Experiment confirms Sun theories

The SNO was constructed to solve a mystery

By Dr David Whitehouse BBC News Online science editor

Neutrinos - some of nature's most elusive sub-atomic particles - do change their properties as they travel through space.

We are much more certain now that we have really shown that solar neutrinos change type

Prof Dave Wark, University of Sussex

New evidence confirms last year's indication that one type of neutrino emerging from the Sun's core does switch to another type en route to the Earth.

This explains the so-called solar neutrino mystery, which has had scientists puzzled for 30 years - why so few of the particles expected to emerge from the nuclear furnace in our star can actually be detected.

The new data mean the reactions put forward by physicists to describe how the Sun works are correct.

The data were obtained from the underground Sudbury Neutrino Observatory (SNO) in Canada.

Going underground

Neutrinos are ghostly particles with no electric charge and very little mass. They are known to exist in three types related to three different charged particles - the electron and its lesser-known relatives, the muon and the tau.

Electron-neutrinos are created in the thermonuclear reactions at the solar core. Because these reactions are understood, it has been possible to estimate the number of electron-neutrinos that should emerge from our star.

But it has baffled scientists for decades as to why just a third of this expected number could actually be detected.

Using the underground Sudbury neutrino detector, an international group of researchers has been able to determine that the observed number of electron-neutrinos is only a fraction of the total number emitted from the Sun - clear evidence that the particles change type en route to Earth.

SNO Project Director, Dr Art McDonald, of Queen's University, Canada, said the number of electron-neutrinos detected combined with the numbers of other types picked up at Sudbury gave a total that was consistent with scientists' understanding of the nuclear reactions occurring at the Sun's core.

All types

The Sudbury Neutrino Observatory is a unique neutrino telescope, the size of a 10-storey building, two kilometres underground, down a mine in Ontario.

The SNO detector consists of 1,000 tonnes of ultrapure heavy water, enclosed in a 12-metre-diameter acrylic-plastic vessel, which in turn is surrounded by ultrapure ordinary water in a giant 22-metre-diameter by 34-metre-high cavity.

The observatory detects about one neutrino per hour

Outside the acrylic vessel is a 17-metre-diameter geodesic sphere containing 9,600 light sensors or photomultiplier tubes, which detect tiny flashes of light emitted as neutrinos are stopped or scattered in the heavy water.

At a detection rate of about one neutrino per hour, many days of operation are required to provide sufficient data for a complete analysis.

Because SNO uses "heavy" water - the hydrogen atom in the water molecule has an extra neutron - it is able to detect not only electron-neutrinos through one type of reaction, but also all three known neutrino types through a different reaction.

Very accurate

Dr Andre Hamer, of the Los Alamos National Laboratory, US, said: "In order to make these measurements, we had to restrict the radioactivity in the detector to minute levels and determine the background effects very accurately to show clearly that we are observing neutrinos from the Sun."

The research not only improves our understanding of the Sun but of the elusive neutrinos as well.

The latest results, entirely from the SNO detector, (and which have been submitted to Physical Review Letters) are said to be 99.999% accurate.

Dr MacDonald said: "The SNO team is really excited because these measurements enable neutrino properties such as mass to be specified with much greater certainty for fundamental theories of elementary particles."

Mass differences

This announcement is confirmation of indications released in June 2001 that suggested that it was highly likely that neutrinos changed type on their way from the Sun.

However those conclusions were always tentative because they were based on comparisons of results from SNO with those from a different experiment, the Super-Kamiokande detector in Japan.

Professor Dave Wark, of the University of Sussex and the Rutherford Appleton Laboratory, UK, commented: "Whenever a scientific conclusion relies on two experiments, and on the theory connecting them, it is twice as hard to be certain that you understand what is going on.

"We are therefore much more certain now that we have really shown that solar neutrinos change type."

Professor Hamish Robertson of the University of Washington, US, added: "There's absolutely no question the neutrino type changes and now we know quite precisely the mass differences between these particles."