GHOST HUNTERS

IN THE TIME IT TAKES TO READ THIS SENtence, millions upon millions of neutrinos, pouring in from outer space, will zip through the body of every human being on earth. It's like a never-ending barrage of subatomic bullets, but with one important difference: unlike bullets, neutrinos are so ethereal that they pass through ordinary matter as though it were not there at all. Unless a neutrino scores a direct hit on an atomic nucleus, it leaves no hint of its passage. And such hits are so unlikely that the average neutrino can easily penetrate a slab of lead a trillion miles thick without grazing a single atom.

Yet for all their elusiveness, neutrinos are among the most important particles in the universe. Because they emerge unscathed from the core of the sun and from the explosive death throes of stars, they carry information about processes that are otherwise unobservable. And because the cosmos is packed with neutrinos--so many that there is no "illion" large enough to count them--their combined mass could be the dominant force in the evolution and eventual fate of the universe.

It could, that is, if neutrinos have any mass. Nobody knows if they do or if they're massless, like the photons that carry electromagnetic force. That is one of the reasons Canada, the U.S. and the United Kingdom are combining their scientific resources and building the world's most powerful neutrino catcher near the shores of Lake Huron in northern Ontario. When the multimillion-dollar Sudbury Neutrino Observatory goes into operation sometime next year, it should settle some of the thorniest questions in the universe. Says John Bahcall of the Institute for Advanced Study in Princeton, New Jersey, a pioneer in modern neutrino theory: "This is the kind of experiment you do every few decades that can literally set the course of physics for many years to come."

The word observatory conjures up images of white domes silhouetted against a mountain sky. Forget them. S.N.O. is buried more than a mile underground, in a tunnel branching off from a working nickel mine. On Earth's surface, cosmic rays and other stray subatomic particles would trigger false signals in sensitive detectors. Nothing but a neutrino, however, can get through 6,000 ft. of earth and rock. The detector is a gigantic spherical vat containing 1,000 tons of heavy water--a form of H2O in which the H, or hydrogen atom, has an extra neutron in its core. The vat in turn is surrounded by a 7,000-ton jacket of ordinary water, which shields it from trace amounts of naturally occurring radiation in the environment.

A neutrino that laughs at lead won't be stopped by a few drops of water, of course. But the awesome penetrating power of these particles applies to the average neutrino, not to all of them. A very tiny percentage of neutrinos do slam into water nuclei. The number is so vanishingly small that even with quadrillions of neutrinos passing through millions of pounds of water every second, only a few thousand collisions will take place every year. That's still 50 times the rate at existing observatories. When one of these rare collisions happens, the impact can force the nucleus to emit an electron. That in turn generates a tiny flash of light that passes through the clear acrylic walls of the containment vessel and is recorded on electronic sensors.