Light dripline nuclei

Artikel i vetenskaplig tidskrift, 2004

Experimental studies of light dripline nuclei are reviewed. Progress in the production of very short lived nuclei and the development of radioactive nuclear beams has given this field the necessary tools for detailed studies of the most exotic nuclei. A well-known feature for some of the light dripline nuclei is that under certain circumstances they may form a neutron or a proton halo with a dilute mass distribution extending far outside the core of the nucleus. The first observation of halo states was made in the middle of the 1980s and it generated an interest in dripline physics, both experimentally and theoretically, that has gone far beyond the study of halo states. The experimental results for halo states are starting to give a fairly complete understanding of their structure in many cases. The data include masses, spins, moments, reaction data over a wide energy range and beta-decays. There are two main classes of halo state: the two-body halos with one nucleon surrounding the core, like the one-neutron halos Be-11 and C-19 and the one-proton halo B-8; and the Borromean three-body halos with two valence nucleons around the core, the key examples being He-6, Li-11 and Be-14. Experimental information about systems lying just outside the dripline play an important role in understanding the structure of the halo states, examples being Li-10 and Be-13, which form two-body subsystems of Li-11 and Be-14, respectively. Unbound resonance states that correspond to exotic unbound quantum systems like H-5, H-7 and He-9 have been identified. There are continuum states existing above the particle separation threshold as well as spectra indicating cluster or molecular structure. The traditional magic numbers valid for more stable nuclei have been found to disappear and be replaced with new ones in the dripline regions. The beta-decays in these regions may give access to halos in excited states and the associated beta-delayed particle decay modes provide information about coupling to the continuum. After a short historical overview, examples on the most recent experimental results from this rapidly growing field of nuclear physics will be given.