7/20/05
CONTACT:
Mark Shwartz, News Service: (650) 723-9296, mshwartz@stanford.edu
COMMENT:
Giorgio Gratta, Physics: (650) 725-6509 and cell (650) 387-9658,
gratta@stanford.edu
Atsuto Suzuki, Research Center for Neutrino Science, Tohoku University,
Japan: +81-22-795-6720 and +81-22-217-5123, suzukia@awa.tohuku.ac.jp
The study, ``Experimental Investigation of Geologically Produced
Antineutrinos with KamLAND,`` appears in the July 28 edition of Nature.
Images are available at
http://hep.stanford.edu/neutrino/KamLAND/Pictures/Pictures.html.
RELEVANT WEB URLS:
THE STANFORD KAMLAND WEBSITE
http://hep.stanford.edu/neutrino/KamLAND/KamLAND.html
EMBARGOED FOR RELEASE until Wednesday, July 27, 2005, at 1 p.m. U.S. Eastern
Daylight Time
Geologically produced antineutrinos provide a new window into the Earth`s
interior
By Dawn Levy
In Jules Verne`s nineteenth century classic Journey to the Centre of the
Earth, an Edinburgh professor and colleagues follow an explorer`s trail down
an extinct volcano to the Earth`s core. Ah, fantasy! Here`s reality: For
more than a century after Verne wrote his novel, geophysicists have had only
one tool with which to peer into our planet`s heart-seismology, or analysis
of vibrations produced by earthquakes and sensed by thousands of instrument
stations worldwide. But now, geophysicists have a new tool for studying the
Earth`s interior, reported in the July 28 issue of the journal Nature.
That tool is a gift from unlikely collaborators-physicists who study
neutrinos, subatomic particles that stars spew out, and their antiparticles,
called antineutrinos, which emanate from nuclear reactors and from the
Earth`s interior when uranium and thorium isotopes undergo a cascade of
heat-generating radioactive decay processes. A detector in Japan called
KamLAND (for Kamioka liquid scintillator antineutrino detector) has sensed
the geologically produced antineutrinos, known as ``geoneutrinos.`` This new
window on the world that geoneutrinos open could yield important geophysical
information, according to the Nature paper`s 87 authors from more than a
dozen institutions and four nations.
``There are still lots of theories about what`s really inside the Earth and
so it`s still very much an open issue,`` said Giorgio Gratta, a Stanford
physics professor who with Stuart Freedman, a nuclear physicist with a joint
appointment at the Lawrence Berkeley National Laboratory and the University
of California-Berkeley, is co-spokesman for the U.S. part of the
collaboration. ``The neutrinos are a second tool, so we`re doubling the
number of tools suddenly that we have, going from using only seismic waves
to the point where we`re doing essentially simple-minded chemical
analysis.``
Said Freedman: ``This is a significant scientific result. We have
established that KamLAND can serve as a unique and valuable tool for the
study of geoneutrinos with wide-ranging implications for physical and
geochemical models of the Earth.``
Added physics Professor Atsuto Suzuki, director of the Research Center for
Neutrino Science, vice president of Tohoku University and a spokesman for
the KamLAND experiment, ``We now have a diagnostic tool for the Earth`s
interior in our hands. For the first time we can say that neutrinos have a
practical interest in other fields of science.``
The Japanese Ministry of Education, Culture, Sports, Science and Technology;
the Japan Society for the Promotion of Science; and the U.S. Department of
Energy funded the experiment.
Receiving their doctorates as a result of work reported in the paper were
two of the authors-Nikolai Tolich, a former Stanford doctoral candidate who
is now a postdoctoral fellow at the Lawrence Berkeley National Laboratory,
and Sanshiro Enomoto of Tohoku University.
In the dark to see the light
``How well do we know our planet?`` Gratta asked. ``We have very few
diagnostics. We only know essentially the crust of our planet. We can
measure mountains. We can sample rocks on the surface of the Earth. We can
drill holes a few kilometers deep and sample stuff down there, but in terms
of chemical analysis or what kind of rocks there are, beyond a few
kilometers, you simply don`t have access.``
What scientists can learn from seismology is limited. Seismic waves travel
through the planet as either compressional waves, which pulse like sound and
can travel through anything, or shear waves, which wobble side-to-side like
shaken jelly but cannot propagate in liquids, which cannot store the energy
needed to generate side-to-side motions. These waves travel at different
speeds and refract differently when they traverse the interfaces between
different types of rocks. So seismology gives information about the
locations of boundaries of different types of rock, Gratta said.
Geoneutrinos, in contrast, provide crude information about chemistry.
``Essentially [geoneutrinos reveal] just the chemistry of how much uranium
and how much thorium is there,`` Gratta said. ``You don`t know anything
about the crystal structure, whether the thorium is thorium oxide or thorium
nitride. But still, when you know nothing, knowing a little bit already
makes a big difference. This is really the first tool to actually do this.``
Scientists originally built KamLAND in 1997 to reproduce in the lab, using
antineutrinos from nuclear reactors, what they saw in nature with solar
neutrinos-the phenomenon that the three ``flavors`` of
neutrinos/antineutrinos ``oscillated,`` or turned into the other flavors, as
they propagated through space. They saw the same thing in both cases.
Previously neutrinos were thought to lack mass, but the oscillations told
them that neutrinos must have a very tiny mass-less than 500,000 times less
than that of an electron, Gratta said.
``That was a big deal because there`s lots of neutrinos in the universe, and
the mass of the universe is to some extent influenced by the mass of those
neutrinos,`` Gratta said.
Unlike the energetic sun, which is a gigantic generator of neutrinos, the
Earth emits only a modest number of antineutrinos-and scientists need a huge
detector to be able to see them. KamLAND was built with the size and
sensitivity required to detect Earth-made antineutrinos. In a cavern
underneath a Japanese mountain shielding the experiment from the background
noise of cosmic radiation, KamLAND consists of about 2,000 photomultiplier
tubes, each 20 inches (51 centimeters) in diameter and contained in a
59-foot (18-meter) vessel, bathed in 1,000 tons of liquid scintillator.
``Scintillator is essentially a mix of baby oil-lots of it-and benzene,``
Gratta explained. ``To this cocktail you add a little bit of fluorescent
material. When particles interact with this cocktail, they make a little
flash of light that then is recorded by light sensors. These are the
photomultiplier tubes.``
The detector sees when particles arrive and measures their energies. Nuclear
reactors produce antineutrinos quickly-the detector sees about one a day.
The Earth is not so prolific-the detector sees about one a month.
Antineutrinos from nuclear reactors have a different energy spectrum than
those from the Earth`s interior, so scientists can tell them apart. Thorium
and uranium also have different energy spectra, so scientists can tell the
geoneutrinos made from each apart, too.
Future possibilities
What`s next? Bigger detectors are on many scientists` wish lists. A larger
detector would allow scientists to spot an event every few days instead of
one a month. Ideally, the detector would be far from nuclear reactors in a
location with well-characterized surface geology. Some scientists have
considered placing large detectors in mines in Australia, South Africa,
Canada and South Dakota. Others favor underwater detectors near island
systems such as Hawaii. ``The ocean water would shield cosmic radiation, and
the very thin oceanic crust would contribute little to the neutrino signal,
giving the best sensitivity to neutrinos from deep inside the planet,``
Gratta explained.
Norman Sleep, a Stanford geophysics professor, thinks geoneutrinos will
bring his field revolution, not evolution. Radioactive heat drives plate
tectonics, he said, and getting accurate ratios of thorium to uranium
isotopes will help scientists better understand deep-Earth processes. The
KamLAND results, while of limited statistical power, show a number of
neutrinos consistent with what`s expected from existing models, the Nature
authors wrote. ``Now we`ll be able to resolve the Earth as a sphere,`` Sleep
said.
``It`s a revolution,`` Gratta agreed, ``but let me temper this a little bit
with the physicists` point of view-that is, those are very difficult
measurements and those detectors are very expensive and large. So before the
revolution really comes to fruition, I think it`ll take some time, I would
imagine one or two decades, before we have more of those detectors and maybe
larger ones built in the appropriate place for geophysics.``
Researchers from the following KamLAND collaborating institutions also
participated in the study: University of Alabama, California Institute of
Technology, Drexel University, University of Hawaii-Manoa, Kansas State
University, Louisiana State University, University of New Mexico, University
of Tennessee, Duke University, University of North Carolina-Chapel Hill,
North Carolina State University, Beijing Institute of High Energy Physics,
University Bordeaux I and CNRS (France).
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