Neutrino detector

The inside of the MiniBooNE 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.^[1] The field of neutrino astronomy is still very much in its infancy – the only confirmed extraterrestrial sources so far are the Sun and supernova SN 1987A. Neutrino observatories will "give astronomers fresh eyes with which to study the universe."^[2]

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.

Theory

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."^[3] Despite this, they are extremely "difficult to detect" and may 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 some speculation by scientists.^[4] There are three types of neutrinos or what scientists term "flavors": electron, muon, and tau neutrinos, which are named after the type of particle that arises after neutrino collisions; as neutrinos propagate through space, the neutrinos "oscillate between the three available flavours."^[3] Neutrinos have only a "smidgen of rest mass" according to the laws of physics, perhaps less than a "millionth as much as an electron."^[1] Neutrinos can interact via the neutral current (involving the exchange of a Z boson) or charged current (involving the exchange of a W boson) weak interactions.

• In a neutral current interaction, the neutrino enters and then leaves the detector after having transferred some of its energy and momentum to a target particle. If the target particle is charged and sufficiently light (e.g. an electron), it may be accelerated to a relativistic speed and consequently emit Cherenkov radiation, which can be observed directly. All three neutrino flavors can participate regardless of the neutrino energy. However, no neutrino flavor information is left behind.
• In a charged current interaction, a high-energy neutrino transforms into its partner lepton (electron, muon, or tau).^[5] However, if the neutrino does not have sufficient energy to create its heavier partner's mass, the charged current interaction is unavailable to it. Neutrinos from the sun and from nuclear reactors have enough energy to create electrons. Most accelerator-based neutrino beams can also create muons, and a few can create taus. A detector which can distinguish among these leptons can reveal the flavor of the incident neutrino in a charged current interaction. Because the interaction involves the exchange of a charged boson, the target particle also changes character (e.g., neutron → proton).

Detection techniques

Scintillators

Antineutrinos were first detected near the Savannah River nuclear reactor 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 cadmium 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.

Radiochemical methods

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).

A similar detector design, with a much lower detection threshold of 0.233 MeV, uses a gallium → germanium 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 because of the reaction sequence (gallium-germanium-gallium) involved. Gallium and germanium are named after France and Germany, respectively, and ownership of the Alsace-Lorraine territory has historically been a point of contention between France and Germany, thus the nickname of the technique. These radiochemical detection methods are useful only for counting neutrinos; no neutrino direction or energy information is available. The SAGE experiment in Russia used about 50 tons, and the GALLEX/GNO experiments in Italy about 30 tons, of gallium as reaction mass. This experiment is difficult to scale up due to the prohibitive cost of gallium. Larger experiments have therefore turned to a cheaper reaction mass.

Cherenkov detectors

"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 a 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.^[6] Scientists detected 19 neutrinos from an explosion of a star inside the Large Magellanic Cloud—only 19 out of the billion trillion trillion trillion trillion neutrinos emitted by the supernova.^[1] 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) uses 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.^[5] 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.

An illustration of the Antares neutrino detector deployed under water.

Located at a depth of about 2.5 km in the Mediterranean Sea, the ANTARES (Astronomy with a Neutrino Telescope and Abyss environmental RESearch) is fully operational since May 30, 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 to 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 is currently being upgraded to the IceCube observatory, eventually increasing the volume of the detector array to one cubic kilometer.^[7]

Radio detectors

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.

Tracking calorimeters

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 NOνA proposal suggests eliminating the absorber planes in favor of using a very large active detector volume. The active detector is often liquid or plastic scintillator, read out with photomultiplier tubes, although various kinds of ionisation chambers have also been used.

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 pions or another charged lepton 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.)

Background suppression

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").

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.

Telescopes

Neutrino detectors can be aimed at astrophysics observations, many astrophysics events being believed to emit neutrinos.

Underwater neutrino telescopes:

• DUMAND Project (1976–1995; cancelled)
• Baikal Deep Underwater Neutrino Telescope (1993 on)
• ANTARES (2006 on)
• KM3NeT (future telescope; under construction since 2013)
• NESTOR Project (under development since 1998)

Under-ice neutrino telescopes:

• AMANDA (1996–2009, superseded by IceCube)
• IceCube (2004 on)^[2]
• DeepCore and PINGU, an existing extension and a proposed extension of IceCube

Underground neutrino telescopes:

• Borexino, in the Gran Sasso National Laboratories (LNGS), Italy
• Soudan Mine, home of Soudan 2, MINOS, and CDMS^[8]

Miscellaneous:

• GALLEX (1991–1997; ended)
• Tauwer experiment (construction date to be determined)

See also

• List of neutrino experiments

References