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. [1] The field of neutrino astronomy is still very much in its infancy – the only confirmed extraterrestrial sources as of 2018 [update] are the Sun and the supernova 1987A in the nearby Large Magellanic Cloud. Another likely source (three standard deviations) [2] 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". [3]
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 was similar, but used 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 used 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: every second, tens of billions of them "pass through every square centimetre of our bodies without us ever noticing." [4] [lower-alpha 1] Many were created during the Big Bang, and others are generated by nuclear reactions inside stars, planets, and by other interstellar processes. [5] According to scientists' speculations, 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". [6] [lower-alpha 2]
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 weak interaction. The two types of weak interactions they (rarely) engage in are neutral current (which involves the exchange of a Z boson and only results in deflection) and charged current (which involves the exchange of a W boson and causes the neutrino to convert into a charged lepton: an electron, a muon, or a tauon, or one of their antiparticles, if an antineutrino). According to the laws of physics neutrinos must have mass, but only a "smidgen of rest mass" – perhaps less than a "millionth as much as an electron" [1] – so the gravitational force caused by neutrinos has so far proved too weak to detect, leaving the weak interaction as the main method of 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 positrons annihilate with electrons, creating 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 than 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.
The SNO+ experiment uses linear alkylbenzene as a liquid scintillator, [8] in contrast to its predecessor Sudbury Neutrino Observatory which used heavy water and detected Cherenkov light (see below).
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 occasionally 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).
A similar detector design, with a much lower detection threshold of 0.233 MeV, uses a gallium (Ga) → germanium (Ge) transformation which is sensitive to lower-energy neutrinos. A neutrino is able to react with an atom of gallium-71, converting it into an atom of the unstable isotope germanium-71. The germanium was then chemically extracted and concentrated. Neutrinos were thus detected by measuring the radioactive decay of germanium.
This latter method is nicknamed the "Alsace-Lorraine" technique in a joke-reference to the Ga → Ge → Ga reaction sequence. [lower-alpha 3]
The SAGE experiment in Russia used about 50 tons of gallium, and the GALLEX / GNO experiments in Italy about 30 tons of gallium as reaction mass. The price of gallium is prohibitive, so this experiment is difficult to afford on large-scale. Larger experiments have therefore turned to a less costly reaction mass.
Radiochemical detection methods are only useful for counting neutrinos; they provide almost no information on neutrino energy or direction of travel.
"Ring-imaging" Cherenkov detectors take advantage of a phenomenon called Cherenkov light. Cherenkov radiation is produced whenever charged particles such as electrons or muons are moving through a given detector medium somewhat faster than the speed of light in that medium. In a Cherenkov detector, a large volume of clear material such as water or ice is surrounded by light-sensitive photomultiplier tubes. A charged lepton produced with sufficient energy and moving through such a detector does travel somewhat faster than the speed of light in the detector medium (although somewhat slower than the speed of light in vacuum). The charged lepton generates a visible "optical shockwave" of Cherenkov radiation. This radiation is detected by the photomultiplier tubes and shows up as a characteristic ring-like pattern of activity in the array of photomultiplier tubes. As neutrinos can interact with atomic nuclei to produce charged leptons which emit Cherenkov radiation, this pattern can be used to infer direction, energy, and (sometimes) flavor information about incident neutrinos.
Two water-filled detectors of this type (Kamiokande and IMB) recorded a neutrino burst from supernova SN 1987A. [9] [lower-alpha 4] Scientists detected 19 neutrinos from an explosion of a star inside the Large Magellanic Cloud – only 19 out of the octo-decillion (1057) neutrinos emitted by the supernova. [1] [lower-alpha 5] The Kamiokande detector was able to detect the burst of neutrinos associated with this supernova, and in 1988 it was used to directly confirm the production of solar neutrinos. The largest such detector is the water-filled Super-Kamiokande. This detector uses 50,000 tons of pure water surrounded by 11,000 photomultiplier tubes buried 1 km underground.
The Sudbury Neutrino Observatory (SNO) used 1,000 tonnes of ultrapure heavy water contained in a 12 metre-diameter vessel made of acrylic plastic surrounded by a cylinder of ultrapure ordinary water 22 metres in diameter and 34 metres high. [7] [lower-alpha 6] In addition to the neutrino interactions visible in a regular water detector, a neutrino can break up the deuterium in heavy water. The resulting free neutron is subsequently captured, releasing a burst of gamma rays that can be detected. All three neutrino flavors participate equally in this dissociation reaction.
The MiniBooNE detector employs pure mineral oil as its detection medium. Mineral oil is a natural scintillator, so charged particles without sufficient energy to produce Cherenkov light still produce scintillation light. Low-energy muons and protons, invisible in water, can be detected. Thus the use of natural environment as a measurement medium emerged.
Since the neutrino flux incoming to earth decreases with increasing energy, the size of neutrino detectors must increase too. [10] Though building a kilometer-sized cube detector underground covered by thousands of photomultiplier would be prohibitively expensive, detection volumes of this magnitude can be achieved by installing Cherenkov detector arrays deep inside already existing natural water or ice formations, with several other advantages. Firstly, hundreds of meters of water or ice partly protect the detector from atmospheric muons. Secondly, these environments are transparent and dark, vital criteria in order to detect the faint Cherenkov light. In practice, because of Potassium 40 decay, even the abyss is not completely dark, so this decay must be used as a baseline. [11]
Located at a depth of about 2.5 km in the Mediterranean Sea, the ANTARES telescope (Astronomy with a Neutrino Telescope and Abyss environmental Research) has been fully operational since 30 May 2008. Consisting of an array of twelve separate 350 meter-long vertical detector strings 70 meters apart, each with 75 photomultiplier optical modules, this detector uses the surrounding sea water as the detector medium. The next generation deep sea neutrino telescope KM3NeT will have a total instrumented volume of about 5 km3. The detector will be distributed over three installation sites in the Mediterranean. Implementation of the first phase of the telescope was started in 2013.
The Antarctic Muon And Neutrino Detector Array (AMANDA) operated from 1996–2004. This detector used photomultiplier tubes mounted in strings buried deep (1.5–2 km) inside Antarctic glacial ice near the South Pole. The ice itself is the detector medium. The direction of incident neutrinos is determined by recording the arrival time of individual photons using a three-dimensional array of detector modules each containing one photomultiplier tube. This method allows detection of neutrinos above 50 GeV with a spatial resolution of approximately 2 degrees. AMANDA was used to generate neutrino maps of the northern sky to search for extraterrestrial neutrino sources and to search for dark matter. AMANDA has been upgraded to the IceCube observatory, eventually increasing the volume of the detector array to one cubic kilometer. [12] Ice Cube sits deep underneath the South Pole in a cubic kilometre of perfectly clear, bubble-free ancient ice. Like AMANDA it relies on detecting the flickers of light emitted on the exceedingly rare occasions when a neutrino does interact with an atom of ice or water. [12]
The Radio Ice Cherenkov Experiment uses antennas to detect Cherenkov radiation from high-energy neutrinos in Antarctica. The Antarctic Impulse Transient Antenna (ANITA) is a balloon-borne device flying over Antarctica and detecting Askaryan radiation produced by ultra-high-energy neutrinos interacting with the ice below. Currently the Radio Neutrino Observatory Greenland is being built, exploiting the Askaryan effect in ice to detect neutrinos with energies >10 PeV. [13]
Tracking calorimeters such as the MINOS detectors use alternating planes of absorber material and detector material. The absorber planes provide detector mass while the detector planes provide the tracking information. Steel is a popular absorber choice, being relatively dense and inexpensive and having the advantage that it can be magnetised. The active detector is often liquid or plastic scintillator, read out with photomultiplier tubes, although various kinds of ionisation chambers have also been used.
The NOνA proposal [14] suggests eliminating the absorber planes in favor of using a very large active detector volume. [15]
Tracking calorimeters are only useful for high-energy (GeV range) neutrinos. At these energies, neutral current interactions appear as a shower of hadronic debris and charged current interactions are identified by the presence of the charged lepton's track (possibly alongside some form of hadronic debris).
A muon produced in a charged current interaction leaves a long penetrating track and is easy to spot; The length of this muon track and its curvature in the magnetic field provide energy and charge (
μ−
versus
μ+
) information. An electron in the detector produces an electromagnetic shower, which can be distinguished from hadronic showers if the granularity of the active detector is small compared to the physical extent of the shower. Tau leptons decay essentially immediately to either another charged lepton or pions, and cannot be observed directly in this kind of detector. (To directly observe taus, one typically looks for a kink in tracks in photographic emulsion.)
At low energies, a neutrino can scatter from the entire nucleus of an atom, rather than the individual nucleons, in a process known as coherent neutral current neutrino-nucleus elastic scattering or coherent neutrino scattering. [16] This effect has been used to make an extremely small neutrino detector. [17] [18] [19] Unlike most other detection methods, coherent scattering does not depend on the flavor of the neutrino.
Most neutrino experiments must address the flux of cosmic rays that bombard the Earth's surface.
The higher-energy (>50 MeV or so) neutrino experiments often cover or surround the primary detector with a "veto" detector which reveals when a cosmic ray passes into the primary detector, allowing the corresponding activity in the primary detector to be ignored ("vetoed"). Since the atmospheric muon incident flux is isotropic, a localised and anisotropic detection is discriminated in relation to the background [20] betraying a cosmic event.
For lower-energy experiments, the cosmic rays are not directly the problem. Instead, the spallation neutrons and radioisotopes produced by the cosmic rays may mimic the desired signals. For these experiments, the solution is to place the detector deep underground so that the earth above can reduce the cosmic ray rate to acceptable levels.
Neutrino detectors can be aimed at astrophysics observations, since many astrophysical events are believed to emit neutrinos.
Underwater neutrino telescopes:
Under-ice neutrino telescopes:
Underground neutrino observatories:
Others:
A neutrino is a fermion that interacts only via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass is so small (-ino) that it was long thought to be zero. The rest mass of the neutrino is much smaller than that of the other known elementary particles. The weak force has a very short range, the gravitational interaction is extremely weak due to the very small mass of the neutrino, and neutrinos do not participate in the electromagnetic interaction or the strong interaction. Thus, neutrinos typically pass through normal matter unimpeded and undetected.
The Sudbury Neutrino Observatory (SNO) was a neutrino observatory located 2100 m underground in Vale's Creighton Mine in Sudbury, Ontario, Canada. The detector was designed to detect solar neutrinos through their interactions with a large tank of heavy water.
Super-Kamiokande is a neutrino observatory located under Mount Ikeno near the city of Hida, Gifu Prefecture, Japan. It is operated by the Institute for Cosmic Ray Research, University of Tokyo with the help of an international team. It is located 1,000 m (3,300 ft) underground in the Mozumi Mine in Hida's Kamioka area. The observatory was designed to detect high-energy neutrinos, to search for proton decay, study solar and atmospheric neutrinos, and keep watch for supernovae in the Milky Way Galaxy.
Neutrino astronomy is the branch of astronomy that gathers information about astronomical objects by observing and studying neutrinos emitted by them with the help of neutrino detectors in special Earth observatories. It is an emerging field in astroparticle physics providing insights into the high-energy and non-thermal processes in the universe.
The Cowan–Reines neutrino experiment was conducted by physicists Clyde Cowan and Frederick Reines in 1956. The experiment confirmed the existence of neutrinos. 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. With neither mass nor charge, such particles appeared to be impossible to detect. The experiment exploited a huge flux of 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.
The Antarctic Muon And Neutrino Detector Array (AMANDA) is a neutrino telescope located beneath the Amundsen–Scott South Pole Station. In 2005, after nine years of operation, AMANDA officially became part of its successor project, the IceCube Neutrino Observatory.
Explorer 11 was a NASA satellite that carried the first space-borne gamma-ray telescope. This marked the beginning of space gamma-ray astronomy. Launched on 27 April 1961 by a Juno II, the satellite returned data until 17 November 1961, when power supply problems ended the science mission. During the spacecraft's seven-month lifespan it detected twenty-two events from gamma-rays and approximately 22,000 events from cosmic radiation.
A solar neutrino is a neutrino originating from nuclear fusion in the Sun's core, and is the most common type of neutrino passing through any source observed on Earth at any particular moment. Neutrinos are elementary particles with extremely small rest mass and a neutral electric charge. They only interact with matter via weak interaction and gravity, making their detection very difficult. This has led to the now-resolved solar neutrino problem. Much is now known about solar neutrinos, but research in this field is ongoing.
The IceCube Neutrino Observatory is a neutrino observatory developed by the University of Wisconsin–Madison and constructed at the Amundsen–Scott South Pole Station in Antarctica. The project is a recognized CERN experiment (RE10). Its thousands of sensors are located under the Antarctic ice, distributed over a cubic kilometer.
ANTARES is a neutrino detector residing 2.5 km under the Mediterranean Sea off the coast of Toulon, France. It is designed to be used as a directional neutrino telescope to locate and observe neutrino flux from cosmic origins in the direction of the Southern Hemisphere of the Earth, a complement to the South Pole neutrino detector IceCube that detects neutrinos from both hemispheres. The experiment is a recognized CERN experiment (RE6). Other neutrino telescopes designed for use in the nearby area include the Greek NESTOR telescope and the Italian NEMO telescope, which are both in early design stages. The data taking of ANTARES was finished in February 2022, after 16 years of continuous operation.
T2K is a particle physics experiment studying the oscillations of the accelerator neutrinos. The experiment is conducted in Japan by the international cooperation of about 500 physicists and engineers with over 60 research institutions from several countries from Europe, Asia and North America and it is a recognized CERN experiment (RE13). T2K collected data within its first phase of operation from 2010 till 2021. The second phase of data taking is expected to start in 2023 and last until commencement of the successor of T2K – the Hyper-Kamiokande experiment in 2027.
Hyper-Kamiokande is a neutrino observatory and experiment under construction in Hida, Gifu and in Tokai, Ibaraki in Japan. It is conducted by the University of Tokyo and the High Energy Accelerator Research Organization (KEK), in collaboration with institutes from over 20 countries across six continents. As a successor of the Super-Kamiokande and T2K experiments, it is designed to search for proton decay and detect neutrinos from natural sources such as the Earth, the atmosphere, the Sun and the cosmos, as well as to study neutrino oscillations of the man-made accelerator neutrino beam. The beginning of data-taking is planned for 2027.
The Kamioka Observatory, Institute for Cosmic Ray Research, University of Tokyo is a neutrino and gravitational waves laboratory located underground in the Mozumi mine of the Kamioka Mining and Smelting Co. near the Kamioka section of the city of Hida in Gifu Prefecture, Japan. A set of groundbreaking neutrino experiments have taken place at the observatory over the past two decades. All of the experiments have been very large and have contributed substantially to the advancement of particle physics, in particular to the study of neutrino astronomy and neutrino oscillation.
SNO+ is a physics experiment designed to search for neutrinoless double beta decay, with secondary measurements of proton–electron–proton (pep) solar neutrinos, geoneutrinos from radioactive decays in the Earth, and reactor neutrinos. It could also observe supernovae neutrinos if a supernova occurs in our galaxy. It is under construction using the underground equipment already installed for the former Sudbury Neutrino Observatory (SNO) experiment at SNOLAB.
A cosmic-ray observatory is a scientific installation built to detect high-energy-particles coming from space called cosmic rays. This typically includes photons, electrons, protons, and some heavier nuclei, as well as antimatter particles. About 90% of cosmic rays are protons, 9% are alpha particles, and the remaining ~1% are other particles.
The solar neutrino problem concerned a large discrepancy between the flux of solar neutrinos as predicted from the Sun's luminosity and as measured directly. The discrepancy was first observed in the mid-1960s and was resolved around 2002.
The Tunka experiment now named TAIGA measures air showers, which are initiated by charged cosmic rays or high energy gamma rays. TAIGA is situated in Siberia in the Tunka valley close to lake Baikal. Meanwhile, TAIGA consists of five different detector systems: Tunka-133, Tunka-Rex, and Tunka-Grande for charged cosmic rays; Tunka-HiSCORE and Tunka-IACT for gamma astronomy. From the measurements of each detector it is possible to reconstruct the arrival direction, energy and type of the cosmic rays, where the accuracy is enhanced by the combination of different detector systems.
The Accelerator Neutrino Neutron Interaction Experiment (ANNIE) is a proposed water Cherenkov detector experiment designed to examine the nature of neutrino interactions. This experiment will study phenomena like proton decay, and neutrino oscillations, by analyzing neutrino interactions in gadolinium-loaded water and measuring their neutron yield. Neutron Tagging plays an important role in background rejection from atmospheric neutrinos. By implementing early prototypes of LAPPDs, high precision timing is possible. The suggested location for ANNIE is the SciBooNE hall on the Booster Neutrino Beam associated with the MiniBooNE experiment. The neutrino beam originates in Fermilab where The Booster delivers 8 GeV protons to a beryllium target producing secondary pions and kaons. These secondary mesons decay to produce a neutrino beam with an average energy of around 800 MeV. ANNIE will begin installation in the summer of 2015. Phase I of ANNIE, mapping the neutron background, completed in 2017. The detector is being upgraded for full science operation which is expected to begin late 2018.
Eugene William Beier is an American physicist.
The diffuse supernova neutrino background(DSNB) is a theoretical population of neutrinos (and anti-neutrinos) cumulatively originating from all core-collapse supernovae events throughout the history of the universe. Though it has not yet been directly detected, the DSNB is theorized to be isotropic and consists of neutrinos with typical energies on the scale of 107 eV. Current detection efforts are limited by the influence of background noise in the search for DSNB neutrinos and are therefore limited to placing limits on the parameters of the DSNB, namely the neutrino flux. Restrictions on these parameters have gotten more strict in recent years, but many researchers are looking to make direct observations in the near future with next generation detectors. The DSNB is not to be confused with the cosmic neutrino background (CNB), which is comprised by relic neutrinos that were produced during the Big Bang and have much lower energies (10−4 to 10−6 eV).
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