Borexino, the Sun neutrino hunter
An international team of more than 100 researchers has used the huge Borexino detector to detect low-energy solar neutrinos for the first time. These results confirm recent 'theories about the nature of neutrinos and the inner workings of the sun and other stars.' In particular, it's now almost certain that neutrinos oscillate between three types, namely electron, muon and tau neutrinos. The Borexino detector used for these discoveries is located near L'Aquila, Italy. It is a 18-meter-diameter dome shielded by 2,400 tonnes of purified water and lying more than a kilometer underground to block interferences with cosmic rays.
Above is a diagram showing the various components of the Borexino detector. This figure comes from a scientific paper signed by 73 researchers belonging to 15 international organizations, "First real time detection of Be7 solar neutrinos by Borexino." It has been published by arXiv on August 16, 2007. Here are two links to the abstract and to the full paper (PDF format, 10 pages, 374 KB).
Here are some quotes from the National Science Foundation news release. "'In making these first direct measurements of low-energy neutrinos coming from the sun, Borexino represents a convergence of our present understanding of neutrino properties and the physics of solar energy generation,' said Brad Keister, program director for nuclear physics in NSF's mathematical and physical sciences directorate. 'The great depth of the laboratory and the incredible purity of the materials used in the detection were critical to the discovery and demonstrated the impact of eliminating background radiation from such experiments,' added Keister."
But why is it so important to detect the behavior of neutrinos coming from our Sun? In "Low-energy neutrinos spotted," Nature explains why. "Previous experiments at labs have detected neutrinos and demonstrated that they oscillate between three types (or 'flavours'): electron, muon and tau neutrinos. That cracked many of the bigger questions about neutrinos. But the earlier experiments were not able to measure solar neutrinos with energy levels of less than 5 mega electronvolts (MeV), as natural radioactivity in the surroundings can obscure results. The Borexino experiment, in the Gran Sasso National Laboratory near L'Aquila in Italy, was designed to get around this problem and detect particles with energies of less than 1 MeV."
So how was the Borexino experiment designed? In "Borexino Awash in Neutrinos, ScienceNOW provides some answers. "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."
And now that the Borexino is operational, what exactly are the first results obtained with it? Here is the answer from ScienceNOW. "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 [Borexino spokesperson] Gianpaolo Bellini of the University of Milan in Italy."
For more information, please visit the Borexino Solar Neutrino Experiment sites at the Gran Sasso Laboratory of the Istituto Nazionale di Fisica Nucleare (INFN, the Italian National Institute of Nuclear Physics) or at Princeton University.
Finally, if you want to see more pictures of the Borexino detector, please browse this INFN gallery. And please note that the most recent ones are located at the bottom of the gallery.
Sources: National Science Foundation news release, August 20, 2007; and various websites
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