Every second, billions of neutrinos fly through your body as if it weren't there. Even planet Earth poses little barrier to these tiny particles. Neutrinos interact so rarely with normal matter that huge detectors are needed to trap just a few.
Only two cosmic sources of neutrinos have been pinned down so far: the interior of the Sun, where nuclear fusion reactions produce prodigious quantities of neutrinos; and Supernova 1987A, in which the neutrinos formed during the collapse of a massive star's core (approximately 99 percent of a supernova's energy is released as neutrinos).
Neutrino detectors work by looking for the carnage left behind when a neutrino does interact with normal matter. In some cases, the neutrino transmutes an atomic nucleus into a different element and the detector watches for that change. More often, the neutrino interacts with matter and produces a high-energy subatomic particle. That particle will produce an eerie bluish light called Cerenkov radiation when it moves faster than the speed of a light in a transparent medium such as water. The speed of light in a vacuum — 186,000 miles (300,000 km) per second — is the highest speed attainable, but water slows it down to a relatively lethargic 140,000 miles (225,000 km) per second.
Since the 1960s, physicists have developed experiments that can detect neutrinos from astrophysical sources. Astronomy.com has complied a list of the larger neutrino "telescopes" that have come on-line or will in the near future.
HomestakeOperations: 1968-2001
Location: Lead, South Dakota
The granddaddy of all neutrino experiments, Homestake used a tank containing 100,000 gallons of perchloroethylene (essentially dry-cleaning fluid) located 4,850 feet underground in the Homestake Gold Mine. Neutrinos from the Sun interacted with a few chlorine atoms, converting them to argon. The experiment found fewer solar neutrinos than expected. Physicists later deduced that this discrepancy arose from neutrinos changing "flavor" and having a small mass. Raymond Davis, Jr., of Brookhaven National Laboratory, won the 2002 Nobel Prize in Physics for his leadership in this experiment.
SAGE (Soviet American Gallium Experiment)Operations: 1990-present
Location: Baksan Laboratory, Russia
Now known officially as the Russian American Gallium Experiment, SAGE uses a tank containing 55 tons of liquid gallium metal located 5,250 feet below the Caucasus Mountains to search for solar neutrinos. The detector finds the neutrinos by watching for interactions that convert gallium atoms into germanium atoms.
GALLEX (GALLium EXperiment)Operations: 1990-present
Location: Gran Sasso Laboratory, Italy
GALLEX uses the same technique as SAGE to search for solar neutrinos (watching for neutrinos to interact with gallium and convert it to germanium). It GALLEX's case, the target consists of a tank containing 30.3 tons of gallium in a liquid gallium chloride solution located 3,700 feet underground in Italy's Abruzzo region.
Kamiokande (KAMIOKA Nucleon Decay Experiment)Operated: 1983-1996
Location: Kamioka, Japan
Kamiokande's detector consisted of 4,500 tons of pure water in a tank located 3,300 feet beneath the Japanese Alps. Photomultiplier tubes then searched for Cerenkov radiation from neutrino interactions in the tank. It was originally developed to search for the decay of protons, as predicted by early Grand Unified Theories, but was later upgraded to allow it to search for neutrinos. It was one of two experiments that detected neutrinos from Supernova 1987A in the Large Magellanic Cloud. Team leader Masatoshi Koshiba shared the 2002 Nobel Prize in Physics with Raymond Davis, Jr.
