Messengers from the stars. Neutrino astronomy looks inside the core of a star
Astronomers have found that, when a star explodes, tiny particles are released called neutrinos. Although these elusive particles are far smaller than atoms, they provide the only direct window into the nuclear processes at the core of a star. Studying neutrinos may help scientists settle the question of whether the universe will expand forever, or collapse in the `big crunch.' WHEN a star exploded in the skies over the Southern Hemisphere in February, it did more than rewrite astronomy texts. It spawned a science.
``Detection of neutrinos from the supernova marked the beginning of the science of extra-solar system neutrino astronomy,'' says John N. Bahcall, a professor of theoretical physics at the Institute for Advanced Studies at Princeton.
Neutrinos are arguably the most elusive of all the known particles of matter. They have virtually no mass. They flit through space at the speed of light. They are electrically neutral, so they are unaffected by charged particles - such as electrons and protons in atoms - or by magnetic fields. And they are far smaller than a single atom. As a result, they rarely interact with other matter. By some estimates, an average neutrino will travel through 3,500 light-years of solid lead before being absorbed. For all practical purposes, once a neutrino comes into being, it is likely to hurtle through the universe forever.
So why bother looking for them? Because, astrophysicists say, they provide the only direct window into the nuclear processes at work at the core of a star. Take the sun, for example. Two byproducts of nuclear reactions that convert the sun's hydrogen fuel into helium are neutrinos and ``packets'' of high-energy electromagnetic radiation known as gamma-ray photons. Because photons interact more readily with other particles than neutrinos do, they wind up ricocheting their way to the sun's surface. In the process, many lose enough energy to become visible light. It may take more than 1 million years for photons to travel from the sun's core to its surface. Neutrinos from the core, on the other hand, pierce the sun's surface in seconds.
For the most part, neutrino hunting is something of a subterranean sport: Detectors consisting of tens to thousands of tons of liquid are generally placed deep underground to shield them from extraneous signals from cosmic rays.
The amount and type of liquid used determines not only what types of neutrinos observers are likely to see, but at what energy levels. These characteristics are valuable keys to understanding how the sources that give rise to the neutrinos work. In addition, the detector's surface area determines the number of neutrinos it's likely to intercept in a given period of time - a crucial point to those trying to spot distant objects whose concentration of neutrinos will have thinned substantially by the time they reach Earth.
In putting such detectors to use, there are two astronomical sources ``where you are guaranteed signals,'' Dr. Bahcall says, neutrinos from the sun and from supernovae.
The longest-lived solar neutrino experiment took place between 1967 and 1985 some 4,850 feet deep in the Homestake Gold Mine in South Dakota. The facility (now being upgraded) is run by the University of Pennsylvania and the Brookhaven National Laboratory. It consists of a tank filled with 133 tons of chlorine in the form of perchlorethylene. Neutrinos from the sun hit neutrons in the chlorine atoms, converting atoms they hit into a radioactive form of argon gas. After a period of time, the argon is flushed from the tank and studied as it decays into a more stable form. The result: estimates on the number and energy of neutrinos passing through the tank.
The idea is to test theories that predict three main types of fusion reactions taking place in the sun. Each reaction is expected to emit neutrinos at specific energy levels and in specific proportions.
The problem, says Kenneth Lande, chairman of the astronomy department at the University of Pennsylvania, is that the experiment is coming up with only one-third to one-fourth of the number of neutrinos that theory predicts.
THAT leads to two possibilities, he says: Either something is wrong with the theory of how the sun works, or ``the solar model is OK but something happens to the neutrinos.''
Loath to give up a fairly successful theory of the sun without a fight, researchers are taking several approaches to the ``solar neutrino problem.''
One is to build telescopes that are more sensitive to neutrinos with lower energy levels. The Homestake site, it turns out, is most sensitive to high-energy neutrinos that account for a relatively small fraction of the sun's neutrino output. As a result, University of Pennsylvania researchers are joining other collaborators in supporting more-sensitive detectors in Europe and the Soviet Union. Instead of chlorine, the European device would use 30 tons of gallium. The Soviet telescope would use 60 tons of the same chemical. If the neutrino count still comes up short, then it's time to reexamine what scientists think they know about neutrinos, Dr. Lande says.
IN the meantime, he adds, the Homestake detector is being modified to test the notion that something happens to the neutrinos en route from the sun. It may be that the so-called electron neutrinos produced in the sun somehow change into other forms of neutrino that can't be picked up in a chlorine-based detector. As they leave the sun's surface, electron neutrinos may turn into muon or tau neutrinos - higher-energy counterparts to the electron neutrino. But once they hit Earth, they may change back into electron neutrinos.
If that's the case, Lande says, a suitably equipped detector should pick up more of the sun's electron neutrinos at night, when they have to pass through Earth's diameter to hit the detector.
But among neutrino-watchers, February's supernova has generated by far the most excitement: Detectors in Japan and the United States simultaneously recorded bursts of neutrinos from the stellar explosion. It marked the first confirmed sighting of neutrinos from outside the solar system.
``We never thought that we would see a supernova,'' says Lawrence Sulak, chairman of the physics department at Boston University and a principal investigator at the US facility involved.
The facility, known as the IMB detector (for the University of California at Irvine, the University of Michigan, and Brookhaven National Laboratory), is the largest of its kind in the world. Instead of detecting neutrinos via radioactive decay, the IMB detector uses 5,000 tons of highly purified water in a cavern lined with photomultiplier tubes.
An incoming particle, in this case an electron antineutrino, collides with a proton in the nucleus of one of a water molecule's two hydrogen atoms. The resulting reaction generates a ring of light that gets picked up by the photomultiplier tubes.
Not only does the light pattern leave a distinctive trace on the tubes, but it allows researchers to estimate the direction from which the antineutrino came.
Dr. Sulak says that he and his colleagues didn't look for the supernova's signature until they had gotten wind that the Japanese water detector at Kamiokande had detected a pulse. Up to that point ``we thought the energy of the neutrinos from the supernova would be too low for our detector, so why look?'' he says. Now, he adds, he's organized a program to look back over three years' worth of data to see if they contain similar signatures from supernovae in our own galaxy that might have been obscured by the Milky Way's nucleus or by interstellar dust in the galaxy.
In the meantime, one of Sulak's colleagues has used supernova neutrino data to try to reconstruct events after the star collapsed.
Using data from the US, Japan, and a neutrino detector underneath Mont Blanc on the French-Italian border, Alvaro De R'ujula offers the intriguing suggestion that the supernova emitted two blasts of neutrinos: one when the parent star became a highly dense and compact neutron star, and one when it collapsed further, perhaps becoming a black hole. In addition, he has used the data to try to deduce the characteristics of the parent star.
REGARDLESS of whether he turns out to be right, Dr. De R'ujula's calculations illustrate the potential that researchers see for neutrino astronomy's ability to help explain such events.
The problem, physicists and astrophysicists say, is money.
``There is no precedent anywhere for funding high-energy astrophysics,'' says George Cassidy, a physics professor who specializes in high-energy astrophysics at the University of Utah. Instead, as Dr. Cassidy's position implies, with few exceptions the experiments that can detect neutrinos were initially set up as physics projects to measure proton decay as a test of theories trying to unify the four forces of nature.
Neither of the detectors that saw neutrinos from the supernova was set up for that. They were looking for proton decay,'' says Bahcall. ``Neutrino astronomy, certainly solar neutrino astronomy, was born and developed here. Almost every advanced country has a major scientific program in the field except the United States. It's a national scandal.''
A FIRST-RATE neutrino telescope dedicated to astronomical observations would cost in the neighborhood of $10 million, he estimates, noting that such a price tag is less than the cost of one experiment at a high-energy particle accelerator.
Ideally, he says, several systems would be set up around the world, not only to corroborate sightings, but to help pin down the source's point of origin. The facilities would run around the clock.
One element of that network is likely to be a new detector scheduled for start-up this summer near the Gran Sasso Tunnel in Italy.
When it begins operation, the facility will have the largest sensing area in the world, making it much more sensitive to relatively distant sources of neutrinos than current facilities. In addition, says Sulak, who is one of the project's collaborators, the MACRO facility, as it's known, will be the world's first telescope for high-energy neutrinos.
A second candidate for the network is under consideration at the University of California at Irvine. If built, it would take over the title of world's largest neutrino telescope, using a volume of water 1,000 feet square and 250 feet deep. Unlike most detectors, however, it would remain on Earth's surface. The estimated price of the project is $15 million.
Meanwhile, out in Hawaii, a team from the University of Hawaii and three other participating universities is preparing to test prototype detectors that, if they work as planned, would form the basis for a neutrino detector placed some 16 miles off shore and 3 miles under the sea. Building up a strip of detectors at a time, the goal is to build a neutrino telescope 1 cubic kilometer (6-10ths of a mile on a side) in size.
And in Japan, researchers have proposed enlarging a detector at Kamiokande to a point where, at 10 times the volume of the IMB detector, it would be the largest-volume detector in the world.
The `little neutron' and the `big crunch'
Neutrinos are particles that literally had to be invented to save three cherished laws of physics. Between 1900 and 1931, physicists noticed with growing unease that electrons emitted in a form of radioactivity known as ``beta decay'' didn't have enough energy to satisfy the law of conservation of energy. Under this principle, the combined energies of the decay particles should equal the energy of the original particle. In 1931, physicist Wolfgang Pauli offered a solution that looked more like bookkeeping than physics: He proposed that the ``missing'' energy was carried off by another particle.
In trying to keep beta decay in step with two other conservation laws, physicists filled in the details about how the new particle should act, and Enrico Fermi christened it the neutrino, or little neutron.
It would take another 25 years before experiments confirmed the neutrino's existence and, in the process, set the stage for the development of ``neutrino telescopes'' to study the sun, cosmic rays, and other extraterrestrial sources of neutrinos.
Neutrinos also provide unique insights into supernovae and may help explain more exotic objects, such as pulsars, energetic galactic cores, and black holes - stellar remnants with gravity so strong that not even light can escape their pull.
To researchers who hold that neutrinos are the dominant form of matter in the universe, knowledge gained from studying extraterrestrial neutrinos could help settle the question of whether the universe will expand forever, or collapse in what has come to be called ``the big crunch.''