Synopsis: Carbon-12 Caught in a Triangle

The discovery of a new excited state in carbon-12 hints at this nucleus’s triangular symmetry.
Synopsis figure
Martin Freer/University of Birmingham

Carbon-12 (12C), like many nuclei, doesn’t conform to the simple picture of the nucleus as a homogenous distribution of protons and neutrons. Instead, its nucleons are thought to cluster into alpha particles (two protons and two neutrons) that arrange into an equilateral triangle, a configuration with D3h symmetry. Several observed nuclear excitations support this picture. Now, Daniel Jose Marin-Labarri at the University of Birmingham, UK, and his colleagues report in Physical Review Letters the measurement of a new state that provides additional evidence for the triangular cluster model of 12C. The finding could point physicists to more accurate models of carbon’s structure, which are needed to better understand carbon nucleosynthesis.

Working at the Birmingham cyclotron facility, the researchers used a beam of high-energy helium-4 to excite and break apart the nuclei in a carbon target. By measuring the momenta of the emitted alpha particles they were able to reconstruct the energy levels of the carbon nuclei. Models of 12C that assume it has D3h symmetry predict a band of rotational states (think of a spinning top) having spin and parity, 0+,2+,3-,4± and 5-; Marin-Lambarri and his colleagues observed the last of these, 5-, at an energy of 22.4 mega-electron-volts.

The measurement could lead to a better understanding of the Hoyle state—an excited state of carbon-12, essential for nucleosynthesis. If, as Marin-Labarri’s data suggest, the D3h cluster model is the right one, the Hoyle state is due to a breathing mode excitation of the triangular configuration. But this contradicts the findings of ab initio calculations (see 9 May 2011 Viewpoint), which suggest that the spatial structure of the Hoyle state is more like a linear chain of three alpha clusters. Future measurements of other states predicted by the two differing models are needed to decide which one best describes carbon. – Kevin Dusling


Features

More Features »

Announcements

More Announcements »

Subject Areas

Nuclear Physics

Previous Synopsis

Related Articles

Viewpoint: Doubly Magic Nickel
Nuclear Physics

Viewpoint: Doubly Magic Nickel

Two independent experiments on the isotope copper-79 confirm that its nuclear neighbor nickel-78 is indeed a doubly magic nucleus. Read More »

Viewpoint: Atom Scattering Picks Out the Heavyweights
Nuclear Physics

Viewpoint: Atom Scattering Picks Out the Heavyweights

Atomic-beam diffraction emerges as a viable approach to separating isotopes within the beam. Read More »

Viewpoint: Watching the Hoyle State Fall Apart
Nuclear Physics

Viewpoint: Watching the Hoyle State Fall Apart

Two experiments provide the most precise picture to date of how an excited state of carbon decays into three helium nuclei. Read More »

More Articles