Volume XXVII Number 1
The cavity for the Sundbury Neutrino Observatory was excavated more than a mile below the Earth's surface.
Photo courtesy SNO
IU South Bend physicist Ilan Levine (right) and student Eric Greiner (left) are now collaborating on the PICASSO Dark Matter Experiment.
Photo courtesy Ilan Levine
IUSB student Ed Behnke is also collaborating on the PICASSO experiment, which is searching for evident of the universe's missing matter.
Photo courtesy Ilan Levine
For five years, Ilan Levine's routine for going to work went like this: He awoke about 4:30 a.m., ate a quick breakfast, and drove half an hour to the parking lot of INCO's Creighton Mine, an active nickel mine near Sudbury, Ontario.
Inside a nondescript building at the mine, Levine joined scores of miners beginning their day. He changed from street clothes into a dirt-covered jumpsuit and boots. He checked his headlamp, clipped on safety gear, "tagged in" to let mine officials know he was going below, and walked down a long hallway to await his scheduled turn for a freight elevator.
Levine took the miners' elevator down well more than a mile--6,800 feet--then hiked about another mile through a dimly lit tunnel to reach the entrance of the lab where he worked, though there was still plenty to do before he could do anything that resembled his job. He had to hose off his muddy boots, undress, and enter a two-sided shower. After leaving the shower from the clean side, he dressed again in a pristine jumpsuit and a paper hair-cover. Then he had to go through an "air shower," designed to remove any remaining dust.
Anything he brought with him--from equipment to notes--had to be subjected to the same rigorous cleaning process or, like Levine's lunch, stored away from the dust-free work area Levine was about to enter. And speaking of lunch, you don't want to forget to bring it: "Once you're down there, you're down there--for at least eight hours," says Levine.
It's an unusual commute for anyone--but for Ilan Levine, the daily journey to work held an additional irony. Levine went to the underground lab, far removed from the light of day, to study the Sun.
The solar-neutrino problem
As a researcher at the Sudbury Neutrino Observatory (SNO), Levine was engaged in answering one of the fundamental questions about the star of our solar system. For decades, scientists have believed they understood the answer to a basic question that has occupied physicists and philosophers alike: how does the Sun shine? Scientists had a theory they thought was sound, but one piece didn't fit. According to the theory, nuclear fusion in the Sun's core ought to produce an enormous number of tiny, mass-less subatomic particles called "neutrinos."
"But there was one problem," says Levine. "Starting in the 1960s, when experimenters measured the number of neutrinos coming from the Sun, they found only a third to a half of the number predicted." Uncovering the reason for the deficit became known as "the solar-neutrino problem."
The reason for the solar-neutrino problem could have been that the favored theory of how the Sun worked was wrong. But measurements of all other solar properties agreed with the theory's predictions. If the theory wasn't wrong, what was the problem?
It was this problem--what happens to solar neutrinos--that the Sudbury Neutrino Observatory was created to answer. SNO was designed to let researchers observe the natural phenomenon of neutrino movement in a highly controlled environment. In the facility, researchers could see--almost literally--the fate of solar neutrinos interacting with matter.
"Another possibility was that electron-type neutrinos (the only kind created in the Sun) were being produced in the predicted numbers, but that they changed into other types of neutrinos after being produced," says Levine. According to the standard theory describing particles, that couldn't happen--but if it did, then it made sense that previous experiments found a deficit, because they were designed to see only the electron-type of neutrino.
SNO's underground location effectively blocks out most solar radiation, the effects of which could throw off scientists' data. But solar neutrinos penetrate the mine without difficulty, traveling to a huge acrylic vat--about 10 stories tall--at SNO's core. The vat contains some 1,000 tons of heavy water, water in which the hydrogen atom has an extra neutron in its nucleus. "The ‘heavy' hydrogen atoms in heavy water--they're called ‘deuterons'--are very easy for solar neutrinos to break up," says Levine.
The heavy-water vat is encased in a grid of light-detecting tubes, so exquisitely sensitive that they could detect a firefly's glow from more than six miles away. As neutrinos pass through the vat, they collide with heavy-water molecules; these collisions create infinitesimal flashes of "Cherenkov radiation." By measuring that radiation, scientists can tell not only how many neutrinos actually make it to Earth, but also what type the neutrinos are. "All neutrinos can shake apart the atom into its constituent proton and neutron," Levine explains. "But electron neutrinos alone can break up the atom by transforming the deuteron's neutron into a proton plus an electron."
SNO had an ingenious design--and one that actually worked. In 2001, scientists, including Levine, were able to announce that they had solved, once and for all, the solar-neutrino problem. The answer: Neutrinos change. Electron neutrinos are produced in the high numbers that the existing model of solar activity predicts, but about two-thirds of them change somewhere en route from the sun, morphing into some combination of the two other neutrino types.
And the fact that they change reveals another important fact about solar neutrinos: "They have mass," says Levine. "So our model of neutrino properties needs fixing."
Accomplishing that fixing is the goal of future SNO experiments. "The differences discovered between predicted and actual neutrino properties may help lead us to the long-sought ‘theory of everything,'" Levine says.
Not surprisingly SNO's work on the solar neutrino problem has received wide recognition: Science magazine named three SNO papers published in 2001 and 2002 as the most important physics breakthroughs of those years.
The what's-next problem
After SNO's history-making solar-neutrino measurements wrapped up, Levine began to look not only for a new cosmic question to answer but for a new home base as well. He had been a postdoctoral fellow at SNO, an important but temporary phase in the life of a scientist. Next for him would be an academic appointment at a university.
In 2002, Levine, who completed his doctorate at Purdue University, landed back in Indiana at IU South Bend as an assistant professor of physics and astronomy, drawn by the strong and collegial faculty and the opportunity to continue his subterranean studies of the universe.
What's next wasn't only a question for Levine. It was also a relevant consideration for SNO, which had been built specifically with the solar-neutrino question in mind. But having successfully completed the $60 million construction of what amounted to a remarkable scientific ship-in-a-bottle, SNO's prime movers weren't about to abandon that ship. They invited researchers to submit proposals for projects that could be fruitfully pursued in the underground confines of SNO.
Answering the solar-neutrino question, though, is a pretty tough act to follow. It would take another pressing cosmic mystery--the missing-matter question--to give SNO a second act worthy of its curtain-opener.
The missing-matter problem
As the solar-neutrino question suggests, scientists have their work cut out for them each time a prediction doesn't match what they really find. And it was, in fact, another gap between prediction and reality that gave rise to Levine's current research focus.
The prediction in question was about the amount of mass in the universe. Using the amount of light coming from, say, a distant galaxy to measure the mass of that galaxy gives scientists a very different number than when they measure the mass by measuring the orbital speeds of matter around the galaxy. In other words, matter that is predicted by one measure fails to appear in another measure.
Where is the missing matter? Many scientists now believe that a majority of the universe's matter--90 percent or more--is in a form that cannot be seen or measured in any conventional way. Confirming the existence of this "dark matter"--and unmasking it as well--is Levine's latest endeavor. And if the project pans out like Levine hopes, it will not only be his next act, but SNO's, and, in some part, IUSB's as well.
WIMPs and PICASSO
With its mysterious ability to go undetected, dark matter sounds almost supernatural. But Levine believes that it is nothing more or less than a special type of subatomic particle. "We call them WIMPs--weakly interactive massive particles," he says. Such particles, "don't interact with light, and they don't interact strongly," which accounts for their invisibility. But they do have tiny amounts of mass, and, Levine says, "they do interact with other matter through the weak nuclear force--like neutrinos."
Levine is by no means the only researcher to have pegged WIMPs as the likely candidate for the universe's dark matter. "Many experiments are trying to search for WIMPs, either through direct searches, where you try to observe WIMP-nucleus collisions, or by indirect means such as looking for high-energy photons or neutrinos from what are called ‘WIMP-WIMP annihilations,'" he notes. "PICASSO is a direct-search experiment."
PICASSO, which stands for "Program in CAnada to Search for Supersymmetric Objects," is Levine's current research focus. Based at the University of Montreal, PICASSO is an international collaboration that includes researchers not only from Montreal, but also from Canada's Queen's University, the University of Pisa, Yale University, and IUSB.
PICASSO aims to detect WIMPs by using "superheating," a state that many of us observe in our own microwave ovens.
"You've probably seen that when you heat water in a clean mug in a microwave, it won't boil until you add something--say, hot chocolate powder," Levine explains. "And then it boils explosively. It's actually heated beyond its boiling point, but if it isn't disturbed, it won't boil or turn to steam."
Following that principle, the PICASSO researchers will suspend droplets of Freon in a gel and then superheat them. On the rare occasions when WIMPs collide with fluorine atoms in these droplets, the result is a tiny explosion into a gas bubble. PICASSO researchers will measure the acoustic waves from the explosion.
Beginning in 2003, Levine and a group of IUSB researchers, including faculty, students, and a staff member, have been pursuing their part of the collaboration: studying acoustic properties of the detector, designing and developing acoustic sensors for the experiment, and designing the amplifiers that will send these signals to a computer for analysis. The IUSB group's efforts will enable the PICASSO team to hear something that rivals the sound of one hand clapping.
As an undergraduate physics student at IUSB, Ed Behnke played a pivotal role in designing the amplifier PICASSO will use. "We've invented an amplification technique that we're patenting," says Levine. "Ed was really the main contributor in that." Another undergraduate, Eric Greiner, is helping with the production and testing of hundreds of ultrasonic sensors designed by Behnke and Levine.
In chemistry, Associate Professor Bill Feighery and undergraduate Cynthia Muthusi are solving another of the key problems associated with the project: devising a way to incorporate lead into the liquid matrix of the detectors, which will help researchers determine which of two types of interactions dominate when WIMPs collide with ordinary matter.
Working with talented undergraduates is a plus of being at IUSB, says Levine. "You find students here with rich and varied backgrounds. They contribute far more than I could at that stage."
Six small modules of the PICASSO experiment, using 40 grams of Freon, are already taking data in the Creighton Mine. Starting this October, the group is installing a much larger version of the detector, which will use a kilogram of Freon. At the end of 2005, an even larger and more sophisticated version of the detector will take its place. Each subsequent version will be about 10 times larger than the one it replaces, and 10 times lower in radioactivity--a reduction in "background" that improves the reliability of the experiment results. One of IUSB's ongoing roles will be to develop better sensing devices that can be placed in the larger detectors and still hear the very faint sounds.
For Ilan Levine, the daily commute to work these days is a breeze. But the lab at the bottom of Creighton Mine still plays a large part in his work--and, he hopes, will continue to do so for many years to come. PICASSO, he believes, has the potential to become "one of the leading astroparticle experiments in the world." It could very well be that what is currently believed about dark matter is ultimately confirmed or refuted by evidence from acoustic detectors designed at IUSB.
But perhaps even more important for this scientist, who earned his stripes helping solve the solar neutrino problem, is the thrill of the hunt.
"Wouldn't you want to solve this problem?" he asks. "Wouldn't anyone?
"It's so much fun."
Elizabeth E. Hunt is editor-in-chief of the Indiana Alumni Magazine.