IceCube opens up a window on energy in the universe

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By Eric Niiler
Special to The Washington Post
Tuesday, February 8, 2011

AMUNDSEN-SCOTT BASE, ANTARCTICA - The world's newest astronomical observatory is defined by a field of 86 colored flags rippling across an ice-covered polar landscape. Each banner marks a line of glass-covered orbs that stretches down a mile and a half into the ice, like beads on a frozen string.

Known as IceCube, this massive underground array is designed to do what no other observatory has done before - catch a glimpse of elusive neutrinos, ghostly particles that are formed in the hearts of supernovas, black holes and other deep-space objects and may give scientists new information about the origins of the universe.

"The idea with IceCube is to do astronomy, but instead of using light, we're using neutrinos," said Greg Sullivan, a physicist at the University of Maryland who is one of the collaborators on the $279 million project.

"It opens up a window on energy in the universe," he explained. "We've seen particles in outer space that are 10 million times more energetic than the ones we can accelerate on Earth. Neutrinos are a way to try and find out what's causing those very high energy [particles]. It's been a mystery for 100 years."

Astronomers have flocked to the South Pole in the winter for decades, drawn by the sunless skies and atmospheric conditions that make superb star-gazing. A permanent U.S. station has been at the pole since 1956, and several telescopes have been built here to take advantage of the darkness that lasts from late February to early October.

But IceCube is something different, an observatory built entirely beneath the ice. Along each of the 86 cables are strung 60 three-foot spherical detectors, called digital optical modules or DOMs. These glass-covered orbs are designed to find evidence of neutrinos - particles formed in the hearts of stars that are so small they pass right through the Earth (and our bodies) without hitting molecules or other matter.

Since neutrinos have almost no mass and are too small to be seen with a normal telescope, researchers instead are looking for the extremely small and extremely brief flashes of bluish light that are given off when a neutrino's energy trail strikes an oxygen atom in the ice and creates a third particle, called a muon.

"We thought that if we could . . . detect that light, we could reconstruct the direction and energy of that muon, which would give us the direction of the neutrinos," Sullivan said during a visit last month to the South Pole sponsored by the National Science Foundation.

In the past, scientists have tried to build neutrino detectors in the deep ocean, abandoned mine shafts and the bottom of deep lakes. All the projects failed for different reasons: salt corroded the detectors, for example, or the muon trails were obscured by the natural light given off by plankton.

Astrophysicists have high hopes for the South Pole location. One advantage of the massive icepack is that it provides a "scaffolding . . . infrastructure for the detectors," holding them steady, Jonathan Feng, a particle physicist and cosmologist at the University of California at Irvine, explained in a phone interview. It also presented extreme challenges: Constructing IceCube involved more than 400 technicians and engineers and took seven summers of tough drilling through polar ice.

IceCube's detectors are pointed northward, toward the center of the Earth, so the planet's mass serves as a filter to block most cosmic rays and other particles. Feng noted that in addition to passing through most matter, neutrinos also are not bent by electric and magnetic fields, which can bend other forms of radiation - potentially bringing information more directly from farther corners of the universe.

The National Science Foundation picked up $242 million of Ice Cube's $279 million price tag. The rest was split among science agencies from Germany, Sweden and Belgium, which also cooperated on construction. The University of Wisconsin at Madison, the project's lead institution, coordinated the design, build and software to run it. The university is also coordinating the data distribution, making information available to scientists around the world.


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