Neutrinos are some of the most abundant yet mysterious particles in our universe. Every second 65 billion neutrinos pass through every square centimeter of our body and the Earth. Neutrinos do not carry electric charge, which means that they are not affected by the electromagnetic forces that act on charged particles such as electrons and protons. They are also extremely tiny because of which they travel mostly undisturbed through matter. This makes neutrinos extremely hard to detect, and the harder a particle is to detect, the more massive and sophisticated the detectors have to be. The Super Kamiokande in Japan is one such neutrino observatory.
The Super Kamiokande or Super-K for short is located 1,000 meters underground in the Mozumi Mine in Hida's Kamioka area. The observatory was designed to search for proton decay, study solar and atmospheric neutrinos, and keep watch for supernovae in the Milky Way Galaxy. The observatory was built underground in order to isolate the detector from cosmic rays and other background radiation.
The observatory consists of 50,000 tons of pure water in cylindrical stainless steel tank that is 41.4 meters tall and 39.3 meters in diameter. This is surrounded by 11,146 photomultiplier tubes (PMT). When a neutrino interacts with the electrons or nuclei of water, it produces a charged particle that moves faster than the speed of light in water (not to be confused with exceeding the speed of light in a vacuum, which is physically impossible). This creates a cone of light known as Cherenkov radiation, which is the optical equivalent to a sonic boom. The Cherenkov light is projected as a ring on the wall of the detector and recorded by the PMTs. The distinct pattern of this flash provides information on the direction and flavor of the incoming neutrino.
Engineers examining instruments inside the half-filled Super-Kamiokande tank in a row boat.
The Super Kamiokande started operation in 1996. It held fifteen times the water and ten times as many PMTs as its predecessor, the Kamiokande Observatory. Just two years later, the observatory scored it’s first success - the first evidence of neutrino oscillation. This was the first experimental observation supporting the theory that the neutrino has non-zero mass, a possibility that theorists had speculated about for years.
On February 23, 1987, the Super-Kamiokande detected, for the first time, neutrinos from a supernova explosion that occurred in the Large Magellanic Cloud. This observation confirmed that the theory of supernova explosion was correct and was the dawn of a new era in neutrino astronomy.
An example neutrino detection event.
Entrance to the Super-Kamiokande observatory. Photo credit.
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