Solving the secrets of the universe

Where do you go when you want to determine the mass of some beyond tiny, beyond fast-moving atomic particles that the sun is emitting daily? Or when you want to produce the first generally accepted evidence that the dark matter that theory says should make up 23 per cent of the universe does, in fact, exist?

What if you need a signal that a distant star has imploded upon itself and become a supernova?

Your first guess might not be more than two kilometres down an often noisy, always dusty working mine in Sudbury. But that is exactly where physicists from around the world are placing their research bets as a Canadian team constructs a complex, superclean facility in the Earth's bowels. Their hope is that SNOLAB will be able to repeat the extraordinary coup of the facility it is replacing - the Sudbury Neutrino Observatory (SNO). In 2002, SNO data provided an incontrovertible resolution to what had been called "the missing neutrino paradox."

The paradox was that measurements made for more than 30 years kept finding that the fusion process that powered the sun was somehow producing only about a third the number of neutrinos the theory said it should. Because neutrinos are known to be created as the result of the nuclear decay that takes place in stars, this inconsistency suggested scientists didn't really understand what was making the sun shine.

Using a 1,000-tonne sphere loaded up with $300 million worth of heavy water, SNO was able to resolve the paradox by demonstrating that missing neutrinos were there but had changed from one form to another as they flew to the Earth. How? Being underground shielded the SNO detector from the untold numbers of cosmic rays that would swamp all neutrino readings with background data noise. As well, the form of hydrogen that makes heavy water heavy was able to interact with types - the physicists call them flavours - of neutrinos that previous detectors had missed.

Prestigious Science magazine hailed the SNO finding as one of the breakthroughs of the year in 2002. And the scientific and media acclaim informed various levels of government that a Canadian science experiment had won the research equivalent of an Olympic gold medal. "That is why we got funded to build the expanded SNOLAB laboratory," says David Sinclair, the director of SNOLAB development.

The new facility's underground unit is four times larger than SNO and will house a clean room facility where the air is 5,000 times more pure than ordinary room air. It will also contain cooling systems to dampen down the mine's 41° C temperatures and devices that will prevent the rocks' natural radioactivity from producing confusing readings.

Some of SNOLAB's experiments will attempt to resolve a paradox that is even greater than the missing neutrinos. Theory suggests that some large part of matter in the universe may be composed of unseen so-called cold dark matter. Cold dark matter itself is thought to be composed of neutralinos - neutral particles that were formed during the big bang; however, neutralinos have never been seen, perhaps because they, like the missing neutrinos, have never interacted with any previous detector technology.

To address this issue, SNOLAB's cold dark matter detector will consist of droplets of a superheated and unstable fluorine-rich liquid. The slightest perturbation in the droplets - think a collision with a neutralino - should trigger an explosive vaporization. "Using sound detectors, scientists hope to record the presence of neutralinos literally from the pops their collisions produce," says Sinclair.

Other experiments will look for the signature of the elusive cold dark matter in other materials.

Another section of the complex is devoted to the upgraded SNO facility, which will continue the study of neutrinos. This research will attempt to get a handle on the exact mass of neutrinos and pick up the huge bursts of them that stars emit when they turn into supernovas.

And, yes, there have been practical applications to SNO's search for basic particles too. SNO's home was the largest cavity ever excavated at a depth of more than two kilometres, says Queen's University physicist Art McDonald, SNO's scientific director. "The geotechnical information Inco gathered from this increased the company's confidence in pushing even deeper in Creighton mine, and they have since excavated large cavities at even greater depths."