engleski

Covalent binding on the femtometer scale : nuclear molecules

Nuclear molecules are objects having two or more individual clusters as centres with extra nucleons (usually neutrons) binding them. The clusters have to be strongly bound themselves, while they get bound into molecules due to the specific properties of the nucleus-nucleus potentials and exchange of nucleons. The molecular, quantum mechanical covalent binding effect via the exchange of neutrons is the dominant source of binding in many light nuclei, overcoming the Coulomb repulsion in such structures. This results in states having valence neutrons in the pi- and sigma- orbitals, known from molecules in atomic physics ; in this paper we discuss the structural properties of such states in light nuclei. The interaction between clusters resulting in nuclear molecules should show a repulsive interaction at smaller distances. This is generally the case for strongly bound clusters, like alpha-particles (but also for the 12C and 16O nuclei) - due to the Pauli principle, nucleons penetrating the second cluster show then a strong repulsion effect. The particular properties of nuclear clusters needed for the formation of bound molecules is their intrinsic stiffness (a large gap to the first excited states), which inhibits their excitation and allows their survival in a two-centre configuration. A large number of strongly deformed nuclear states in light nuclei with neutron excess have been experimentally identified in the last decades, and some of them have been associated with covalent structures, mainly via their grouping into rotational bands. These results have been confirmed in many theoretical studies with nuclear cluster models, but also in model independent calculations, e.g. in the Antisymmetrized Molecular Dynamics approach, where no a priory cluster structure is assumed.

nuclear structure, nuclear reactions

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