New Type of Dipole Vibration in Nuclei

Abstract

l) Included among these are some neutron dripline nuclei, e.g., l1Li, 14Be and 17B. All of these nuclei have been shown to have extremely large root mean square radii. The large radius is clearly related to the very small two-neutron separation energy: This suggests that several valence nucleons are coupled to a core very weakly. In those nuclei with neutron or proton excess it is tempting to explore a new type of motion in which the core part and the loosely bound excess part play a characteristic role. Let us consider the electric dipole mode as an example. The Goldhaber-Teller (GT)2) and Steinwedel-Jensen (SJ)3) models are known as macroscopic models for the giant dipole resonance (GDR). The GT model treats a displacement mode of the bulk of protons against the neutrons assuming that the proton and neutron fluids are incompressible, while the SJ model considers the GDR as the acoustic mode of density oscillations assuming that both the fluids are compressible but the total fluid is incompressible. Both of the GT and SJ models consider the out-of-phase density oscillation of all the protons and all the neutrons, and relate the restoring force against the vibration to the energy of the GDR. It appears possible, however, that the neutrons inside the core of neutron-rich nuclei move together with the protons because of their tight binding but the excess neutrons move against the protons leaving the center-of-mass at rest. The restoring force corresponding to this kind of dipole vibration is expected to be smaller than that of the GDR. This dipole oscillation, which we call pygmy dipole resonance (PDR), will thus appear at lower excitation energy, although more detailed examination is needed to be definitive because the mass paramet~r of the vibration also becomes smaller. Hansen and J onson 4 ) speculated about the existence of a similar soft dipole mode, in an attempt to discuss the possible enhancement of Coulomb dissociation cross sections in a loosely bound nucleus such as l1Li.