Scintillators 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, in contrast to its predecessor
Sudbury Neutrino Observatory which used heavy water and detected Cherenkov light (see below).
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 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 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 reaction sequence. 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.
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
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. 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. 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.
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 as cosmic ultra-high-energy neutrinos travel through the ice below and produce a shower of secondary charged particles, which emits a cone of coherent radiation in the radio or microwave part of the electromagnetic spectrum. Currently the
Radio Neutrino Observatory Greenland is being built, exploiting the Askaryan effect in ice to detect neutrinos with energies >10 PeV.
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 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 suggests eliminating the absorber planes in favor of using a very large active detector volume. 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 ( versus ) 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.)
Coherent Recoil Detector 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. This effect has been used to make an extremely small neutrino detector. Unlike most other detection methods, coherent scattering does not depend on the flavor of the neutrino. ==Background suppression==