More than a mile deep within the bowels of a Canadian nickel mine,
scientists for the first time have counted all the solar neutrinos that
are hitting the Earth, researchers announced yesterday.
Neutrinos are elusive, subatomic particles so small that thousands of
them pass unimpeded through every human being every second.
The research, at the Sudbury Neutrino Observatory, near Sudbury,
Ontario, demonstrated conclusively that there is no "solar neutrino
deficit," scientists said. Neutrinos produced by the sun are reaching the
Earth instead of mysteriously vanishing en route, as some scientists had
"Previous experiments have only seen one-third to one-half of what
there is," said the University of Pennsylvania's Eugene Beier, one of an
army of physicists involved in the experiment. "For the first time, we've
been able to measure all of the neutrinos at once. You've got it all."
The new neutrino research has important implications for two of the
enduring quandaries of physics. The Standard Model proposes a massless
neutrino as part of its description of how nature works at its most basic
level, but yesterday's announcement provided fresh confirmation that
neutrinos have mass. Now, Beier said, "the Standard Model will need new
At the same time, even though neutrinos have a tiny mass -- only as
much as one ten-millionth of an electron -- they may contribute to an
understanding of "dark matter," the view shared by astronomers that 90
percent of the matter in the universe is in an unknown, invisible
Yet neutrinos' contribution is not large, Beier said. "Neutrinos
probably have enough mass to be comparable to the stars in the universe.
That may seem like a lot, but the stars don't really make up a lot of the
Results of the Sudbury research were presented yesterday in Albuquerque
at a joint meeting of the American Physical Society and the American
Astronomical Society. A paper with 178 co-authors has been submitted for
publication in Physical Review Letters.
Scientists have known of the existence of neutrinos since 1959, and
subsequently confirmed the existence of three neutrino forms, or "flavors"
-- electron neutrinos, muon neutrinos and tau neutrinos. Besides solar
fusion, cosmic rays in the Earth's atmosphere also create neutrinos.
Neutrinos are so small that they can pass almost unimpeded through
significant thicknesses of virtually any material on Earth. When
scientists tried to measure them, they found many fewer than calculations
based on solar fusion would suggest. This was the origin of the
The neutrino has begun surrendering its secrets in recent years,
however, as scientists brought on-line special underground laboratories
that block access to all but the most persistent neutrinos, whose
activities can be measured and photographed as they smack into
In the late 1990s scientists confirmed that atmospheric neutrinos
"oscillate," or change flavors, as they journey from the sun to the Earth.
Physicists agree that oscillation would not be possible unless neutrinos
In 1999, a team of scientists from Canada, the United States and the
United Kingdom completed construction and calibration of the Sudbury
observatory, built inside Canada's Creighton Nickel Mine.
The lab is the equivalent of a 10-story building constructed 6,800 feet
below the Earth's surface. It contains a spherical tank 12 meters in
diameter filled with 1,000 metric tons of heavy water -- water composed of
heavy isotopes of hydrogen. The tank is monitored by about 10,000 light
Last year, in its first report since the observatory began operation,
the research team showed that the "deficit" existed not because the
neutrinos were mysteriously disappearing, but simply because muon and tau
neutrinos could not be reliably counted.
But that experiment was based on tabulations comparing Sudbury data
that measured only electron neutrinos with data from another underground
lab that was measuring electron neutrinos and a bit of something else --
presumably muons and taus.
Then, however, the Sudbury scientists dug deeper into their data. By
screening out residual radioactivity and other unwanted interference, they
isolated the reaction they wanted.
"When the neutrino enters the tank, it can collide with a heavy water
nucleus and knock a neutron loose," explained University of Washington
physicist Hamish Robertson, another member of the team. "The neutron will
bounce around, hit another nucleus and combine to form a tritium nucleus
[another type of heavy water] and give off a gamma ray at the same
By counting the gamma rays, scientists know how many neutrinos there
are, because "any flavor of neutrino will cause this to occur," Robertson
said. "This is a very direct and obvious way to measure all the neutrinos
in a single operation."