Neutrinos are among the tiniest particles in the universe. They’re also
among the more perplexing problems physicists face.

Scientists working at huge underground laboratories in Japan and Canada
have made major strides in understanding neutrinos during the last three
years. Now a team working with a particle accelerator at the University
of Washington has added another significant finding, determining with
the greatest precision yet just how many energetic neutrinos are
generated in the sun’s nuclear furnace.

They found that the fusion rate producing those subatomic particles in
the sun is 17 percent greater than previously thought, said Kurt Snover,
a UW research professor in physics who heads the team. And, he added,
the new number is accurate to within 3 or 4 percentage points, compared
to 15 points for the old standard. The findings mean the sun must be
producing 17 percent more energetic neutrinos (the highest-energy solar
neutrinos) than scientists previously thought.

The research, published in the Jan. 28 edition of Physical Review
Letters, was done by Snover, Arnd Junghans, Erik Mohrmann, Tom Steiger,
Eric Adelberger, Jean-Marc Casandjian and Erik Swanson, all of the UW
Center for Experimental Nuclear Physics and Astrophysics. Also taking
part were Lothar Buchman, Sehwan Park and Alex Zyuzin from Canada’s
national particle and nuclear physics laboratory in Vancouver, British
Columbia.

Neutrinos come from several natural sources. The highest-energy
neutrinos are produced by cosmic rays outside the solar system, while
solar neutrinos have lower energies. Either way, the particles come in
three types — what physicists call “flavors” — electron, muon and
tau. A fourth type, called a sterile neutrino, also could be a factor.

Experiments at the Super-Kamiokande detector in Japan and the Sudbury
Neutrino Observatory in Canada in the last three years have shown that
these particles can change from one flavor to another, proof that
neutrinos have mass. That finding is important for researchers trying
to find the so-called “missing mass” of the universe, mass that is
theorized to have resulted from the Big Bang but which so far has
been unaccounted for.

For more than two decades there has been a physics problem associated
with solar neutrinos. An experiment begun in 1965 at Homestake Gold
Mine in South Dakota proved that the particles were bombarding Earth,
but at far lower levels than expected.

“People didn’t know if the calculations of how many neutrinos coming
from the sun was wrong, whether there was a problem with the experiment
or a third possibility, that neutrinos traveling from the sun changed
their character,” Snover said.

Together, the Sudbury and Super-Kamiokande experiments demonstrated the
third possibility was the correct one. All solar neutrinos start in the
electron “flavor,” but the two underground experiments showed that some
of those change to the tau or muon varieties, and that the total of all
three varieties coming from the sun totals roughly what physicists
would expect to see.

The solar neutrino production rate, determined from calculations
performed at other institutions, is figured from the sun’s temperature;
the amounts of beryllium, hydrogen, helium and other materials used for
fuel; and the rates at which the materials combine with each other,
including the fusion rate of beryllium-7 with hydrogen.

“We determined this fusion rate much more precisely with our experiment,”
Snover said.

The UW team used a particle accelerator to fire protons (nuclei of
hydrogen atoms) in a particle beam at a tiny piece of beryllium-7, a
radioactive metal isotope. In the experiment, the beryllium-7 is held
on a rotating arm that moves the metal in front of the particle beam,
where it is bombarded by high-speed protons and transformed into
boron-8. The arm then quickly swings the metal fragment in front of a
detector that verifies that boron-8 has been produced. From there the
scientists deduce neutrino production, since the new isotope has a
half-life of less than a second before it gives off a neutrino. By
studying the boron-8 production in the experiment, Snover’s group is
able to determine a key factor in calculating the solar neutrino
production rate.

The U.S. Department of Energy and the Natural Sciences and Engineering
Research Council of Canada finance the work, which is a step in
improving the understanding of particle physics.

“It helps us understand better the differences in mass, as well as other
properties, among these character-changing neutrinos,” Snover said.

“It’s factors like these that go into the soup pot when you’re trying to
figure out what are the properties of a neutrino.”

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For more information, contact Snover at (206) 543-4022, (206) 543-4080
or snover@npl.washington.edu

IMAGE CAPTION:

[http://www.washington.edu/newsroom/news/images/neutrino.jpg (391KB)]
A small piece of beryllium-7 is affixed to the top of the rotating bar
(center), swings into the path of the beam (right) and then immediately
moves in front of the detector (left) to verify boron-8 production.
Neutrino emission quickly follows boron-8 production. (Photo by Arnd
Junghans)