Why would a group of scientists spend more than a decade building an enormous cave deep underground just to hold a large bottle of water?

Because that bottle of water would help them solve a nearly 30-year-old puzzle concerning the nature of the sun, a solution that made Science magazine’s list of the 10 Top Breakthroughs of 2001.

The cave was built so that 112 physicists from 11 universities and laboratories, including Penn, could search for neutrinos. The tiny particles are produced whenever radioactivity occurs, as in nuclear fusion, the process that fuels the sun. Like neutrons, protons and electrons, neutrinos are one of the elementary particles found in the universe.

The reason the scientists were looking, said Professor of Physics Eugene Beier, was because about a half century ago, some calculations projected the number of neutrinos produced by the sun over time. But in the 1960s, when physicists first started looking for these neutrinos, they could only detect one-third the number predicted in the calculations.

Thus began nearly three decades of efforts to find the other two-thirds, culminating in the construction of the Sudbury Neutrino Observatory (SNO), the huge cave mentioned above, located in an Ontario nickel mine, 6,800 feet beneath the surface to eliminate interference from other radioactive particles.

The cave is so huge—the size of a 10-story building—and the vessel so large—it holds 1,000 metric tons of deuterium, or heavy water—because neutrinos, despite being so plentiful, are very hard to detect.

“At this instant,” Beier said, “a paper coffee cup has a couple of tens of thousands of neutrinos from the Big Bang, about a thousand from the sun and about one from the earth’s core in it.” Since they interact weakly at best with matter, he said, “you need a lot of material for them to interact with.”

The heavy water provides that material. When neutrinos interact with electrons in the water, they produce light that is detected by an array of photoreceptor tubes surrounding the vessel.

In June of 2001, not quite two years after it began collecting data, the SNO team announced that they had solved the puzzle. As it turned out, earlier experiments failed to find all the missing neutrinos because they were mainly looking for one variety, or flavor, the electron neutrino, a direct product of the sun. But somewhere between the sun and the Earth, some of the neutrinos changed flavor, becoming muon or tau neutrinos instead.

Actually, said Beier, the U.S. co-spokesman for the SNO project, the Penn physicists on the team knew ahead of time that it would succeed. “One of our graduate students did a thesis simulating the calibrations [of the SNO detector] and came up with the result a year before we had the data. So we knew where we were going, but we didn’t have all the work done.”

Now that they do, they and other scientists around the world are discussing what the results tell us about neutrinos and the sun. And the SNO team itself is busy replicating some of the other neutrino-detection experiments that allowed them to solve the puzzle so they can confirm their own results.