Neutrino detector
[2] 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.
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.
The proposed acoustic detection of neutrinos via the thermoacoustic effect is the subject of dedicated studies done by the ANTARES, IceCube, and KM3NeT collaborations.
"[5][a] Many were created during the Big Bang, and others are generated by nuclear reactions inside stars, planets, and by other interstellar processes.
[7][b] Despite how common they are, neutrinos are extremely difficult to detect, due to their low mass and lack of electric charge.
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).
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.
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,[9] 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 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.
In a Cherenkov detector, a large volume of clear material such as water or ice is surrounded by light-sensitive photomultiplier tubes.
This detector used photomultiplier tubes mounted in strings buried deep (1.5–2 km) inside Antarctic glacial ice near the South Pole.
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.
AMANDA has been upgraded to the IceCube observatory, eventually increasing the volume of the detector array to one cubic kilometer.
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.
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.
Since the atmospheric muon incident flux is isotropic, a localised and anisotropic detection is discriminated in relation to the background[21] betraying a cosmic event.
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.
