Neutrino detector

The Sudbury Neutrino Observatory, a 12-meter sphere filled with heavy water surrounded by light detectors located 2000 meters below the ground in Sudbury, Ontario, Canada.

A neutrino detector is a physics apparatus designed to study neutrinos. Because neutrinos are only weakly interacting with other particles of matter, neutrino detectors must be very large in order 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 SN1987A. 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 while the proposed NOνA detector will use 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 and IceCube collaborations.

Theory

Neutrinos are omnipresent in nature such that in just one 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 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 only have a "smidgen of weight" 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 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, the 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. Solar and reactor neutrinos 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 antineutrino oscillations from 53 Japanese nuclear power plants. A smaller but more pure Borexino detector was able to measure the Beryllium neutrinos from the Sun.

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. 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" detectors take advantage of the Cherenkov light produced by charged particles moving through a medium faster than the speed of light in that medium. In these detectors, a large volume of clear material (e.g., water or ice) is surrounded by light-sensitive photomultiplier tubes. A charged lepton produced with sufficient energy typically travels faster than the speed of light in the detector medium (though slower than the speed of light in a vacuum). This generates an "optical shockwave" known as Cherenkov radiation which can be detected by the photomultiplier tubes. The result is a characteristic ring-like pattern of activity on the array of photomultiplier tubes. This pattern can be used to infer direction, energy, and (sometimes) flavor information about the incident neutrino.

Two water-filled detectors of this type (Kamiokande and IMB) recorded the neutrino burst from supernova 1987A.^[6] Scientists detected only 19 neutrinos from an explosion of a star inside the Large magellanic Cloud -- only 19 out of a billion trillion trillion trillion trillion neutrinos which "flew from the supernova".^[1] Kamiokande 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 which are contained in a 12-metre-diameter vessel made of acrylic-plastic, and this structure is surrounded by ultrapure ordinary water cylinder which is 22 metres in diameter and 34 metres high.^[5] In addition to the neutrino interactions available in a regular water detector, the deuterium in the heavy water can be broken up by a neutrino. The resulting free neutron is subsequently captured, releasing a burst of gamma rays which are 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 can still produce scintillation light. This allows low energy muons and protons, invisible in water, to be detected.

An illustration of the Antares neutrino detector deployed under water.

Located about 2.5 km under the Mediterranean Sea, the ANTARES (Astronomy with a Neutrino Telescope and Abyss environmental RESearch) has been fully operational since May 30, 2008. Consisting of an array of twelve separate 350 meter long vertical strings 70 meters apart from each other with 75 photomultiplier optical modules, this detector uses the sea water as the detector mass.

The Antarctic Muon And Neutrino Detector Array (AMANDA) operated from 1996 to 2004. This detector used photomultiplier tubes mounted on strings, buried deep (1.5–2 km) inside the glacial ice at the South Pole in Antarctica. The ice itself is used as the detector mass. The direction of incident neutrinos is determined by recording the arrival time of individual photons using a three-dimensional array of detector modules containing one photomultiplier tube each. This method allows detection of neutrinos above 50 GeV with a spatial resolution of approximately 2 degrees. AMANDA has been used to generate neutrino maps of the northern sky in order to search for extraterrestrial neutrino sources and in searches for dark matter. AMANDA is currently in the process of being upgraded to the IceCube observatory, eventually increasing the volume of the detector array to one cubic kilometer.^[7]

Radio detectors

The Radio Ice Cerenkov Experiment uses antennas to detect Cerenkov radiation from high-energy neutrinos in Antarctica. The Antarctic Impulse Transient Antenna (ANITA) is a balloon-born 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 physics signals. For these experiments, the solution is to locate the detector deep underground so that the earth above can reduce the cosmic ray rate to tolerable levels.

Telescopes

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

Underwater neutrino telescopes:

• DUMAND (1976–1995, cancelled)
• Baikal (1993–)
• ANTARES (2006–)
• Km3net (proposed)
• NESTOR Project (under development since 1998)

Underice neutrino telescopes :

• AMANDA (1996–2009, superseded by IceCube)
• IceCube (2004–)^[2]

Underground neutrino telescopes:

• Soudan lab, in Soudan, Minnesota^[8]

Miscellaneous :

• GALLEX (1991–1997, ended)

See also

• List of neutrino experiments

References

1. ^ ^{a b c} KENNETH CHANG (April 26, 2005). "Tiny, Plentiful and Really Hard to Catch". The New York Times. http://www.nytimes.com/2005/04/26/science/26neut.html?pagewanted=print&position=. Retrieved 2011-06-16. "In 1987, astronomers counted 19 neutrinos from an explosion of a star in the nearby Large Magellanic Cloud, 19 out of the billion trillion trillion trillion trillion neutrinos that flew from the supernova." 
2. ^ ^{a b} Ian Sample (23 January 2011). "The hunt for neutrinos in the Antarctic". The Guardian. http://www.guardian.co.uk/science/2011/jan/23/neutrino-cosmic-rays-south-pole. Retrieved 2011-06-16. "The $272m (£170m) IceCube instrument is not your typical telescope. Instead of collecting light from the stars, planets or other celestial objects, IceCube looks for ghostly particles called neutrinos that hurtle across space with high-energy cosmic rays. If all goes to plan, the observatory will reveal where these mysterious rays come from, and how they get to be so energetic. But that is just the start. Neutrino observatories such as IceCube will ultimately give astronomers fresh eyes with which to study the universe." 
3. ^ ^{a b} Pierre Le Hir (22 March 2011). "Tracking down the crafty neutrino". Guardian Weekly. http://www.guardian.co.uk/science/2011/mar/22/france-science-nuclear-neutrino-lehir. Retrieved 2011-06-16. "But they are nevertheless almost undetectable: in just one second several tens of billions of neutrinos pass through every square centimetre of our bodies without us ever noticing. ... No magnetic field diverts them from their course, shooting straight ahead at almost the speed of light. ... Almost nothing stops them. ... Neutrinos are remarkably tricky customers. There are three types or flavours: electron, muon and tau neutrinos, named after three other particles to which they give rise when they collide with an atom." 
4. ^ Dr David Whitehouse, BBC News Online science editor (15 July, 2003). "Icebound telescope probes the Universe". BBC News. http://news.bbc.co.uk/2/hi/science/nature/3068359.stm. Retrieved 2011-06-16. "Sensors in the ice have detected the rare and fleeting flashes of light caused when neutrinos interact with the ice. ... Amanda 2 (Antarctic Muon and Neutrino Detector Array - 2) is designed to look not up, but down, through the Earth to the sky of the Northern Hemisphere." 
5. ^ ^{a b} Dr David Whitehouse, BBC News Online science editor (22 April, 2002). "Experiment confirms Sun theories". BBC News. http://news.bbc.co.uk/2/hi/science/nature/1943837.stm. Retrieved 2011-06-16. "New evidence confirms last year's indication that one type of neutrino emerging from the Sun's core does switch to another type en route to the Earth. ... The data were obtained from the underground Sudbury Neutrino Observatory (SNO) in Canada. ... Neutrinos are ghostly particles with no electric charge and very little mass. They are known to exist in three types related to three different charged particles - the electron and its lesser-known relatives, the muon and the tau. ..." 
6. ^ MALCOLM W. BROWNE (February 28, 1995). "Four Telescopes in Neutrino Hunt". The New York Times. http://www.nytimes.com/1995/02/28/science/four-telescopes-in-neutrino-hunt.html. Retrieved 2011-06-16. "NEUTRINO astronomy was given a strong push in 1987 when a supernova in a galaxy only one-quarter of a million light-years away from Earth flared into view -- the closest supernova in 400 years." 
7. ^ J.P. (Dec 1st 2010). "Hang on, that's not a neutrino". The Economist. http://m.economist.com/babbage-tech-21013466.php. Retrieved 2011-06-16. "The largest, IceCube , sits deep underneath the South Pole in a cubic kilometre of perfectly clear, bubble-free ancient ice and is set to start working in earnest early next year. All rely on detecting the flickers of light emitted on the exceedingly rare occasions when a neutrino does interact with an atom of ice or water." 
8. ^ "Minnesota neutrino project to get under way this month". USA Today. Feb. 11, 2005. http://www.usatoday.com/tech/science/mathscience/2005-02-11-neutrino-detector_x.htm. Retrieved 2011-06-16. "Later this month, Fermi National Accelerator Laboratory near Chicago will begin shooting trillions of subatomic "neutrino" particles through 450 miles of solid earth, their target a detector at the Soudan Underground Laboratory beneath this Iron Range town."