They are very much like ghosts. Billions pass through your body every second with no more effect than a fleeting thought. These poltergeists are “neutrinos,” subatomic particles of matter that are electrically neutral and rarely interact with any other particle.
Despite the fact that neutrinos are the second most populous resident in the universe after photons, they remain very much a mystery because of their ghostlike qualities. But these ghosts are being “busted”; their secrets are coming out thanks to international teams of scientists such as Berkeley Lab physicist Kevin Lesko.
Born and raised in the San Francisco Bay Area, Lesko earned his B.S. from Stanford University and his Ph.D. from the University of Washington. After a two year stint at Argonne National Laboratory, he came to Berkeley Lab in 1985 for a post-doctoral fellowship in the study of nuclear astrophysics. In 1987 he became a staff scientist with Berkeley Lab’s Nuclear Sciences Division and, shortly thereafter, was immersed in a historic project designed to catch and analyze neutrinos with unprecedented sensitivity. This project, a collaboration involving research teams from the United States, Canada, and the United Kingdom, is called the Sudbury Neutrino Observatory (SNO).
You can think of SNO as a neutrino telescope. Located about a mile underground in a Canadian nickel mine, this “telescope” consists of a geodesic sphere18 meters in diameter, suspended in a huge pool filled with purified water. The outer steel surface of the geodesic sphere is studded with a total of 9,456 ultrasensitive light sensors called photomultiplier tubes. Inside the sphere is an acrylic vessel filled with heavy water (deuterium oxide or D2O).
When a neutrino passing through the heavy water interacts with a deuterium atom, a flash of light called Cerenkov radiation is emitted. The photomultiplier tubes detect these light flashes and convert them into electronic signals that scientists can analyze. These flashes of light signal the real-time presence of neutrino poltergeists.
"It’s vital that we detect as many light flashes as possible and all of the light emitted in each interaction," says Lesko, who oversaw the design and construction of the support structure for SNO's elaborate web of photomultiplier tubes. “Therefore, we had to squeeze as many photomultiplier tubes as possible onto the geodesic sphere while controlling costs and maintaining an installation underground.”
Berkeley Lab engineers solved this challenge with a tesselated sphere surface made up of 700 panels that come in five different shapes constructed from repeating patterns of hexagons. The result was a honeycomb pattern covering 70 percent of the sphere’s surface with photomultiplier tubes. SNO is the only neutrino detector in the world able to catch and measure all three types of neutrinos.
“Completing Berkeley Lab's component of SNO was a big accomplishment," Lesko says. “It was very rewarding after all that work and nearly a year underground to see how well SNO has performed.”
Lesko now heads a Berkeley Lab team that analyzes data from SNO.