New experiments based on recent advances in radioactive ion beams and detectors reveal a wealth of structural phenomena in exotic nuclei. Having theoretical nuclear models capable of describing the properties of these exotic nuclei far from the region of stability is crucial to understand the interactions and configurations of nucleons in the exotic nuclei, to guide future experiments, and to predict the radioactive decays of exotic nuclei.
In a study published in Physics Letters B, the researchers from the Institute of Modern Physics (IMP) of the Chinese Academy of Sciences (CAS) and the Technische Universit？t München proposed an optimized point-coupling interaction for the relativistic Hartree-Bogoliubov framework to quantify the nuclear bulk properties. The outcome serves as a stepping stone to the nuclear physics community to further investigate and predict nuclear properties of exotic nuclei. The covariant density functional theory is one of the most successful models for the study of nuclear structure. The relativistic Hartree-Bogoliubov framework belongs to the covariant density functional theory.
"We benefit from covariant density functional theory such as the advantage of the natural inclusion of the nucleon spin degree of freedom. The consistent treatment of isoscalar Lorentz-scalar and -vector self-energies provides a unique parametrization of time-odd components of the nuclear mean field. A natural explanation of the empirical pseudospin symmetry is also permitted. Moreover, we can get a distinction between scalar and four-vector nucleon self-energies for a new saturation mechanism of nuclear matter,” said LIU Zixin, the first author of the paper and a Ph.D. student at IMP.
The researchers had taken more observables into account for optimizing the point-coupling interaction, i.e., the binding energies of 91 spherical nuclei, charge radii of 63 nuclei, and 12 sets of mean pairing gaps consisting of 54 nuclei in total. To better reproduce the experimental odd-even oscillations of the binding energies of nuclei, the researchers optimized the separable proton and neutron pairing strengths together with the point-coupling interaction.
"The results of the fit for the pairing strength show that the effective pairing-constant for the protons is smaller than that for the neutrons. This could be connected with the fact that the repulsive contribution of the Coulomb force to the pairing correlations reduces the size of the effective attractive pairing strength for protons,” said Peter Ring, one of the authors of the paper and the professor emeritus of Technische Universit？t München.
Besides, the results indicated that the newly optimized point-coupling interaction, with the acronym PC-L3R, is able to predict the best description on the experimentally determined binding energies of 91 spherical nuclei among all point-coupling interactions, and the calculated charge radii of 63 nuclei satisfactorily agree with the respective experimental findings.
"The PC-L3R point-coupling interaction provides a reliable alternative set of references for future studies based on the covariant density functional theory. We expect that the PC-L3R interaction is capable of describing the properties of exotic nuclei and guiding future experiments. These properties include the binding energies, radioactive decays, and radii, of which can be measured in future advanced experiments,” said LAM Yi Hua from IMP, one of the authors of the paper.
91 spherical nuclei (red dots) are plotted against the nuclear chart consisting of all experimentally discovered nuclei (background squares). (Image by LU Ning and LAM Yi Hua)
