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It may look like the eye
of a giant reptile, but in fact it is a 12-meter-wide acrylic vessel,
now filled with a thousand tons of heavy water, at the core of
the SNO experiment. Surrounding the vessel, mounted on a geodesic
steel support structure, are the photomultiplier tubes which will
record neutrino
events. |
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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.
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Nearly 10,000 ultrasensitive
photomultiplier tubes form a snakeskin-looking sheath around SNO’s
18-meters-in-diameter geodesic sphere. These tubes enable SNO to detect
and identify all three types of neutrinos |
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"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.
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