Search

We investigate tensor effects in pygmy dipole excitations for the case of neutron-rich nuclei 68 Ni and 124 Sn using effective nucleon–nucleon Skyrme interaction. We use the Hartree–Fock–Bogoliubov (HFB) theory and employ the quasiparticle random phase approximation (QRPA). We calculate and compare the PDR and also GDR strength in the PDR–GDR energy region for QRPA calculations with and without tensor correlations. The most obvious results for the dipole excitations calculations are strongly dependent on the tensor terms. We see that the tensor correlations are more active at around 14–20 MeV , especially for the neutron-rich nuclei 68 Ni . We also compare the PDR calculations with their experimental results for the different proton–neutron tensor coupling constants.

Vibrational states built on the K π = 9− isomer and on the ground state (K π = 1+) in 180Ta are calculated within the quasiparticle-phonon nuclear model using the 178Hf nucleus as a core. A procedure for calculating the rates of K-allowed γ-ray transitions from vibrational states built on the isomer to those built on the ground state is presented. The probabilities of two-step processes consisting of a dipole excitation of the isomer and successive E1 and E2 transitions from them to vibrational states built on the ground state of the 180Ta nucleus are calculated. Two-step transitions from the isomer to vibrational states below 2.7 MeV and to the vibrational states built on the ground state appear to be very weak. There are many E1 transitions from the vibrational states built on the isomer to the vibrational states built on the ground state. They are weak and cannot be responsible for the strong deexcitation of 180m Ta in the relevant (γ, γ′) reaction. A decisive role is played by collective E2 transitions from dipole excitations in several excitation energy intervals ranging between 2.7 and 4.0 MeV. These highly intense K-allowed two-step γ-ray transitions can be responsible for the strong deexcitation of the 180m Ta state in the (γ, γ′) reactions.

Highlights of RIKEN radioactive beam experiments are reviewed. In particular the phenomena of giant soft dipole excitation of halo nuclei are closely discussed.

Coulomb excitations using radioactive beams are discussed. Applications of such reactions are described including soft dipole excitation of neutron halo nuclei, collective E2 excitation of 32 Mg and astrophysically interesting 8 B breakup.

A general method for the calculation of second order effects in the Coulomb dissociation is presented in the small ξ-limit avoiding the introduction of intermediate states. The amplitudes for a dipole excitation are analytically calculated in a simple model for the projectile with a zero-range force. The model is applied to the excitation of 11 Li at projectile energies of 28 and 800 MeV nucleon . The angular and energy distribution of the fragments and post-acceleration effects are examined in first and second order perturbation theory as well as the sudden approximation.

The mass-dependent structure of the composite nucleus is shown based on three-dimensional timedependent Hartree-Fock calculations with Skyrme interactions (SLy4d and SkM*). One remarkable result is that the isovector monopole excitation dominantly appears for collisions of heavy nuclei, and the isovector dipole excitation for those of light ones. Such a difference found in the dynamical structure of composite nucleus plays a role in the equilibration of charge.

We look for collective excitations originating from the strong surface pairing in unstable nuclei near the neutron drip-line. The soft dipole excitation is such a pairing mode as it exhibits a character of core-vs-dineutron motion. Possibility of the hydrodynamic phonon mode (the Anderson-Bogoliubov mode) is also discussed., Comment: 9 pages, a talk presented at Collective Motion in Nuclei under Extreme Conditions (COMEX2), June 20-23, 2006, St. Goar, Germany

The soft dipole modes in neutron rich even-even Ni-isotopes are investigated in the quasiparticle relativistic random phase approximation. We study the evolution of strengths distribution, centroid energies of dipole excitation in low-lying and normal GDR regions with the increase of the neutron excess. It is found in the present study that the centroid energies of the soft dipole strengths strongly depend on the thickness of neutron skin along with the neutron rich even-even Ni-isotopes., 14 pages, 7 figures

A brief description of exotic states of nuclei involving radioactive beam technology is presented with the emphasis being on the nucleus 11 Li. The status of experiment and theory associated with soft dipole excitation are discussed and suggestions for theoretical improvements are given.

We use a three-body model to discuss the role of dineutron correlation in the ground state and the dipole excitations of typical weakly-bound Borromean nuclei, 11Li and 6He. We first show that the wave function for the neutron pair in the ground state is strongly concentrated on the nuclear surface for both nuclei. We then show that the energy distributions for the two emitted neutrons from the dipole excitation, on the other hand, are considerably different between the two nuclei, even though they show similar strong dineutron correlations in the ground state to each other. Our calculation indicates that this different behaviour of the energy distribution primarily reflects the interaction between the neutron and the core nucleus, rather than the interaction between the valence neutrons.

Inelastic excitations have been studied in 48 MeV α-scattering from 208 Pb. A concentration of isoscalar L = 1 strength is observed at E x ≈ 5.5 Mev which is interpreted as a dipole excitation mode representing a dipole oscillation of the nuclear diffuseness. A consistent description of inelastic excitations and electromagnetic widths is obtained.

Bremsstrahlung-weighted cross-section (bwcs) measurements may be a decisive step to obtain information about the difference between neutron and proton radii. A model-independent relation is discussed between the total bwcs, the relative intensity of the bwcs of the upper ( T > ) fragment of the dipole excitation and the neutron and proton radii.