursts from exploding stars, and/or violent events at the cores of distant galaxies,' according to speculation by scientists. Despite how common they are, neutrinos are extremely 'difficult to detect' due to their low mass and lack of electric charge. Unlike other particles, neutrinos only interact via gravity and the neutral current (involving the exchange of a Z boson) or charged current (involving the exchange of a W boson) weak interactions. As they have only a 'smidgen of rest mass' according to the laws of physics, perhaps less than a 'millionth as much as an electron,' the gravitational force caused by neutrinos has proven too weak to detect, leaving the weak interaction as the main method for detection: Antineutrinos were first detected near the Savannah River nuclear reactor by the Cowan–Reines neutrino experiment in 1956. Frederick Reines and Clyde Cowan used two targets containing a solution of cadmium chloride in water. Two scintillation detectors were placed next to the water targets. Antineutrinos with an energy above the threshold of 1.8 MeV caused charged current 'inverse beta-decay' interactions with the protons in the water, producing positrons and neutrons. The resulting positron annihilations with electrons created pairs of coincident photons with an energy of about 0.5 MeV each, which could be detected by the two scintillation detectors above and below the target. The neutrons were captured by cadmium nuclei resulting in delayed gamma rays of about 8 MeV that were detected a few microseconds after the photons from a positron annihilation event. This experiment was designed by Cowan and Reines to give a unique signature for antineutrinos, to prove the existence of these particles. It was not the experimental goal to measure the total antineutrino flux. The detected antineutrinos thus all carried an energy greater 1.8 MeV, which is the threshold for the reaction channel used (1.8 MeV is the energy needed to create a positron and a neutron from a proton). Only about 3% of the antineutrinos from a nuclear reactor carry enough energy for the reaction to occur. A more recently built and much larger KamLAND detector used similar techniques to study oscillations of antineutrinos from 53 Japanese nuclear power plants. A smaller, but more radiopure Borexino detector was able to measure the most important components of the neutrino spectrum from the Sun, as well as antineutrinos from Earth and nuclear reactors. Chlorine detectors, based on the method suggested by Bruno Pontecorvo, consist of a tank filled with a chlorine containing fluid such as tetrachloroethylene. A neutrino converts a chlorine-37 atom into one of argon-37 via the charged current interaction. The threshold neutrino energy for this reaction is 0.814 MeV. The fluid is periodically purged with helium gas which would remove the argon. The helium is then cooled to separate out the argon, and the argon atoms are counted based on their electron capture radioactive decays. A chlorine detector in the former Homestake Mine near Lead, South Dakota, containing 520 short tons (470 metric tons) of fluid, was the first to detect the solar neutrinos, and made the first measurement of the deficit of electron neutrinos from the sun (see Solar neutrino problem).