January 20, 1998
New Window on Cosmos Opening a Mile Down
By MALCOLM W. BROWNE
s the world's first heavy-water neutrino detector nears completion, scientists eagerly await results expected to open a new window on the universe and settle one of the thorniest questions in physics: whether or not the elusive neutrino particle has mass.
A definitive answer to this question would solve the mystery of the "missing" neutrinos -- those which are believed to be created by the sun in vast numbers but which have failed to turn up in the numbers predicted by theory. Proof that neutrinos have mass would also force a revision of the "standard model" theory of fundamental particles and could shed light on the distribution of gravitating matter in the universe.
Heavy water is expensive but exists naturally as a constituent of ordinary water. It is made of molecules consisting of oxygen and heavy hydrogen. An atomic nucleus of heavy hydrogen, or deuterium, consists of a proton and a neutron; an ordinary hydrogen nucleus contains only one proton and no neutrons.
The extra neutron of the heavy water offers a new kind of scientific target for a variety of neutrinos that may be bombarding the Earth from outer space, advancing the hunt for these ghostly particles produced by many nuclear processes. A neutron knocked loose by a neutrino impact can be readily detected.
As finishing touches are being applied to the multinational Sudbury Neutrino Observatory more than a mile underground in a Canadian nickel mine, some physicists wax rhapsodic.
"It's one of the most exciting times in the life of any scientist," said Dr. John Bahcall, a leading astrophysical theorist at the Institute for Advanced Study, in Princeton, N.J. "A more thrilling moment you can't imagine."
If the experiment works as expected, Bahcall said, theorists will have found what he calls a "smoking gun" proving that neutrinos can transform themselves from one type to another, and therefore have mass.
Some theorists have suggested that even if neutrinos have only very tiny masses compared with the more familiar array of subatomic particles, neutrinos are so numerous they might comprise most of the mass of the universe. This mass, many theorists believe, might have provided enough gravitational glue to slow or even halt the expansion of the universe, contrary to the findings of astronomers who recently estimated the rate by observing the apparent motion of distant exploding stars.
But Bahcall said the Sudbury detector, if successful, has only limited ability to measure the overall mass of neutrinos in the universe, and that the detector's findings are unlikely to have a significant effect on cosmological questions.
During the next few weeks, technicians working within Inco Ltd.'s Creighton nickel mine, near Sudbury, Ontario, will begin trucking in 1,000 metric tons of heavy water valued at $210 million. Rail cars hauling this precious water will take about three months to complete the operation. Nine months after the purified heavy water has been pumped into a transparent spherical tank, a remotely controlled submarine will install sensors within the tank, and the world's first heavy-water detector -- and the first one capable of detecting all three types of neutrinos -- will be ready to work.
The Sudbury Neutrino Observatory (or SNO, pronounced "snow") is expected to register the arrival of electron neutrinos, muon neutrinos, and tau neutrinos -- not just the electron neutrinos that are known to be created by the sun's hydrogen fusion process. Physicists collaborating in the project predict that the detector will record the impacts of about 20 neutrinos a day -- a minuscule proportion of the astronomical flood of neutrinos from the sun, but still a good detection rate compared with those achieved by other observatories.
This unique detector was made possible by Canada's commercialization of its CANDO nuclear power reactors. Unlike reactors built in the United States, the Canadian reactors, many manufactured for sale abroad, use heavy water rather than ordinary light water to moderate neutrons created by the uranium fission reaction. This enables the Canadian-built reactors to use ordinary uranium as fuel rather than the expensive enriched uranium required by American reactors.
But this system required Canada to create a large stockpile of heavy water to supply the reactors it sells. Heavy water is present in ordinary water in a ratio of about one part in 7,000, and it can be separated by chemical and physical processes, but these are expensive. The heavy water about to be poured into the Sudbury detector has been loaned to the observatory by Atomic Energy of Canada Ltd.
The 65 scientists working on the experiment represent 13 institutions in the United States, Britain, and Canada. The U.S. Department of Energy has contributed to the $50-million construction cost.
Neutrinos, whose existence was proposed in 1930 by the Austrian physicist Wolfgang Pauli, have been compared with ghosts. They are known to be everywhere throughout the universe, and yet neutrinos have never been detected directly. A typical neutrino flying along at nearly the speed of light could easily pierce a slab of lead one light-year thick without being hindered, and neutrinos are therefore harder to snag and count than any particle known.
Fortunately for science, however, a minuscule proportion of neutrinos collide with more palpable particles, and when such a collision occurs in a detector, its effects reveal the neutrino's existence.
A neutrino (Italian for "little neutral one") has no electric charge and a mass, if any, so small that it has never been measured. Two of nature's fundamental forces -- electromagnetism and the strong nuclear force -- have no effect on neutrinos, and neutrinos are influenced only by the weak nuclear force. (If neutrinos have any mass, they would also be influenced by gravity.)
The elusive particle was not actually discovered until 1956, when two Americans, Dr. Clyde Cowan and Dr. Frederick Reines, recorded flashes of light induced in tanks of water by the impacts of neutrinos coming from a nuclear reactor.
Enormous effort has been spent during the last four decades to measure and characterize neutrinos, but their shy natures have made progress slow. Complicating matters, it is known that there are six types of neutrinos and three "flavors." A separate neutrino type is associated with each of three other fundamental particles -- the electron, the muon, and the tau. Moreover, each neutrino flavor has a corresponding antimatter partner.
The sun and all sun-like stars that burn hydrogen in a nuclear fusion process are believed to convert protons (hydrogen nuclei) to helium in a nine-step process. In three of these steps -- the fusion of two protons, the decay of beryllium 7 atoms, and the decay of boron 8 atoms -- electron neutrinos are believed to be created. These neutrinos zip through the sun to its surface, and about 10 minutes later some of them reach Earth, where detectors filled with various liquids -- cleaning fluid, mineral oil, water, and molten gallium metal, among others -- may register their arrival.
Most of the neutrinos reaching the Earth are created by the sun, our nearest thermonuclear neighbor. But even allowing for the fact that only a few neutrinos out of the vast flood from the sun are ever detected, too few are found to satisfy theoretical predictions. Even the most efficient detectors have registered no more than 60 percent of the solar neutrinos predicted by theory.
This mystery of the "missing" solar neutrinos has two possible solutions. One is that the standard theory of solar fusion is wrong. The other possibility is that the electron neutrinos emitted by the sun undergo oscillating transformations that change them into either muon neutrinos or tau neutrinos, or both. Traditional apparatus can detect neither type.
But the Sudbury observatory can. Its charge of heavy water will enable it to clock the arrivals not only of pristine electron neutrinos from the sun but of the two other neutrino flavors as well: unchanged electron neutrinos plus, if they exist, transformed electron neutrinos taking the forms of muon neutrinos or tau neutrinos. No other detector can register all three.
Like all major neutrino detectors, the Sudbury observatory is deep underground to shield it from cosmic rays that could mask the faint neutrino signals. The outer chamber of the detector will be filled with ordinary water, which further shields the detector's core from cosmic rays and radiation from terrestrial minerals. Within this chamber, a transparent inner chamber is suspended: an acrylic sphere 39 feet in diameter containing the heavy water.
Surrounding it, 10,000 very sensitive light detectors are arrayed to register the tiny flashes of blue light created when an electron neutrino hits a heavy hydrogen atom's nucleus (a proton plus a neutron), and sends an electron hurtling through the water.
About one year from now, a remotely controlled submarine will enter the heavy-water sphere and install another set of sensors. These, containing the isotope helium 3, will be sensitive to all three neutrino types, not just electron neutrinos.
The part of the detector that measures electron neutrinos alone looks for their impacts on heavy-hydrogen atoms. Each such impact splits the neutron in a heavy-hydrogen nucleus into a proton and an electron. As the electron speeds through the water faster than the speed of light in water, it must shed its excess speed and energy as a blue flash of "Cherenkov radiation," which is counted by the detector as representing an electron neutrino.
In the part of the experiment that records all types of neutrinos, a neutrino breaks up the nucleus of heavy hydrogen into a neutron plus a proton. The neutron is then captured by the helium 3 in the sensors, and is counted as a neutrino impact.
After the detector has been running for a year or so, its operators may add several tons of salt to the heavy water. Chlorine atoms in the salt make easy targets for neutrons produced by neutrino impacts, so the detection rate might improve.
If the experiment works as planned, the detector will register many more neutrinos of all three types than electron neutrinos alone. If this proves to be the case, said Dr. Arthur McDonald, director of the observatory, "it would be a clear indication that there are more than just electron neutrinos arriving from the sun." With luck, the total number of neutrinos from the sun will equal the number predicted by theory.
This will mean that neutrinos change from one flavor to another and must therefore have mass -- a demonstration likely to be recognized by one or more Nobel Prizes, if it comes to pass.
But what are the conditions under which electron neutrinos from the sun undergo this curious transformation?
Physicists believe that neutrinos might change flavor (or "oscillate") in either a vacuum or while passing through some substance. But the oscillation rates under these two conditions might differ.
"That's one of the questions the new Super Kamiokande neutrino detector in Japan may help to answer," Bahcall said. "If neutrinos oscillate at different rates while passing through matter or a vacuum, we would expect to see different proportions of the three types during the day, when the sun is overhead, compared with those at night, when solar neutrinos must pass through the Earth to reach the detector."
Scientists are cautious about forecasting a timetable for major discoveries at Sudbury. Dr. Douglas Cowen, of the University of Pennsylvania, one of the leading physicists working on the project, said that "much depends on the rate of background noise impinging on the detector, and we won't know that rate until we turn the detector on."
The noise in question consists mainly of radiation produced by uranium, thorium, and other radioactive substances in the ground. To filter out these minerals and other contaminants, elaborate purification plants will constantly filter both the heavy water in the detector's inner chamber and the ordinary water outside it.
Men and women working in the control room of the detector must walk through a long, muddy tunnel, shed their clothes, take showers, put on ultraclean garments, and proceed through air locks into a "clean room" like those in which electronic chips are manufactured.
"We've gone to enormous lengths to keep things as clean as possible," Cowen said, "but we won't know if we've done enough until we start up. Our fingers are crossed, but if we succeed all our efforts will be amply repaid."