Doubles match. In some atomic nuclei with triaxial symmetry, a lone proton and neutron whizzing around a whirling nuclear core give different nuclei mirror-image values of momentum.

Nuclei Crash Through the Looking Glass

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.

Related sites

Frauendorf's home page
An upcoming conference on high spin physics, where triaxial symmetry will be discussed

Posted in Physics