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Hunting the Wild Nutrinos

April 1, 1997

The forbidding Antarctic ice cap has become a new Mecca for astronomers looking to take advantage of the continent’s many months of darkness and pristine skies. Yet perhaps the most revolutionary astronomy project now under way at the South Pole plans to make use not of its clear views but its surprisingly clear ice.

For four years, an international team of scientists has been drilling holes up to 2 kilometers deep into the ice in order to build Amanda, the Antarctic Muon and Neutrino Detector Array. By spring, the principal investigators say, they should obtain their first indication of whether the premise behind their project is valid: that it is possible to do astronomy based not on light or any other form of electromagnetic radiation but on neutrinos.

If they are right, the repercussions would be enormous. Neutrino astronomy could give scientists a view straight to the heart of some of the most violent and energetic processes in the universe, including quasars and active galactic nuclei (distant galaxies believed to be powered by massive black holes), as well as the sources of mysterious gamma ray bursts and perhaps even the universe’s origin in the Big Bang.

What makes neutrinos such a good subject for astronomy is that unlike visible light or other forms of radiation they zip through the universe virtually unimpeded. Produced as a byproduct of the nuclear fusion that occurs at the heart of every star, they have no electric charge and-as far as anyone can tell so far-no mass. So if astronomers could detect neutrinos and measure their energy levels they could learn more about what goes on in those stars.

Neutrino detection is also important for studying high-energy sources such as active galactic nuclei. The powerful gravitational pull of these objects and the interstellar dust and gas surrounding them prevent most other forms of energy from escaping. It is as if astronomers, even with radio telescopes and instruments that capture other wavelengths of radiation, were viewing them through a dense fog. Researchers expect that Amanda will see through this turbulence to reveal a sky dotted with heretofore unknown sources
of intense energy, thus opening a new chapter in astronomy.

The only problem with neutrinos is that the very property that makes them such valuable sources of information also makes them devilishly difficult to detect. The earth is constantly bathed in a flood of neutrinos, yet astronomers have no way of detecting them directly. Instead, they look for evidence that high-energy neutrinos have collided with the atomic nuclei of surrounding matter. When collisions occur, they give off muons-negatively charged particles that are like electrons but have more than 200 times the mass. These muons give off a bluish light called Cherenkov radiation that cascades away from the crash site like waves from the bow of a boat.

Fortunately, the direction of muons aligns closely with the direction of the neutrinos that produced them. So a three-dimensional Cherenkov light detector, such as that provided by Amanda’s array of detectors buried beneath the South Pole, could not only confirm that high-energy neutrinos passed through, but also identify their trajectory with better than a 1-degree resolution: good enough to pinpoint their sources in
the sky.

An early indication of the promise of muon detection came on February 24, 1987, when astronomers at an observatory in Chile spotted a new supernova. At the same time, neutrino detectors in Kamioka, Japan, and Cleveland, Ohio, registered a sudden flurry of activity, confirming that high-energy neutrino showers are associated with violent astronomical events.

Encouraged by these findings, the U.S. Department of Energy funded DUMAND, the Deep Underwater Muon and Neutrino Detector, which attempted to drop detectors into the deep ocean off Hawaii to hunt for neutrinos in December 1993. But technical problems stalled the project, which was eventually canceled last fall.

Another deepwater project is now under construction by European and U.S. scientists in the Mediterranean Sea. Yet, as Amanda scientists point out, the problem all oceanic detectors face is in filtering out the noise of background radiation from radioactive potassium, which is present in small amounts in ocean water, as well as from bioluminescent bacteria and higher organisms, not to mention the dangers to the instruments from currents and storms.

Conversely, Antarctic ice, which is essentially pure freshwater containing no such contaminants, produces virtually no background radiation. Moreover, once detectors are frozen in the ice, they will not be disturbed.

Amanda research teams, including those from the University of Wisconsin, the University of Delaware, the University of California campuses at Irvine and Berkeley, the Lawrence Berkeley Laboratory, and Swedish universities in Stockholm and Uppsala, originally placed some 80 detectors a kilometer deep in the ice. But initial tests showed that at that depth the ice, which fell as snow some 18,000 years ago, had bubbles that interfered with the transmission of the Cherenkov radiation emitted by the muons. The team later found that the bubbles disappeared at around 1,500 meters, so in subsequent years detectors were placed at depths of around 2 kilometers. The faint bluish light travels more than 300 meters and should be more easily picked up by the detectors, 400 of which by February are scheduled to be distributed in a cylindrical pattern with a radius of 60 meters and a depth of 2 kilometers.

Faint Traces

For a project with such grand hopes, the technology it uses is fairly simple. The detectors are commercial photomultiplier tubes packed inside thick glass globes. They are like “flashlights in reverse,” explains Francis Halzen, an astrophysicist at the University of Wisconsin-Madison and one of the leaders of the project. The tubes can detect the faintest traces of light given off by neutrino collisions, amplify them more than 100 million times, and transmit the signals back to the surface as electrical pulses through coaxial cables for analysis.

To drill the holes for the strings of detectors, scientists use a technique borrowed from glaciologists called hot-water drilling. The rig uses a device similar to a massive shower head that gushes scalding water to melt the ice. As the ice melts, the water is recirculated, heated, and fed back down the hole. Since the drill is steered by gravity, its fall is straight, deviating less than 1 meter over a kilometer. Because of the ice’s natural insulating properties, the hole doesn’t refreeze for about four days, giving the crews enough time to deploy the strings of detectors.

The detectors are already hard at work. They record some 25 events per second, but nearly all are too weak to be of interest to Amanda scientists. The researchers are looking for the signature of neutrinos that have traveled all the way through the earth to arrive at detectors from below, figuring that the earth will filter out the radiation from less energetic sources and leave only the signature of high-energy neutrinos. In fact, Halzen says the detectors have picked up such readings, which the collaborators believe are traces of the high-energy neutrinos the project is being built to detect.

To measure actual energy levels more precisely, Halzen makes use of the very air bubbles in the ice that originally seemed to pose a problem. It turns out that the bubbles gather and scatter light in a predictable fashion. By measuring how far and in what direction the light emitted by excited muons scatters throughout the detector array, scientists can calculate the energy levels of incoming neutrinos.

In his public talks, Halzen likes to point out how astronomy has succeeded in exploring radiation arriving from space at wavelengths spanning 18 orders of magnitude, from languid radio waves to intense gamma rays. Neutrinos-which are emitted by radiation with wavelengths up to eight orders of magnitude smaller than gamma rays-are in essence a messenger that astronomers hope to use to describe that radiation. It’s hard to know what energy sources astronomers would find at that level, but, Halzen says, “it’s inconceivable that there’s nothing out there.”

Ironically, one fear of the Amanda scientists is that their findings might put them too far ahead of the rest of the astrophysics community. As Halzen says, if no other instrument is able to replicate Amanda’s observations, “I’m afraid one day we’ll see all these wonderful things and no one will believe us.”

Scientists familiar with the project, however, are expecting a better fate. “Our experience in astronomy in the last 50 years is that when a new technique for observing the universe is applied, nature always turns out to have surprises in store for us,” says John Bahcall, a physicist who studies neutrinos at the Institute for Advanced Study in Princeton, N.J. “What we observe,” he says, “is not what we expect to observe.”

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