Getting to the heart of the matter

Testing the tank that will hold 1,000 tonnes of water

One of the world's strangest observatories has been inaugurated 2,000 metres underground, where scientists will look for particles thrown out from the heart of the Sun. Our science correspondent David Whitehouse reports.

The Sudbury Neutrino Observatory (SNO), situated below ground at a Canadian nickel mine, is designed to detect the most elusive and ghostly particles in the universe, neutrinos.

Neutrinos are given off by certain nuclear reactions. Their existence was first suspected in 1930 but they were not observed until 1956 when American scientists detected them from a nuclear reactor.

The particles barely interact with ordinary matter. They could pass unhindered through light-years of lead, scientists have calculated, and billions pass through us harmlessly all the time.

The Sudbury Neutrino Observatory lies 2,000m below ground

Despite its elusiveness the neutrino is one of the most important particles in the universe. Astronomers do not know what most of the universe is composed of - neutrinos could just be the answer.

Neutrinos are also given off by reactions taking place at the heart of the Sun. The particles then escape into space as if the rest of the star did not exist. Nine minutes later some of them arrive at Earth, which they pass through almost unhindered.

The observatory consists of 1,000 tonnes of an isotope of water, which is underground to shield it from outside interference.

Excavating the cavity

Of the trillions of neutrinos that pass through the water each day just 20 are estimated to react with the water, to produce a tiny flash of light.

To pick this up the inside of the water tank is covered with light detectors connected to a computer.

Previous observations of solar neutrinos have revealed a problem, though. Fewer of them are being given off by the Sun than theory predicts. So, either the theory is wrong or we do not understand the Sun as well as we thought we did.

Until now other neutrino observatories have been able to detect only one type of neutrino. But scientists know there are three types.

The SNO will be the first to look for all three types from the Sun and so will tell us if the solar neutrino problem remains.