Frozen Telescope

red pole into frozen ground
Doug Cowen

AMANDA, the Antarctic Muon and Neutrino Detector Array, consists of 700 light sensors strung onto 19 cables, each cable lowered half a mile into the ice.

Bundled in brightly colored cold-weather gear, Doug Cowen, professor of physics and astronomy and astrophysics at Penn State, boards a small, noisy cargo plane in Christchurch, New Zealand. "Christchurch is hot enough without a coat made of goose down," Cowen says, but he needs the warm attire for the tail-end of the turbulent eleven-hour flight. The plane's destination is the South Pole. There, Cowen and a team of astrophysicists from universities around the globe will work to develop a telescope that will enable them to map the far reaches of the universe.

November through February are Antarctica's warmest and most hospitable months, though they're not quite comfortable by any conventional standards.

During Cowen's time there, the sun won't set; instead, it will circle the vast and icy landscape, hanging low like the average temperature at Amundsen-Scott Station, the National Science Foundation's research base just a short walk from the geographic South Pole.

Though Cowen's interest is in mapping the heavens, he doesn't spend his time gazing up. The telescope he has traveled so far to use looks down, deep into Antarctica's thick, clear ice. AMANDA—the Antarctic Muon and Neutrino Detector Array—is Cowen's buried lens to space. Whereas conventional telescopes scan the various wavelengths of the electromagnetic energy spectrum pulling in light from stars and planets, AMANDA examines the neutrino sky.

When stars explode or collide they release tremendous energy in the form of subatomic particles. The better known of these particles, protons and photons, are quickly absorbed or deflected from their paths as they interact with magnetic fields or matter in the universe. Neutrinos, however, move like ghosts through the cosmos. Possessed of neither electric charge nor substantive mass, they pass through matter and magnetic fields unscathed, traveling millions of light-years in straight lines from their sources. "They don't bounce, they don't get absorbed, they don't scatter," says Cowen. "They're produced with a lot of energy and they just coast through space."

Though hundreds of billions of neutrinos are constantly passing through every cubic yard of the Earth, their ghostly nature makes them difficult to detect. But occasionally, when neutrinos crash into atoms of matter, such as water, they produce a perceivable residue: charged particles called muons, electrons, or taus that give off an eerie blue light known as Cherenkov radiation. The trail of Cherenkov light follows the path the charged particle takes through a body of water or ice. As Cowen puts it, "the light is a daughter of the neutrino's interaction with atoms of water. It's a footprint the neutrino leaves as it passes through the ice."

The idea for a neutrino telescope has been around for a long time," says Cowen. "The challenge," he explains, "has been finding a place to build one." A good spot would be a large body of natural water—large enough to increase the chances for neutrino interactions with water molecules to detectible levels. Accordingly, attempts to build neutrino telescopes have been made in oceans—off the coast of Hawaii, and in the waters of the Mediterranean. But seawater contains potassium, a material with slight but detectable radio-activity that creates what Cowen calls "white noise" for a neutrino telescope. Polar ice, however, is relatively free of radioactivity because it's composed of evaporated water that fell as snow, which lacks heavier radio-active contaminants contained in evaporated water that falls as rain. "Polar ice is the ideal detector medium," Cowen says.

When AMANDA was built during the austral summer of 1999-2000, Cowen, then on the faculty of the University of Pennsylvania, quickly became involved in the project. "They weren't really looking for anyone at the time, but I got a good feeling about the project and got involved," says Cowen. His prior experience in high-energy particle physics had included work done as a postdoctoral researcher at Caltech to determine the masses of tau neutrinos as well as participation in an experiment conducted at the Sudbury Neutrino Observatory in Canada that discovered solar neutrino oscillations—the phenomenon by which one type of neutrino can convert itself into another type.

AMANDA detects Cherenkov light with over 700 photomultiplier tubes, or light sensors, each of which is individually encased in a basketball-sized glass sphere. The spheres are strung like necklace beads on 19 half-mile-long cables. Each beaded cable is buried vertically in a hole dug a mile and a half below the ice surface. "We want the sensors to detect only Cherenkov light," Cowen explains. "The deeper we bury the sensors, the less likely it is that other cosmic sources will cause interference."

To bury the sensors, a large steel drill powered by airplane fuel mines through the firn, the roughly 100 feet of snow that hasn't yet hardened into ice. From that depth, where using a steel drill becomes too cumbersome, the digging process is completed by a high-pressure hot water drill. After the drill reaches the desired depth, it is pulled out and a cable of sensors is lowered into the hot column of water. "The surrounding ice serves as a good insulator that keeps the water column from immediately refreezing," Cowen observes. Within 48 hours, the water freezes around the sensors, fixing them in place. Electrical power is then applied to activate them.

The sensors are designed to pick up the distinctive Cherenkov radiation patterns produced by muons, electrons, and taus. Muons, according to Cowen, produce "skinny tracks," important because they point directly back to a neutrino's cosmic sources, helping to pinpoint location. Electrons and taus, whose energy can be better measured, produce "cascades" that appear in the ice as expanding spheres of light. "Cascades are valuable," says Cowen, "because they provide more conclusive information about energy levels associated with cosmic neutrino interactions in the ice." Studying cascades will allow researchers to gain a better sense, for example, of the nature of the black hole that produced a given neutrino.

So far, Cowen reports, AMANDA has proved useful primarily for detecting neutrino-induced muons. As for taus and electrons, he says, "None have been detected yet. We believe the reason is simply that AMANDA is too small."

IceCube, an expansion of AMANDA that is led by astrophysicist Francis Halzen of the University of Wisconsin-Madison, should solve that problem. Slated for completion in 2009, IceCube will include a total of 5000 sensors on 80 cables frozen into a cubic kilometer of ice (thus the name), creating a telescope with more than twenty times AMANDA'S neutrino-catching capacity.

Cowen heads the group in charge of commissioning IceCube—coordinating the outputs from its 80 cables of sensors so that they function smoothly together. The group also works on verification—calibrating and monitoring detector signals from afar to make sure the detector is working properly.

Additionally, Cowen is responsible for developing the online data-acquisition software that will be used to organize the data that IceCube collects. Computer programs he and his group at Penn State will develop identify and filter out "white noise" radiation, so that only "clean" neutrino data are transmitted to the northern hemisphere, where researchers will view and analyze them in the warmth of their offices.

"We're probing at a new energy range," Cowen says, "and our goal is to construct the very first neutrino map of the universe. Comparing our map with existing electro-magnetic maps of space, we'll see plenty of correlations—regions where we already know that events have occurred. But we'll certainly also see differences that could give us entirely new information about phenomena we currently know nothing about."

Doug Cowen, Ph.D., is associate professor of physics and astronomy and astrophysics, 303D Osmond, University Park, PA 16802; 814-863-5943; cowen@phys.psu.edu. AMANDA and IceCube are funded by the National Science Foundation.

Last Updated May 01, 2004