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A model of the Borexino experiment.

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Borexino Awash in Neutrinos

A huge tank of liquid buried deep in the Italian Apennine mountains has made the first accurate measurement of low-energy neutrinos coming from the heart of the sun. The results generally confirm physicists' theories of how the sun's nuclear furnace generates its heat and support recent findings about the strange nature of neutrinos.

The nuclear fusion that powers the sun takes place in its deep core. It's hard for physicists to know what's going on down there because it takes photons many thousands of years to reach the surface. But neutrinos, which are a byproduct of many nuclear reactions, only very rarely interact with matter and so can zip out of the core and on to Earth in about 8 minutes. The problem is catching them.

Enter Borexino. Located at Italy's Gran Sasso National Laboratory--the world's largest underground facility for the study of subatomic particles from space--Borexino is a spherical vessel containing 300 tons of a liquid called pseudocumene with a small amount of a fluorescent material mixed in. When a passing neutrino hits an atomic nucleus in the liquid, the recoiling nucleus creates a brief flash of light, which is picked up by 2200 photodetectors surrounding the sphere. The detector is surrounded by shielding layers of liquid and is sited deep underground to protect it from other particles that might also create flashes.

Solar neutrinos were first detected in the 1960s, but the experiment caught only high-energy ones from a relatively unimportant nuclear reaction, which produces just 0.01% of all solar neutrinos. Lower energy ones were detected from the 1980s onward, but with detectors that could not differentiate between neutrinos from different reactions. Borexino, for the first time, is able to focus on neutrinos with an energy of 862 kilo-electron volts, which come from the more important nuclear reaction involving beryllium-7. These neutrinos are thought to account for 12% of the total.

"We have started to scan the more interesting part of the neutrino spectrum," says Borexino spokesperson Gianpaolo Bellini of the University of Milan in Italy. "Before Borexino, all this would have not been possible." According to Bruce Vogelaar of the Virginia Polytechnic Institute and State University in Blacksburg, "the fact that you're able to [detect the beryllium-7 neutrinos] is in itself an accomplishment."

Earlier detectors had found fewer solar neutrinos than expected, and it took many years to figure out why: They oscillate between one type of neutrino and another as they travel from their source, and the detectors were only designed to detect one type. Theorists expected the numbers of beryllium-7 neutrinos to be similarly reduced and predicted a detection rate of 49 per day. Borexino appears close to that prediction. "With 47 plus or minus 14 neutrinos per day, we are confident that these results confirm the so-called Standard Solar Model," says Bellini. "But we will have to wait and decrease the level of errors to be really sure."

The Borexino team now hopes to go on to detect other important nuclear reactions in the sun's core, including the proton-proton, PEP, and CNO reactions. Another target is geoneutrinos originating from Earth's core. "A promising season is waiting for us," says Eugenio Coccia, director of the Gran Sasso Laboratories.

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