Monopole Excitation and Nuclear Compressibility: Present and Future Perspectives

Abstract

Isoscalar giant resonances are nuclear collective excitations associated with the oscillation in phase of protons and neutrons according to a certain multipolarity L . In particular, the isoscalar giant monopole resonance ( L = 0 ) is the strongest nuclear compression mode, and its excitation energy is directly related to the compression modulus for finite nuclei. Typically, microscopic calculations are utilized to establish a relationship between the experimental compression modulus and the nuclear incompressibility that is a crucial parameter of the equation of state (EOS) for nuclear matter. The incompressibility of nuclear matter has been determined with an accuracy of 10–20% using relativistic and non-relativistic microscopic models for describing the monopole distributions in 208Pb and 90Zr isotopes. However, the same theoretical models are not able to describe data for open-shell nuclei, such as those of tin and cadmium isotopes. In fact, only effective interactions with a softer nuclear-matter incompressibility are able to predict the centroid energy of monopole distributions for open-shell nuclei. A unified description of the monopole resonance in 208Pb and other open-shell nuclei remains unsolved from the theory side. Most of this uncertainty is due to poor knowledge of the symmetry energy, which is another essential component of the EOS of nuclear matter. Therefore, experimental data on isotopic chains covering a wide range of N : Z ratios, including neutron-deficient and neutron-rich nuclei, are of paramount importance for determining both the nuclear-matter incompressibility and the symmetry energy more precisely. Novel approaches using inverse kinematics have been developed to achieve giant resonance experiments with unstable nuclei. The active target and storage ring are potentially the most feasible methods for measuring giant resonances in nuclei far from stability. The combination of these techniques with high-intensity radioactive beams at new accelerator facilities will provide the means to explore the nuclear-matter properties of the most exotic nuclei.