Located deep underground under kilometers of rock ice, or deep underwater in the abysses, particle physics experiments can look at the deep universe in a unique way, promising challenging discoveries.
The presence of a thick layer of rock or ice strongly reduce particles coming from the cosmos, allowing to observe only neutrinos, a subatomic particles among the most abundant in the universe, but difficult to study since they very rarely interact with matter.
Neutrino was first postulated by Wolfgang Pauli in 1930 in order to explain the continuous spectrum of beta decay, however it remained unobserved until 1956 where Clyde Cowan and Frederick Reines carried out a revolutionary experiment which used the Savannah River nuclear reactor in South Carolina as a source of beta decay.
Neutrino properties make it a unique probe of the deep universe since, arriving undeflected on earth, they will help to pinpoint distant sources of powerful radiation, like quasars or gamma-ray bursts and will help to further investigate the mechanism under catastrophic events in the universe like supernovae explosion and gravitational waves.
Since neutrinos weakly interact with matter and are neutral particles, they can only be detected using huge detectors and looking at the Cherenkov light emitted by the secondary charged particle created by the interaction within the detector volume. The Cherenkov light emission by a charged particle traveling in a medium is possible when it passes through it at a speed greater than the phase velocity of light in that medium.
Super-Kamiokande in Japan, IceCube in Antarctica and the KM3NeT project in the Mediterranean sea are important examples of present and future experiments designed to give an important contribution in astro-particle physics
Super-Kamiokande inner detector
Super-Kamiokande is a neutrino observatory located 1,000 m underground in Mount Ikeno at the Kamioka Observatory of the Institute for Cosmic Ray Research, University of Tokyo.
Designed to study neutrino oscillations and seek for the decay of the nucleon, it began in 1996. In more than 20 years it has produced important results in the field of atmospheric and solar neutrino oscillations, and has set stringent limits on the decay of the nucleon, the existence of dark matter and astrophysical sources of neutrinos.
Super-Kamiokande, for the first time definitively demonstrated that neutrinos have mass and undergo flavor oscillations. For such important result, in 2015 Professor Takaaki Kajita and the Super-Kamiokande collaboration have been awarded of the Nobel Prize in Physics along with Arthur B. McDonald and the SNO collaboration, another neutrino oscillation experiment located in Canada.
The detector has a cylindrical shape, 41.4 m tall and 39.3 m in diameter, holding 50 ktons of ultra-pure water and is divided into an inner detector and outer detector which is used to veto all the possible external background. The inner detector is faced by 11,129 Hamamatsu R3600 50 cm diameter photomultiplier tubes and 1,885 Hamamatsu R5912 20 cm diameter that face the outer detector.
Neutrinos interacting with the electrons or nuclei of water produce a charged particle that creates a cone of Cherenkov light. The cone of light is projected as a ring on the wall of the detector and recorded by the photomultiplier tubes.
An upgrade of this detector, Hyper-Kamiokande, is under discussion among an international collaboration and will be about ten times bigger than Super-Kamiokande and splitted in two tank instead of one.
The IceCube Neutrino Observatory is a neutrino telescope located at the Amundsen–Scott South Pole Station in Antarctica. It consists of 5,610 spherical optical sensors called Digital Optical Modules, each with a 10-inch diameter photomultiplier tube Hamamatsu R7081-02.
DOMs are arranged along 86 vertical cables, each of which has sixty modules that are located in a volume of one cubic kilometer of highly transparent ice, situated at depths from 1450 to 2450 meters in the ice.
The DOMs will detect the optical light emitted by fast-moving electrically-charged particles, each of which is the result of a collision with a high-energy neutrino that penetrated the earth. The Antarctic neutrino observatory, also includes the surface array IceTop and DeepCore. IceTop is used to study the cosmic ray composition and to identify the muon coming from a cosmic-ray which could be misidentified as a neutrino event in the ice. DeepCore extends the observable energies below 100 GeV.
The strings of DeepCore are deployed at the center of the larger array, between 1760 and 2450 meters into the ice.
In November 2013 the IceCube collaboration announced the detection of 28 neutrinos with an energy ranging from 30 to 1200 TeV that likely originated outside of the Solar System and substantially more than the expected from backgrounds.
KM3NeT – so named because it will encompass an area of several cubic kilometers – is a future experiment that will be located in the deepest seas of the Mediterranean in two different places: ORCA in front of Toulon (France) and ARCA, in front Portopalo di Capo Passero (Italy).
The main objective will be the detection of neutrinos from distant and catastrophic astrophysical events like supernova remnants, gamma-ray bursts or colliding stars. It will also search for dark matter in the universe.
Like in IceCube it will consist of Digital Optical Module (DOM) arranged along vertical cables along the KM3NeT array and will be able to identify the Cherenkov light given off by muons produced by neutrinos interactions in the sea. Each DOM is a standalone module with 31 3-inch Hamamatsu photomultiplier tubes R12199-02 in a 17-inch glass sphere .
The experiment will also house instrumentation for Earth and Sea sciences, and on-line monitoring of the deep sea environment at a depth of several kilometers.
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