Neutrino Detectives : Ice Becomes ‘Telescope’ in Quest for Stellar Answers
They are ghostly particles without electrical charge and little or no mass, and they travel near the speed of light.
In theory, these subatomic particles, known as neutrinos, are bombarding the Earth continuously. Yet millions of dollars in elaborate experiments--including a chamber of purified water in a salt mine below Lake Erie and one in the world’s deepest gold mine in South Africa--have been able to detect relatively few of these interstellar voyagers emitted by distant stars.
UC Irvine astrophysicist Steven Barwick is one of a team of researchers who hope to revolutionize the hunt by turning the ice of Antarctica into a neutrino “telescope” that for the first time could allow astrophysicists to see the nuclear reactions occurring within such mysterious objects as neutron stars, pulsars, quasars and possibly black holes.
On Saturday, Barwick will head for the U.S. research station at the South Pole to oversee the drilling of holes more than a half-mile deep in the Antarctic ice pack, bored with hot water from high-pressure hoses. Down the two holes will go ultra-thin stainless-steel cables, each bearing glass spheres containing highly sensitive photographic equipment to detect reactions triggered when a neutrino strikes another atomic particle.
If the prototype works, Barwick and team members from UC Berkeley and the University of Wisconsin hope in succeeding years to add strands of 20 sensors each across more than a half-mile of polar ice.
“The advantage of looking at neutrinos is that you can see the inside of things,” said Barwick, 32, an assistant professor of physics at UCI who is one of the researchers with project AMANDA--the Antarctic Muon and Neutrino Detector Array. “We’d be looking into the center of more exotic objects like neutron stars, binary systems . . . perhaps even quasars. We could look at the very core of nuclear reactions.”
Neutrinos of the sort AMANDA seeks are believed to be produced by massive nuclear interactions within these stellar bodies, then spun beyond their immense gravitational field at close to the speed of light. Because they have little or no mass or charge, they could move unimpeded through planets and whole galaxies without stopping. By tracing these neutrinos back to their source, physicists hope to prove theories about the inner workings of pulsars, quasars and the rest.
Said fellow team member, particle physicist Francis Halzen of the University of Wisconsin at Madison: “We are literally trying to X-ray the universe, something that has never been done. . . . And it is a very good bet that we will see things that have never been seen before.”
The elusive neutrino was first theorized in 1931 by Austrian physicist Wolfgang Pauli in order to balance a formula for a radioactive interaction known as beta decay. A few years later, Italian physicist Enrico Fermi coined the name, which means “the little neutral one.”
But their existence was not proven until 1956, when physicists Frederick Reines and the late Clyde L. Cowan Jr. measured and detected the reaction of neutrinos with other particles in a vat of transparent liquid at New Mexico’s Los Alamos Scientific Laboratory.
In the 1960s, scientists began looking for neutrinos produced not by man-made nuclear reactors, but by forces inside our sun and other stars. They went deep below the Earth’s surface in an effort to screen out most of the other cosmic particles that shower the surface constantly.
The first successful neutrino event was recorded in a South African gold mine in 1966 by an international team that included Reines, who came to UCI the same year. Underground detectors are now set up in the United States, Japan, the Soviet Union, Italy and elsewhere.
Project AMANDA’s primary advantage, when completed, is that it would be at least 50 times bigger than any existing detector, Barwick said. Size is important since the odds are 1 in 10 billion that a neutrino traveling the equivalent of the Earth’s diameter will interact with another atomic particle.
“You can’t just scale up in size in underground mines,” Barwick said. “So the innovation here is to use a medium that already exists: these vast expanses of Antarctica’s ice cap. It’s virtually free if you can get down to it.”
A similar project called DUMAND--Deep Underwater Muon and Neutrino Detector--will place strings of sensors 4.8 kilometers under water off Hawaii. Particle astrophysicist John Learned of the University of Hawaii said they hope to have the underground cable lines laid in the next few months and to have the $10-million DUMAND project up and running by 1993--about the same time the AMANDA group hopes to have enough data to begin mapping neutrino sources in the universe.
Learned, who left Reines’ group at UCI in 1980 to work on DUMAND, claims a “fatherly interest in AMANDA” as one of the first to consider ice and water as mediums 15 years ago due to the size limits of underground chambers.
“If you dig out too big a hole in the ground, it will collapse under pressure of the ground above it,” Learned said. “So the biggest you could have would be the equivalent of about 100 meters across in each dimension. Whereas in the deep ocean or the ice you can make it as big as you could afford to and you could keeping adding onto it.”
At the time, however, not enough was known about the clarity of deep ice and the logistics of developing such a detector seemed prohibitively expensive. Learned and his team proceeded with plans for DUMAND, which will involve nine strands of underwater detectors anchored at the ocean floor 25 kilometers off the big island of Hawaii.
The ice medium was revived less than two years ago with a group of scientists and graduate students led by UC Berkeley physicist P. Buford Price, who already had a research “in” at the U.S. government-sponsored station at the South Pole, which is run by the National Science Foundation.
And so the proposal hatched with Halzen and the Berkeley team in early 1990 became a scramble to raise money to build the detectors and a successful test of Greenland ice as a medium to record neutrino interactions.
Barwick, who left Price’s laboratory for an assistant professor’s post at UCI in July, 1990, unabashedly says that AMANDA has been able to ride the funding coattails of the DUMAND project--and much more cheaply at that. He estimates they have spent about $50,000 on equipment design and development so far. A National Science Foundation official said the agency is underwriting an estimated $50,000 in costs associated with the drilling and related work.
Yet there is no rivalry between the two projects.
“If I had my druthers, I’d say let’s do it in both mediums,” Learned said.
In fact, both are needed to cross-check results, Halzen said.
Both men also are watching with interest similar Soviet projects, including neutrino detectors being placed at the bottom of the world’s deepest freshwater lake in Siberia and in ice at the Soviet research station in Antarctica.
Both AMANDA and DUMAND work on the same principle: measuring faint blue light known as Cerenkov radiation. When a high-energy neutrino interacts with an atomic particle near the array, it will generate a particle called a muon and the wave of blue light, which will pass at least two of the sensors. By checking the time each sensor recorded the flash of light, researchers will be able to calculate the path of each neutrino event to its source, as well as its energy.
By pointing the sensors down to the center of the earth, they hope to screen out all particles except for these high-energy neutrinos.
It will take Barwick two or three days to reach the South Pole station, depending on the weather between New Zealand and the Antarctic port of McMurdo. When he arrives, clad in government-issue down parka, thermal trousers, insulated boots and special gloves and a hat, it will be to a sun that stays low in the sky 24 hours a day. Temperatures should be about zero degrees to 10 degrees Fahrenheit, but could range from minus 30 degrees to 32 degrees.
He’ll join UCI graduate student John Lynch, who left for the South Pole in mid-November, and a few colleagues from UC Berkeley and the University of Wisconsin. With luck, the strands will be lowered into the holes by mid-December. Once the ice refreezes around the specially tempered glass orbs, it will be a matter of monitoring the data received by equipment at the surface and transmitted by cables to their computer equipment in a nearby hut.
“We should know right away whether it will work,” Halzen said.
Barwick was bursting with excitement at the prospect during a recent interview in his UCI laboratory. Yet the downside is that this can be done only in the Antarctic summer--the height of the Northern Hemisphere holiday season. His wife, Lauri, and their nearly 2-year-old daughter, Krista, are not thrilled.
“We’ve got to celebrate Christmas early or late--we haven’t decided which,” said Barwick, who will be gone more than a month on the project.
Why is such research so important?
“This isn’t going to make better Teflon for cooking pots or solve the environmental problems of the world,” Learned said. “Why we do it is to try to understand the universe, where we came from and where we’re going.”
Peering Into Cosmic Secrets
A team of scientists--including a UC Irvine astrophysicist--hopes to turn the South Pole ice pack into a unique “telescope” to reveal the inner workings of some unusual stellar bodies. Researchers with the Antarctic Muon and Neutrino Detector Array project hope that their discoveries will answer fundamental questions about the origins of the universe.
1) Mysterious Objects: AMANDA researchers hope to detect high-energy subatomic particles called neutrinos that are emitted by such little-understood cosmic phenomena as neutron stars, quasars and black holes, objects that reveal little to optical and radio telescopes.
2) Telltale Neutrinos: Neutrinos have no electrical charge and travel near the speed of light. If they have mass, it is infinitesimal. They can pass through solid matter unhindered. By tracing these particles back to their stellar sources, scientists hope to create a “sky map” that can help them test theories about the mechanics powering them.
3) Natural Filter: Using the Earth’s mass to screen out cosmic rays and other interference, sensors will look through the planet’s core toward the northern sky. Scientists say neutrinos should be the only particles streaming past the detector orbs.
4) Icebound ‘Telescope’: Two strands of wire, each carrying four sensors encased in glass spheres, will be lowered down holes bored in the ice pack. If the prototype works, more strands will be added to expand the area used to detect evidence of neutrinos.
5) Watching for Flashes: When a neutrino collides with another atomic particle, it triggers a bluish flash of radiation easily seen by the sensors in the transparent ice. The flash, recorded by multiple sensors and carried by cables to surface equipment, can be charted to show the path the neutrino traveled.
SOURCE: UC Irvine and the AMANDA project
Mystery Objects
Scientists on the Antarctic Muon and Neutrino Detector Array project hope to learn more about several stellar phenomena with a specially created “telescope.”
Quasars, short for quasi-stellar objects, are so luminous they can be seen even though they may be more than 10 billion light-years distant. Many scientists theorize that the extreme brilliance of quasars is produced by gas clouds spiraling into black holes at their center.
Neutron stars are small, extremely dense stars no more than 12 miles in diameter. They are believed formed when the center of a violently exploding star--called a supernova--collapses inward.
Pulsars, short for pulsating radio stars, are believed to be rapidly spinning neutron stars surrounded by an intense magnetic field. Radiation released from their magnetic poles as they spin are detected on Earth as a series of evenly spaced pulses.
Black holes are believed to be invisible objects in space with such strong gravitational pull that almost nothing, not even light, can escape. They are thought to be formed when a large star exhausts itself and collapses inward. As the star becomes extremely dense, its surface gravitational pull increases dramatically, pulling more matter toward the core. Some astronomers believe that black holes may form up to a third of the matter in the Earth’s galaxy, the Milky Way.
