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Thursday, 22 June, 2000, 10:48 GMT 11:48 UK
Deep focus on neutrino mystery
This type of detector has to be built underground
By BBC News Online science editor Dr David Whitehouse

An underground detector designed to "look" into the heart of the Sun is working well say scientists and may soon solve one of the greatest solar mysteries.

The facility, which has only recently begun work, is trying to pick up neutrinos as they stream through the Earth.

Neutrinos are the ghostly particles that are released when the Sun fuses hydrogen to make helium - the basic nuclear process that provides the energy to drive life on Earth.

Previous observations have only detected about a third of the quantity of the particles predicted by current theory. Finding the missing neutrinos is the task of the Sudbury Neutrino Observatory (SNO).

Undergound protection

The SNO is situated two kilometres (1.2 miles) underground at the Creighton Mine in Ontario, Canada.

The location is crucial. It is very difficult to detect neutrinos because they hardly interact with matter and putting the SNO inside the Earth means other particles that might otherwise lead to spurious results are kept out of the instrument.

Even so, the Sunbury detector will capture only a tiny fraction of the neutrinos passing though it.

It is a headache for scientists because neutrinos potentially provide a very direct test of what we believe goes on inside stars and how they evolve over millions of years.

Previous observations have implied a rate of neutrino production in our Sun that is only about one third of that expected from solar theories.

Detect SNO
Only a few neutrinos will be detected by the SNO
It is possible our understanding of the energy-generation processes in the Sun is wrong, or it could be that neutrinos change on their way to the Earth into a type of particle that has hitherto been undetectable.

Physicists think that neutrinos can switch between three types of particle and the SNO is the first detector that should detect all three kinds.

Data from the first six months of full SNO operation show that it has captured some neutrinos from the Sun.

But it will need to pick up all three types to explain the long-standing mystery of the "missing" neutrinos, and also confirm that neutrinos, although extremely lightweight, must have some small mass.

Missing mass

Without mass, the neutrinos could not change from one type to the other and back again.

Physicists say because neutrinos are the second most common particles in the Universe (after photons, the "particles" of light), the question of their mass also has cosmic importance.

Astronomers do not know what 90% of the Universe is composed of and neutrinos may provide part of the answer.

"We are proceeding with careful studies of systematic experimental effects and will present quantitative results for neutrino fluxes in the future," said Professor Art McDonald, the SNO's director.

Neutrino bursts could also provide astronomers with an early (up to 2 hours advance) warning of a supernova explosion in our galaxy. A supernova marks the catastrophic collapse of a big star that has come to the end of its life.

Images courtesy of Ernest Orlando Lawrence Berkeley National Laboratory

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