Neutrinos are like cosmic ghosts. Each and every second, billions of the elusive subatomic particles pass through the Earth and everything on the planet. They pass through unnoticed. Neutrinos can travel 10,000 light years without disturbing anything.

A collaboration of scientists would love to detect a neutrino’s passing. Researchers from Arizona State University, the California Institute of Technology, Stanford, and the University of Alabama are using a 200-ton underground facility to do just that. The scientists want to determine whether these harmless building blocks of the universe have any mass.

The neutrino detector is built at the Palo Verde Nuclear Generating Station located 60 miles west of Phoenix.

Why bother?

“The whole fate of the universe, in a sense, hinges on what this mass is,” says physicist Barry G. Ritchie, who leads ASU participation in the project. “The number of neutrinos is just so mind-boggling that even if their mass is minuscule, it would mean they form a significant amount of mass in the universe.”

Such a discovery could help account for a “missing” source of mass in the universe, Ritchie explains. Currently, much of the matter that makes up the universe is invisible to telescopes and space probes.

“Does the universe continue to expand, or does it eventually stop expanding?” Ritchie asks. “If it stops, does it actually stop and then reverse? The answer to these questions is linked to how much mass exists in the universe.”

Stars such as our sun produce enormous numbers of neutrinos at their cores. But experiments that measure the number of neutrinos passing through us each second disagree greatly with predictions made by the best theories of how the sun actually works.

“If you don’t understand the star that’s right next to you, how can you hope to understand the processes of billions of stars throughout the universe?” Ritchie says.

The three nuclear reactors at Palo Verde will provide a more reliable and intense source of neutrinos, he adds. Scientists hope to gather data about neutrinos from the cores of the reactors. Such data could help determine whether detectors are unable to count neutrinos accurately because the uncharged particles change form as they travel.

Buried about 75 feet underground, the laboratory vault is a concrete box more than 40 feet long, 30 feet high, and 30 feet wide. Inside, a central detector contains some 10,000 gallons of mineral oil. The oil is laced with a fluid that emits light as radiation passes through it.

The participating universities share costs for the two-year project with the Department of Energy. Additional support is provided by Arizona Public Service Corporation, which operates the Palo Verde reactors and shares facility ownership with Salt River Project and other utility companies in the Southwest.—Erik Ellis

Go to: Physical Science: Physics

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