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Carbon wears a halo

Discovery of the heaviest known Borromean nucleus provides a new testing ground for fundamental nuclear models

In addition to lithium-11 and berrylium-14, the neutron-rich isotope carbon-22 (22C) could also be a Borromean ‘halo’ nucleus, a team of researchers from Japan has reported in Physical Review Letters. The finding will allow physicists to test fundamental nuclear models in nuclei containing a high ratio of neutrons to protons.

To a good approximation, the atomic nucleus is a uniformly dense distribution of protons and neutrons packed into a spherical drop a few femtometers (10-15 m) in radius. However, isotopes that contain more than 2 to 3 neutrons for every proton start to ‘leak’ neutrons. For a very few of these neutron rich nuclei, one or two excess neutrons form a loosely bound orbit—or halo—about the nuclear core.

The two-neutron halo nucleus is a special quantum three-body system: if one of the neutrons in the halo is removed, the remaining part falls apart. This interdependent system of two neutrons and a core is called a ‘Borromean’ nucleus, because of its similarity to the three, interlocked Borromean rings.

“[Previously], only the instability of 21C suggested that 22C might be a Borromean nucleus, and hence have a two-neutron halo,” explains Kanenobu Tanaka from the RIKEN Nishina Center for Accelerator-Based Science in Wako. “To study [whether 22C has the halo structure], we assembled a large-scale collaboration among institutions with expertise on many techniques. For example, special detector settings had to be prepared and creating the beams of carbon isotopes required careful tuning.”

Nuclei with a high neutron-to-proton ratio are unstable and can only be made artificially. Using the RIKEN projectile fragment separator (RIPS) the researchers produced three isotopes of carbon—19C, 20C and 22C—from the fragments of a high-energy beam of argon that impinged on a tantalum target. They then bombarded the carbon nuclei against a liquid hydrogen cell. Since larger nuclei are more likely to strike the hydrogen protons in the liquid, the researchers could determine the size of each carbon isotope by measuring its frequency of collision.

Tanaka and colleagues found that the radius of the 22C was about 5.4 fm, which is more than 50% larger than theoretical predictions, providing strong evidence that 22C is a halo nucleus and making it the heaviest Borromean nucleus ever observed.

“This finding opens the possibility to find halo nuclei in a more extended region of the nuclear chart and will give us greater insight into the mechanism of halo formation,” says Tanaka.

The corresponding author for this highlight is based at the Research Instruments Group BigRIPS Team, RIKEN Nishina Center for Accelerator-Based Science

Journal information

1. Tanaka, K., Yamaguchi, T., Suzuki, T., Ohtsubo, T., Fukuda, M., Nishimura, D., Takechi, M., Ogata, K., Ozawa, A. Izumikawa, T. et al. Observation of a large reaction cross section in the drip-line nucleus 22C. Physical Review Letters 104, 062701 (2010)

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