observatories are located in deep mines or other subsurface locations where hundreds of feet of material shield out the cosmic rays that would completely swamp the tiny effects due to neutrinos. The largest such observatory is in the ice at the South Pole, .87 mi (1.4 km) below the surface; the detector array of the IceCube South Pole Neutrino Dectector forms a cube with edges that are .6 mi (1 km) in length. Neutrinos pass as easily through the overlying material as they pass through the star, but react with the material in the detector. In one form of detector, they react with chlorine to produce a radioactive isotope of argon, which is detectable. Other detectors, such as the IceCube, use a large volume of clear water or ice; when a neutrino interacts with the water or ice to produce a lepton that is moving faster than the speed of light in the water or ice detectable Cherenkov radiation is emitted.
Because of its proximity, the sun was expected to be by far the most intense source of neutrinos and was the initial object of study. However, several neutrino detectors observed a rush of neutrinos from Supernova 1987A in a nearby galaxy called the Large Magellanic Cloud. Although their journey from the exploding star began at the moment its core collapsed, they did not move quickly at first since the gravity of the core was so strong. When the shock wave from the explosion reached the neutrinos, it freed them to travel between galaxies, and they arrived on earth about three hours before the first visible light of the explosion appeared.
The Columbia Electronic Encyclopedia, 6th ed. Copyright 2012, Columbia University Press. All rights reserved.
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