Elusive Neutrinos Open a Cosmic Window
An observatory at the bottom of the sea will capture neutrinos and enable astrophysicists to look at nuclear events in space
ASTRONOMERS are turning to the most elusive form of matter to help them peer into some of the most inaccessible places in the universe. They are looking to a fundamental particle called the neutrino to learn what's happening in the deep interiors of stars, in supernova star explosions, in the violent centers of "active" galaxies, or wherever hidden, cosmic nuclear processes occur.
Neutrinos have no electric charge and little or no mass. They race along at or near the speed of light. They interact so weakly with other forms of matter that they generally zip through a star or planet as though it were not there. This enables neutrinos, which are products of nuclear reactions, to stream directly out of the very core of a star where its nuclear "fires" burn.
If astronomers had instruments to capture cosmic neutrinos as telescopes capture light, they could, in a sense, look directly at cosmic nuclear events. This has been hard to do, given the neutrino's elusiveness. But now observatories are being planned and built that can detect these cosmic ghosts. What is more important, they can fix the direction from which the particles come accurately enough to be called neutrino telescopes.
"It's a brand-new window on the universe," says astrophysicist Demosthenes Kazanas at the Goddard Space Flight Center in Greenbelt, Md. He adds: "If [a neutrino signal] proves to be there, it will give new insights. It will direct our thinking in a new direction."
Dr. Kazanas explains that astrophysicists generally had thought that nuclear processes were not important in producing the radiation seen from cosmic objects. Stars shine with light produced in their outer layers by nonnuclear mechanisms, even though their internal nuclear "fires" provide the energy. Radio noise, X-rays, and other radiation that astronomers use to study cosmic objects also arise from nonnuclear processes. But Kazanas says that if astronomers find streams of neutrinos coming from these ob jects, they will know that nuclear processes are involved.
Actually, astronomers have been studying extraterrestrial neutrinos for several decades. University of Pennsylvania physicist Raymond Davis Jr. began looking for solar neutrinos in 1968 when he was at the Brookhaven National Laboratory on Long Island. His detector was a large tank of cleaning fluid (perchloroethylene) installed in the Homestake Gold Mine at Lead, S.D. A few of the passing neutrinos were expected to interact with chlorine 37 in the fluid, changing it into radioactive argon 37. Dr. Davis p eriodically analyzed the fluid for argon and estimated the flux of neutrinos through his tank.
This study began an 18-year experiment, which has shown that the sun appears to emit far fewer neutrinos than the standard theory of solar physics predicts.
Other experiments in Japan, Russia, and the United States have confirmed the neutrino deficit in recent years. This has become what P. Buford Price of the University of California/ Berkeley calls "a famous mystery that may have profound implications for our understanding of physics."
Astrophysicists now suspect that either they don't understand how the sun works as well as they thought they did or that physicists don't understand how neutrinos behave.
Dr. Price and other pioneers of the new neutrino astronomy are not primarily interested in the sun, however. They want to study the entire universe by the "light" of neutrino radiation.
To do this, they need mammoth detectors that can sense weak neutrino signals from remote sources. They need to put those detectors in an environment that is as free as possible from the "noise" of general cosmic rays and other phenomena that would swamp the sensors. And they need to do it cheaply. Price and his colleagues would like to put their sensors in Antarctic ice while another neutrino astronomy team is heading for the bottom of the sea.
Most incoming neutrinos would whiz past the detectors without leaving a trace. But a few would interact with water or ice to produce other particles called muons, which release bursts of light. The detectors consist of arrays of photomultiplier tubes that detect that light. Computer analysis of the sequence in which the photo tubes fire and of the intensity of the light each tube sees will determine the energy of the incoming neutrino and the direction from which it came.
The international DUMAND - Deep Underwater Muon and Neutrino Detector - program is preparing to set up an array of nine buoyant strings, each with 24 photo tubes, off Hawaii. (See illustration above.) University of Hawaii physicist Victor Stenger, DUMAND's deputy director, says he expects three strings of the array to be in position next year. That should be enough for the project to begin to collect data.
Meanwhile, Price told a session at the recent annual meeting of the American Association for the Advancement of Science in Chicago that his group hopes to deploy three strings of photo tubes in kilometer-deep holes at the South Pole next year. This project - called AMANDA for Antarctic Muon and Neutrino Detector Array - could also be taking data within another year or two. Price said that tests of the detection scheme in Greenland in 1990 and this past summer season in Antarctica were encouraging.
At this point, astrophysicists can only speculate about what kinds of cosmic phenomena the neutrinos may reveal. Price says the telescopes will likely find sources of which scientists were not even aware, "which is what you always expect to see when you open a new window on the universe." Dr. Stenger agrees, saying, "we're counting on surprises." But he cautiously adds that astrophysicists know so little about this new field that "I can't promise anything."