Helium-8 study gives insight into nuclear theory, neutron stars
(Download printer-friendly, PDF version.)
ARGONNE, Ill. (January 25, 2008) — The most neutron-rich matter that can be
made on Earth—the nucleus of the helium-8 atom—has been created, trapped
and characterized by researchers at the U.S. Department of Energy's Argonne
National Laboratory. This new measurement gives rise to several significant
consequences in nuclear theory and the study of neutron stars.
"This result will help us test the best nuclear structure theories that
are out there right now, including work from the Physics
Division's own theory
group," said Argonne physicist Peter Mueller, who, along with Ph.D. student
Ibrahim Sulai and other Physics Division colleagues, used an innovative laser
trap to confine individual helium-8 atoms long enough to precisely determine
their nuclear charge distribution, a quantity that indicates how the atom's
two protons and six neutrons arrange themselves to form the nucleus.
Unlike stable helium, which usually has two and occasionally one neutron that
pack closely and symmetrically with two protons, the element's unstable isotopes—helium-6
and helium-8—have additional neutrons that form "halos" around the
compact central core. In 2004, the Argonne team had determined that the two
extra neutrons in helium-6 arrange themselves asymmetrically on one side of
the nucleus, a few trillionths of a millimeter away from the core.
In their recent study, however, the researchers discovered that helium-8's
four extra neutrons group themselves differently from helium-6's. The four
helium-8 neutrons in the halo arrange themselves in a less lopsided way around
the core, altering the dynamics of the nucleus.
Helium-6 and helium-8 are both radioactive and decay quickly, complicating
efforts to measure their properties. Helium-8 has a half-life of only a tenth
of a second, meaning that samples of the atom have to be measured "on-line," or
immediately after they are produced, which is not easy in the first place.
Scientists require high-power accelerators to create even a tiny quantity of
these atoms.
In this experiment, the Argonne scientists teamed up with Antonio Villari
and his colleagues from the GANIL cyclotron facility in northern France, one
of a handful of facilities that could generate a sufficient quantity of helium-8.
Still, helium-8 represents only a small fraction of all the atoms that the
cyclotron produces, so scientists needed a way to separate the target atoms
from the rest of the atom stream and to observe each helium-8 atom long enough
for an accurate study.
In order to do so, the scientists created an "atom trap" using six
laser beams to restrain the helium-8 atoms. While other particles in the beam
would fly right past the trap, about once every two minutes one helium-8 atom
would fall into it. The laser beams functioned as the bars of a small cage—if
the atom moved too much to one side, then one of the beams would push it back
towards the middle.
Once the atom was trapped, the scientists shined another pair of laser beams
onto it. By tuning this laser's frequency, they matched the atom's resonant
frequency, causing it to glow bright enough so that Mueller and his colleagues
could tell they had collected it.
Because the atom's resonant frequency depends on its nuclear structure, each
helium isotope glows at a slightly different frequency. With the help of precision
atomic theory calculations provided by collaborator Gordon Drake from the University
of Windsor, Ontario, the researchers were able to use the measured frequency
data to reveal helium-8's nuclear structure.
While the team carried out the experiment at an accelerator in France, Argonne
will soon submit a bid for a new facility that could produce far greater quantities
of helium-8 and other rare isotopes, attracting students and scientists from
all over the world to Illinois.
The proposed Facility for Rare Isotope Beams (FRIB), for which Argonne will
submit a bid, could, for example, generate more than 1,000 times the number
of unstable helium nuclei that researchers are now able to produce in the same
time. "Having access to a facility like FRIB would open up many new possibilities
for research into types of matter nearly impossible to examine otherwise," Mueller
said. "This result shows that we have reached a scientific frontier, and
FRIB would enable us to expand it even further."
A scientific paper on this work, "Nuclear
Charge Radius of 8He," was published
in the December 21 edition of Physical
Review Letters, and was selected as
an “Editors'
Suggestion” to
promote reading across fields.
Argonne National Laboratory brings the world's brightest scientists and engineers
together to find exciting and creative new solutions to pressing national problems
in science and technology. The nation's first national laboratory, Argonne
conducts leading-edge basic and applied scientific research in virtually every
scientific discipline. Argonne researchers work closely with researchers from
hundreds of companies, universities, and federal, state and municipal agencies
to help them solve their specific problems, advance America 's scientific leadership
and prepare the nation for a better future. With employees from more than 60
nations, Argonne is managed by UChicago
Argonne, LLC for the U.S.
Department of Energy's Office
of Science.
By Jared Sagoff.
For more information, please contact Steve McGregor
(630/252-5580 or media@anl.gov)
at Argonne.
|