The Surreal World of Neutrino Detectors

Jul 27, 2015 0 comments

Neutrinos are one of the fundamental particles which make up the universe, but not in the way electrons, protons and neutrons are. These particles are extremely tiny, nearly massless and electrically neutral so they are not affected by electromagnetic forces and react very weakly with other particles of nature. Neutrinos are produced by the decay of radioactive elements in nuclear reactions such as in the core of the sun or exploding stars. Once born, they travel in straight lines at the speed of light passing through solid matter almost entirely unhindered. Although tiny, they carry a colossal amount of energy — some of these carry the same amount of energy as a well hit tennis ball. To detect these particles using the same technology they use at the Large Hadron Collider in Switzerland, one would require a ring of magnets the size of Earth's orbit around the Sun.

Neutrino detectors therefore use entirely different kind of science and technology. Some detectors use large tanks filled with water and surrounded by photomultiplier tubes that watch for radiation emitted when an incoming neutrino creates an electron or muon in the water. Other detectors have tanks filled with chlorine or gallium or other liquids. Neutrino detectors are often built underground, to isolate the detector from cosmic rays and other background radiation.



Engineers examining instruments inside the half-filled Super-Kamiokande tank in a row boat. Photo credit

The Super-Kamiokande neutrino detector is located 1,000 meters under Mount Kamioka near the city of Hida, in Japan. The detector consist of a cylindrical stainless steel tank 41 meters by 39 meters holding 50,000 tons of ultra-pure water and surrounded by more than 11,000 photomultiplier tubes (PMT). It is one of the largest detector of its kind.

When a passing neutrino interacts with the electrons or nuclei of water, it can produce a charged particle that moves faster than the speed of light in water. 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. Using this data scientists can determine the direction of the source and the flavor of the incoming neutrino.


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Liquid Scintillator Neutrino Detector

The Liquid Scintillator Neutrino Detector (LSND) at Los Alamos National Laboratory operated between 1993 and 1998. The detector consisted of a tank filled with 167 tons (50,000 gallons) of mineral oil mixed with an organic scintillator material, and was equipped with 1220 photomultiplier tubes. The results of the LSND, however, were controversial and were refuted by later tests by other laboratories.


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MiniBooNE detector at Fermilab was designed to unambiguously verify or refute the LSND controversial result in a controlled environment. The detector is a 40-foot (12-meter) diameter sphere filled with 800 tons of mineral oil and lined with 1520 photomultiplier tubes. The detector sees one neutrino collision every 20 seconds, amounting to approximately 1 million neutrino events per year.


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Borexino is a particle physics experiment located nearly a mile below the surface of the Gran Sasso mountain about 60 miles outside of Rome. The detector is a 59-foot steel sphere filled with a liquid scintillator. The primary aim of the experiment is to make a precise measurement of the beryllium-7 neutrino flux from the sun and comparing it to the Standard solar model prediction. This will allow scientists to further understand the nuclear fusion processes taking place at the core of the Sun and will also help determine properties of neutrino oscillations.


The Borexino detector showing inner sphere of scintillator, buffer sphere, and detectors. Photo credit


The exterior of the neutrino detector's 59-foot steel sphere. Photo credit


The interior of the Borexino detector. Photo credit

Sudbury Neutrino Observatory

The Sudbury Neutrino Observatory is located 2,100 meters underground in Vale Inco's Creighton Mine in Sudbury, Ontario, Canada. The SNO detector target consisted of 1,000 tons of heavy water contained in a 6-meter-radius acrylic vessel. The detector cavity outside the vessel was filled with normal water to provide both buoyancy for the vessel and radiation shielding. The heavy water was viewed by approximately 9,600 photomultiplier tubes. The cavity housing the detector is reportedly the largest man-made underground cavity in the world.


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IceCube Neutrino Observatory

The IceCube Neutrino Observatory is located at the Amundsen-Scott South Pole Station in Antarctica. It consist of thousands of sensors distributed over a cubic kilometer of volume under the Antarctic ice. Each sensor, a spherical glass globe called Digital Optical Modules (DOMs), consist of a photomultiplier tube and a computer, and is suspended by “strings” into holes drilled into the ice at depths ranging from 1,450 to 2,450 meters. The IceCube Neutrino Observatory is the largest neutrino telescope in the world.


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A Digital Optical Module (DOM). Photo credit


A Digital Optical Module (DOM) is lowered into a 2,500 meter-deep hole in the ice. Photo credit


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KamLAND Detector

The Kamioka Liquid Scintillator Antineutrino Detector (KamLAND) is an experimental device that was built at the Kamioka Observatory, an underground neutrino detection facility near Toyama, Japan. Its purpose is to detect electron antineutrinos emitted by the 53 Japanese commercial nuclear reactors that surround the detector.

The KamLAND detector's outer layer consists of an 18 meter-diameter stainless steel containment vessel with an inner lining of 1,879 photo-multiplier tubes, each 50 centimeters in diameter. Its second, inner layer consists of a 13 m-diameter nylon balloon filled with a liquid scintillator composed of 1,000 metric tons of mineral oil, benzene, and fluorescent chemicals.


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Daya Bay Reactor Neutrino Experiment

The Daya Bay Reactor Neutrino Experiment is located at Daya Bay, approximately 52 kilometers northeast of Hong Kong, in China. The experiment consists of eight antineutrino detectors, clustered in three locations within 1.9 km of six nuclear reactors. Each detector consists of 20 tons of liquid scintillator surrounded by photomultiplier tubes and shielding.

The innards of each cylindrical antineutrino detector consist of one transparent acrylic vessel nested inside another one, both of which sit inside a third vessel made of stainless steel. This is filled with clear liquid scintillator.


Photo credit: Roy Kaltschmidt, LBNL


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