It may be the most common particle in the universe. Constantly whizzing through every corner of the cosmos, the neutrino - first identified more than 30 years ago - would be expected by now to have given up most of its secrets. But it is still one of the most mysterious players on the subatomic stage.
All that may change with the help of the Sudbury Neutrino Observatory (SNO), a new underground laboratory set to begin collecting data in Canada this spring.
The observatory, unique in its ability to capture the elusive neutrino, is expected to shed light on some important questions: Do neutrinos have mass? Did they play a role in the evolution of the universe? What can they reveal about the core of the sun, where many originate?
"Neutrinos are remarkable objects," says physicist Lawrence Krauss of Case Western Reserve University in Cleveland. If SNO is successful, he says, it will herald a new age of neutrino astronomy, "an incredible new window on the universe."
Financed by Canada, the United States, and Britain, the $49 million observatory lies at the bottom of one of North America's deepest mines, about a mile below the surface just outside Sudbury in northern Ontario. In addition to opening up the universe, what's studied at SNO could have down-to-earth applications. "Neutrinos are a part of our universe that are outside of people's normal experience," says physicist Art McDonald, director of SNO. "On the other hand, the real purpose is to use neutrinos to understand our sun, and in terms of an object that has an impact on our lives, you can't have anything that is more central than the sun."
Pinning it down underground
Despite advances in telescopes - those that collect ultraviolet and infrared rays are now common - catching the slippery neutrino remains a daunting challenge. The average neutrino, produced in the core of the sun and other stars, could slide through a slab of lead one light-year across with only a 50 percent chance of being stopped. That's where the observatory's underground location comes in: The layers of Canadian Shield bedrock will block out cosmic rays and other unwanted particles. Only the neutrinos will penetrate the rock and reach the SNO detector.
Some of those neutrinos will be coming from the sun and will be carrying information from deep within its core as they enter the detection chamber. "By observing the neutrinos, we'll be getting information about the sun's interior," says Peter Sturrock, a physicist at Stanford University in Palo Alto, Calif. "It's information we couldn't get any other way."
At the heart of the SNO detector is a spherical vat, 40 feet across, filled with 1,000 tons of heavy water. A molecule of ordinary water contains an oxygen atom and two hydrogen atoms; a molecule of heavy water contains deuterium instead of hydrogen. Though deuterium and hydrogen each have a proton at their center, deuterium has an extra particle: a neutron. The heavy water is a neutron-rich environment - and that will spell the end of the road for about 20 neutrinos every day.
"That extra neutron is basically the target for the neutrinos," says Professor McDonald. Neutrinos produced in the core of the sun, he explains, will not do anything when they strike a proton.
When they hit a neutron, however, an electron is given off; this, in turn, produces a faint flash of light. Those flashes will be recorded by some 10,000 photodetectors surrounding the vat. With its use of heavy water, McDonald says, the detector will have a sensitivity substantially higher than that of previous experiments.
Though heavy water occurs in nature in small amounts, finding 1,000 tons of it is no simple matter. Fortunately, it is used as a moderating fluid in Canadian nuclear reactors, and Atomic Energy of Canada Ltd. is lending the water to the Sudbury team.
Straight from the sun's core
Among the neutrino's mysteries, perhaps the most puzzling is the "solar neutrino problem." Astronomers have a good idea of the processes taking place in the sun's core, where neutrinos are produced, and, therefore, of how many of the particles should be reaching Earth. Only about half that number appear to be arriving.
One possibility, physicists suggest, is that neutrinos change "flavor" en route to Earth - that is, they turn from one kind of neutrino into a different kind at some point. The phenomenon - which could only happen if neutrinos have mass - is known as oscillation. But existing detectors have only been able to record one kind of neutrino. The Sudbury detector will be sensitive enough to record all types - and therefore settle the flavor-switching issue.
Equally important to physicists is the "dark matter problem." Astronomers have found that as much as nine-tenths of the universe is made of something other than visible matter. Neutrinos - if they have mass - could provide a substantial chunk of the missing mass. This is important because if there's enough matter in the universe (including dark matter), gravity will eventually stop the universe from expanding, and it will collapse in on itself in a "big crunch" billions of years from now. If the total mass is smaller, the universe will continue to expand indefinitely.