Academic Press
Gloves do
it. Toupees do it. Even twists of DNA do it. And now, for the
first time, physicists have discovered that some atomic nuclei
come in right- and left-handed models, too. In the 5 February
issue of Physical Review Letters, a team of researchers reports
observations of rapidly spinning nuclei morphing into mirror-image
forms. In the process, the physicists also uncovered solid evidence
that a long-disputed type of nuclear symmetry really does exist.
The discovery
springs from work by nuclear theorist Stefan Frauendorf of the
University of Notre Dame in Indiana, who was exploring a hypothetical
property of atomic nuclei called triaxial symmetry. It describes
a possible arrangement of neutrons and protons that would cause
the nucleus to be oblong, sort of like a kiwi fruit. In 1997,
Frauendorf suggested that certain triaxial nuclei should come
in left- and right-handed varieties. His calculations showed that
the development of handedness should occur in rapidly rotating
"odd-odd" nuclei--those containing both an odd number
of neutrons and an odd number of protons.
Protons and
neutrons in the center of the atom pair up, like with like, to
create their own structures inside the nucleus. In an odd-odd
nucleus, however, one neutron and one proton are left over. They
spin, as does the nuclear core of paired neutrons and protons.
Because the core can spin in either of two directions, the nucleus's
overall momentum can take on two different values--which Frauendorf
said would establish left-handed and right-handed states.
The catch
was that nobody knew whether triaxial nuclei really exist. Nuclei
with three axes of symmetry had been predicted in the 1960s and
hotly debated ever since, but no one had definitively observed
one. Some physicists suspected that the triaxial shape might be
a fleeting oscillation of the nucleus, too unstable to have a
measurable effect.
To find out,
a team led by Krzysztof Starosta of the State University of New
York, Stony Brook, shot beams of heavy ions--carbon, boron, and
magnesium--into targets of tin and antimony. The smashups initiated
fusion reactions that created odd-odd nuclei and pumped them up
to the right spin states. As the nuclei settled down, they emitted
gamma rays with various energies. Some of the gamma rays clustered
into pairs of closely related frequencies corresponding to right-
and left-handed states. This could have happened only if triaxial
symmetry is a stable feature of the nuclei.
"These
results are causing quite a stir among nuclear structure physicists,"
says Rod Clark of Lawrence Berkeley National Laboratory in California.
Although more work is needed to nail down the conclusions, Clark
says, "it is tremendously difficult to come up with an alternative
interpretation" of the findings.
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