A carbon halo

In most nuclei the protons and neutrons form a roughly spherical core of approximately uniform density. But along the edges—the so-called drip lines—of the chart of nuclides a handful of light nuclei have more nucleons than can be accommodated in the nuclear core. The excess, usually one or two neutrons, form a dilute distribution called a halo that extends far beyond the core. At the RIKEN Nishina Center for Accelerator-Based Science, a Japanese team has studied the reaction of heavy carbon nuclei with hydrogen and identified the extremely neutron-rich carbon-22, with its 6 protons and 16 neutrons, as a halo nucleus, the heaviest one yet found. Nuclear radii generally scale as the cube root of the total number of protons and neutrons, yet based on their cross-section data, the researchers calculated the radius of ²²C to be twice that of the much more common isotope ¹²C; indeed, at 5.4 fm it exceeds the radius of lead-208. The halo of ²²C comprises two valence neutrons; determining their distribution and other aspects of the halo structure will require experiments with different target nuclei and different beam energies. (K. Tanaka et al., Phys. Rev. Lett., in press.)—Richard Fitzgerald