Measuring solar neutrinos

Every second each centimetre of the surface of the Earth is bombarded by 6×10^10 solar neutrinos –produced by nuclear fusion and the decay of isotopes such as beryllium-7. Writing in Physical Review Letters, an international collaboration of scientists has announced the unprecedented accuracy of new solar neutrino data from the BOREXINO experiment at the Laboratori Nazionale del Gran Sasso, in central Italy.

The BOREXINO neutrino detector consists of a special organic liquid called a scintillator. When neutrinos collide with electrons in the liquid at high velocity the electrons start moving faster. These fast moving electrons cause the scintillator to emit light, which is detected by the surrounding array of 2200 photomultiplier tubes. Since neutrinos are so weakly interacting – being only affected by gravity, which is insignificant at subatomic scales, and the weak nuclear force – they are very difficult to detect and so a large mass of scintillator, approximately 278 metric tons, is required.

A major problem with detecting neutrinos is background radiation. The BOREXINO team solved this by building the detector 1.4km beneath the Apennine Mountains. A thick layer of ultra-pure water surrounding the scintillator blocks any radiation that makes it through the mountains.

This shielding has allowed the number of solar neutrinos from beryllium-7 decay reaching the detector to be measured with an unprecedented 5% uncertainty and has made it possible for the first time to detect neutrinos with energies of under 1MeV.

The data collected has already provided evidence for the theory that neutrinos can oscillate between their three ‘flavours’ — electron, muon and tau. This suggests that neutrinos have mass, which goes against the Standard Model — the leading theory explaining the subatomic world. The detector will also be used to explore the composition of the sun. Laura Cadonati, part of the collaboration and based at the University of Massachusetts Amherst, said: “BOREXINO is the only detector capable of observing the entire spectrum of solar neutrinos at once."

"Our results, the culmination of 20 years of research, greatly narrow the observation precision. The data confirm the neutrino oscillations, flavour changes and flow predicted by models of the sun and particle physics.”

There also exists hope that the experiment may yield some more unusual results, such as the discovery of a new flavour of neutrino. Another University of Massachusetts member of team, Andrea Pocar, has commented on this, saying, “You always have the hope of seeing surprises, some small deviation from the expectations. And this you can only have if your accuracy and precision are good enough to see very small variations.”