Neutrino Project

Most people would no doubt balk at having to stand on the roof of an elevator as it drops slowly into a dark mine shaft sunk more than a mile into the ground. Not physicist Duncan Hepburn, 53, who shrugs off the task as just another part of his job. Some job.

Neutrino Project

Most people would no doubt balk at having to stand on the roof of an elevator as it drops slowly into a dark mine shaft sunk more than a mile into the ground. Not physicist Duncan Hepburn, 53, who shrugs off the task as just another part of his job. Some job. Hepburn is site manager at the Sudbury Neutrino Observatory, a unique, $74-million research facility that could profoundly change scientists' understanding of the universe. After a number of delays, the observatory is finally nearing completion, housed in a 10-storey cavern blasted out of rock 2,000 m underground in Inco Ltd.'s Creighton Mine outside the Northern Ontario city of Sudbury. In the run-up to the site's inauguration this week, Hepburn is looking for a break in the fibre-optic communications cables linking the facility to the surface. And the only way he can follow the cable, millimetre by millimetre, is from the top of the elevator. "You get used to it quick," Hepburn says of the unusual demands of his job. "When you're working on SNO, you learn to adapt."

The project has been like that: scientists shifting gears constantly to solve myriad problems associated with building something that has never been built before. Begun in 1990, SNO is the first so-called heavy-water neutrino detector in the world, the only facility capable of detecting all three types of neutrinos, one of the tiniest particles known to science. What neutrinos lack in size, they more than make up for in number. They are produced in almost unimaginable quantities by nuclear fusion and decay occurring in the core of stars such as Earth's sun - which, incidentally, blasts out 200 trillion trillion trillion neutrinos every second.

The Canadian government, through two federally funded agencies - the Natural Sciences and Engineering Research Council of Canada and the National Research Council of Canada - as well as the Northern Ontario Heritage Fund Corporation, are picking up most of the tab. But two other countries have helped, with the U.S. department of energy covering about 25 per cent of construction and Britain's Particle Physics and Astronomy Research Council contributing about 10 per cent. The sponsors hope that unprecedented research to be carried out at SNO starting in July will help explain why the mass of the universe as calculated so far is only one-tenth of what scientists think it ought to be. As well, the research should provide more insight into when - in the far distant future - the universe will collapse.

It might also help with the search for a so-called grand unification theory, which would link the four forces of nature - gravity, electromagnetism, the weak force (which acts on quarks, electrons and neutrinos) and the strong force (which holds atomic nuclei together). Some of the theories already postulated assume neutrinos have mass, a fact that many expect SNO to confirm. "There is this smell of new physics here, of making a significant contribution," says Richard Van Berg, director of engineering at the University of Pennsylvania's department of physics and astronomy, and one of about 70 PhDs working at SNO.

Scientists widely agree that research at the observatory has Nobel Prize potential, with its promise of revolutionizing the understanding of subatomic particles. Even the renowned cosmologist Stephen Hawking, best-selling author of A Brief History of Time, is paying attention. Confined to a wheelchair by a degenerative neurological disease and forced to used a computerized voice synthesizer, Hawking is to make a special visit to the facility on the day before the inauguration. Queen's University physicist Art McDonald, SNO's director, met with Hawking in California in March. "When I discussed the SNO project and our scientific objectives with him," McDonald relates, "his comment - and as you know, he speaks in very short sentences - was, 'Very important science.' "

As grand a scheme as the observatory is, its researchers face the unwelcome prospect that they could be beaten to some of the information they seek. The Super Kamiokande neutrino detector is already operating half a mile underground in Kamioka, Japan, 250 km northwest of Tokyo. While it cannot detect all types of neutrinos, it is capable of determining whether they have mass. But the Japanese detector has been hampered by unexpectedly high levels of background radiation over the past year, says McDonald, expressing confidence that the Sudbury facility will get there first. "The question," he adds, "is whether the Japanese will be able to make an accurate measurement, one which is convincing."

The existence of the elusive neutrino was first postulated in 1931, but it was not actually detected until 1956. Italian physicist Enrico Fermi gave it its name, which means "little neutral one." Because neutrinos pass through just about everything - including Earth - unimpeded, they are exceedingly difficult to observe. About 100 billion zip through the average person's index finger, trillions more through the entire body, every second. They are simply the most numerous entity in the universe. Yet several studies suggest there are actually too few of them out there.

Experiments designed to corroborate theoretical models of how the sun works detect only about half the number of neutrinos expected. A likely explanation lies in the fact that there are three types - or, as physicists call them, "flavors." Besides the so-called electron-neutrinos and muon-neutrinos (detected with pre-SNO equipment), there are also tau-neutrinos, which have not been directly observed, but have been inferred from other observations. According to a widely held theory, some electron-neutrinos transform themselves into the muon and tau forms after being discharged from the sun - which would explain why scientists have not found as many electron-neutrinos as they had expected. SNO, with its ability to detect all three types, will test the theory.

Confirmation that neutrinos change from one flavor to another would hold profound implications. Physicists say it would prove that neutrinos have mass, which in turn would shed new light on another enduring mystery of astrophysics: dark matter. That is the name scientists have given to the enormous amount of material they say has to be out there - given the pattern of movement of galaxies - even though no one has ever seen it. At the moment, when totalling the mass of all known heavenly bodies, astrophysicists find the universe's mass is only 10 per cent of what it should be. If neutrinos have even a miniscule mass, their astronomical numbers, when taken together, could go a long way towards accounting for the discrepancy. What is more, their collective mass could be exerting such a huge gravitational pull that, at some far distant time in the future, it could halt the expansion of the universe, leading to its eventual collapse and demise. "If you want to understand the universe at its smallest and largest level, then you have to know whether neutrinos have mass," says Dave Wark, a particle physicist from Oxford University working at SNO.

But why seek those answers in Sudbury, a city better known for producing nickel than for astrophysics? The answer is the deep mines found in the area. The observatory had to be built more than a mile underground to shield it from cosmic radiation, which would disrupt its detectors - so sensitive they can spot the faint glow of a firefly six kilometres away. Another important consideration in the observatory's location: Canada is the only country in the world with easy access to the necessary quantities of so-called heavy water, a critical substance in neutrino detecting. "There's only one place in the world that this could have been done," says Wark. "And the Canadians rose to the challenge."

Once the various federal and provincial agencies decided to go ahead with the project, the hard work began. It was to have taken two years to blast out and excavate the keg-shaped cavern that houses the observatory in a sector of the 96-year-old Creighton Mine. It took three. Then in August, 1995, the success of the project was threatened with the first discovery of half a dozen imperfections in the 12-m-wide acrylic sphere at the heart of SNO. Filling most of the cavern, the sphere is made of 125 curved panels of ultra-pure, transparent acrylic, six centimetres thick and glued together by hand. During several tense months that it took technicians to eliminate unwanted bubbles from the seals, SNO's future hung in the balance. "That had the potential for stopping the project," says Queen's physicist Barry Robertson.

Although dust hangs thick in the air elsewhere in the mine, technicians must keep the facility spotless to avoid interference with the sensors. On entering, workers and visitors must shower, dress in garments washed in filtered water, don a hair net and pass through an air shower. Adhesive floor mats keep even the soles of construction boots clean.

On April 15, workers began flooding the cavern space outside the vessel with ultra-pure water to further shield against ambient radiation. Next, the acrylic vessel will be filled with 1,000 tonnes of heavy water - normally used to aid nuclear fission in CANDU reactors - on loan from Atomic Energy of Canada Ltd. and valued at $300 million. Both the cavern and vessel should be full by early July. Heavy water, known as deuterium oxide, or D2O, differs from ordinary water (H2O) in having a neutron as well as a proton in each of its two hydrogen nuclei. When an electron-neutrino strikes a deuterium neutron, the impact frees an electron from the perimeter of the deuterium atom. Travelling through the heavy water faster than light can, that electron will create a cone-shaped flash of blue light, known as Cherenkov radiation, which some of the 9,500 light sensors surrounding the acrylic vessel will detect.

Initially, scientists expect to see that phenomenon about 20 times a day. In about a year, they plan to use a remote-controlled submarine to install helium-filled sensors that should capture heavy-water neutrons jarred free by impacts with muon- and tau-neutrinos.

All this technology comes at a price. Based on a 1984 proposal by University of California physicist Herb Chen, the project was initially expected to cost $46 million and be built by 1995. Almost three years behind schedule and close to 60 per cent over budget, SNO is finally almost ready. The cost overruns, says facility director McDonald, are due to several factors - the original proposal's failure to account for inflation; the weakening Canadian dollar; and the departure of some Canadian research institutions that were to have participated in the project. "We're proceeding as economically as we can," McDonald adds. After all the frustrations of getting so far, McDonald is ready to get down to the business of research. Or as he puts it: "Now I can spend some time in front of a computer to see what it all means." The world of astrophysics will be watching.

How to See a Neutrino

Every second, each of the approximately 100 billion stars in our galaxy, including Earth's sun, discharges trillions upon trillions of neutrinos - one of the tiniest particles known to science. As far as physicists now understand, neutrinos are elementary entities, meaning they are indivisible. They also pass through just about everything unhindered, which means they have been almost impossible to observe. That is about to change. Starting in July, the new, $74-million Sudbury Neutrino Observatory will allow physicists, if not to see the mysterious subatomic particles directly, at least to observe what they can do.

The facility is designed to reveal the flashes of light emitted when neutrinos smash into molecules of heavy water and break off an electron. If SNO scientists can then confirm their assumption that neutrinos have mass, that would go a long way towards explaining an enduring astrophysical mystery: why the total mass of all known celestial bodies is only a fraction of what scientists calculate the mass of the universe ought to be.

The SNO home page on the Internet: https://www.sno.phy.queensu.ca/

Natural Sciences and Engineering Research Council of Canada home page: http://www.nserc.ca/

Stephen Hawking's page: http://www.damtp.cam.ac.uk/user/hawking/home.html

The Super Kamiokande Observatory: https://www.phys.washington.edu/~superk/

Physics Around the World: http://www.physics.mcgill.ca/deptdocs/physics_services.html

Maclean's May 4, 1998