montreal

Sun spotter: heavy-water detector could provide clues to the mass density of the Universe. Credit: SNO

Scientists and government representatives from many countries gathered last week near the northern Ontario mining town of Sudbury for the opening of the Sudbury Neutrino Observatory.

The observatory, in a nickel mine 2 km deep, consists of 9,500 light sensors surrounding a 12-metre diameter acrylic flask that will hold 1,000 tonnes of heavy water. The heavy water comes from Atomic Energy of Canada, which had a surplus of the expensive material as a result of a decline in sales of the CANDU reactor in the 1980s.

Interactions between neutrinos and the deuterium in the heavy water will produce light flashes. The observatory's detector will make an independent measurement of all three types of neutrinos (electron, tau and muon), so should be able to determine whether neutrinos produced inside the Sun change type as they travel outwards. If they do, they must have mass, and may make a substantial contribution to the Universe's total mass density.