Dipole Excitations In Nuclei Research Articles

Quasiparticle random phase approximation predictions of the gamma-ray strength functions using the Gogny force

Dipole excitations of nuclei are crucial since they play an important role in nuclear reaction modeling in connection with the photoabsorption and the radiative capture processes. We present here results for the gamma-ray strength function obtained in large-scale axially-symmetric deformed quasiparticle (qp) random phase approximations approach using the finite-range Gogny force, with a particular emphasis on the E1 mode. The convergence with respect to the number of harmonic oscillator shells adopted and the cut-off introduced in the 2-quasiparticle excitation energy space is analyzed. The microscopic nature of our self-consistent Hartree-Fock-Bogoliubov plus QRPA (HFB+QRPA) calculation has unfortunately to be broken, some phenomenological corrections being needed to take into account effects beyond the standard 2-qp QRPA excitations and the coupling between the single-particle and low-lying collective phonon degrees of freedom. The corresponding phenomenological parameters are adjusted on experimental photoabsorption data. In such a procedure, a rather satisfactory description of experimental data is obtained. To study the sensitivity of these phenomenological corrections on the extrapolation, both at low energies and towards exotic neutron-rich nuclei, three different prescriptions are considered. They are shown to lead to rather similar predictions of the E1 strength at low energies as well as for exotic neutron-rich nuclei. The Gogny-HFB+QRPA strength is finally applied to the calculation of radiative neutron capture cross sections and the predictions compared with those obtained with more traditional Lorentzian-type approaches.

Magnetic dipole excitations in nuclei: Elementary modes of nucleonic motion

The nucleus is one of the most multi-faceted many-body systems in the universe. It exhibits a multitude of responses depending on the way one 'probes' it. With increasing technical advancements of beams at the various accelerators and of detection systems the nucleus has, over and over again, surprised us by expressing always new ways of 'organized' structures and layers of complexity. Nuclear magnetism is one of those fascinating faces of the atomic nucleus we discuss in the present review. We shall not just limit ourselves to presenting the by now very large data set that has been obtained in the last two decades using various probes, electromagnetic and hadronic alike and that presents ample evidence for a low-lying orbital scissors mode around 3 MeV, albeit fragmented over an energy interval of the order of 1.5 MeV, and higher-lying spin-flip strength in the energy region 5 - 9 MeV in deformed nuclei, nor to the presently discovered evidence for low-lying proton-neutron isovector quadrupole excitations in spherical nuclei. To the contrary, we put the experimental evidence in the perspectives of understanding the atomic nucleus and its various structures of well-organized modes of motion and thus enlarge our discussion to more general fermion and bosonic many-body systems.

Low-energy dipole excitations in nuclei at the N = 50, 82 and Z = 50 shell closures as signatures for a neutron skin

Low-energy dipole excitations have been investigated theoretically in N = 5088Sr and 90Zr, several N = 82 isotones and the Z = 50 Sn isotopes. For this purpose a method incorporating both HFB and multi-phonon QPM theory is applied. A concentration of one-phonon dipole strength located below the neutron emission threshold has been calculated in these nuclei. The analysis of the corresponding neutron and proton dipole transition densities allows us to assign a genuine pattern to the low-energy excitations and make them distinct from the conventional GDR modes. Analyzing also the QRPA wave functions of the states we can identify these excitations as pygmy dipole resonance (PDR) modes, recently studied also in Sn and N = 82 nuclei. The results for N = 50 are exploratory for an experimental project designed for the bremsstrahlung facility at the ELBE accelerator.

Pairing correlations and soft dipole excitations in nuclei on the neutron-drip line

Paring correlations and soft dipole excitations in weakly bound nuclei on the edge of neutrondrip line are studied by using a three-body model. A density-dependent contact interaction is employed to calculate the ground state of halo nuclei 6He and 11Li, as well as a skin nucleus 24O. Dipole excitations in these nuclei are also studied within the same model. We point out that the dineutron-type correlation plays a dominant role in the halo nuclei 6He and 11Li having the coupled spin of the two neutrons S = 0, while the correlation similar to the BCS type is important in 24O. Contributions of the spin S = 1 and S = 0 configurations are separately discussed in the low-energy dipole excitations. The calculated results are compared with recent experimental data of 6He and 11Li.

Low-lying dipole excitations in nuclei in the mass regions of the new dynamical symmetries

Experimental indications for the nuclear phase transitions described by the new symmetries E(5) and X(5) are discussed for the stable Ba, Nd, and Gd isotopes. Their low-lying dipole excitations, systematically investigated at the bremsstrahlung facility of the 4.3-MV Stuttgart Dynamitron accelerator, provide indications for the phase transitional behavior, including the proposed critical point candidates.

Transport theory of deeply-inelastic heavy-ion collisions based on a random-matrix model II. Study of a one-dimensional model: C. M. Ko, D. Agassi and H. A. Weidenmüller. Max Planck Institut für Kernphysik, Heidelberg, West Germany

This review paper concerns the research devoted to the study of the properties of dipole excitations in nuclei. The main focus is on questions related to isospin effects in these types of excitations. Particular attention is given to the experimental and theoretical efforts made to understand the nature and the specific structure of the low-lying dipole states known as the Pygmy Dipole Resonance (PDR). The main experimental methods employed in the study of the PDR are reviewed as well as the most interesting theoretical aspects. The main features of the experiments and of theoretical models are reported with special emphasis on the reaction cross sections populating the dipole states. Results are organized for nuclei according to different mass regions. The knowledge of the isovector dipole response as well as its low energy part is important in order to deduce the nuclear polarizability as accurate as possible. This issue is discussed in this paper together with the connection with the neutron skin and the nuclear equation of state. The important role played by the dipole response to deduce other physical quantities of general interest is discussed in the last two chapters. One concerns the level density and the other the isospin mixing in nuclei at finite temperature and its relation with beta decay.

Correlations and dipole excitations in nuclei

A microscopic calculation of the effect of two-body correlations on the distribution of dipole strength in nuclei is presented. Large corrections are found for the bremsstrahlung-weighted cross section. An isospin decomposition of the cross section is made and the effect of correlations on the isoscalar, isovector, and isotensor parts is studied. Large corrections to the isotensor part, initially small in the oscillator model, are confirmed. The effects on the photoabsorption cross section of coupling the ground state to the giant isovector monopole vibration are also investigated. This component describes the relative distortion of corresponding neutron and proton wave functions and is shown to be included in an analog spin function. The dominant effect on ground state expectation values of this distortion is shown to be the result of dynamic distortion rather than isospin impurity.NUCLEAR REACTIONS Effect of dynamical correlations on isospin projected sum rules for dipole absorption.

Isobaric splitting of 1− levels in nuclei

Isospin fragmentation of the dipole excitations of nuclei with neutron excess is discussed. Simple expressions are derived for the energy splitting and for the sharing of dipole strength between the two isospin components. The effect of relaxing some assumptions is investigated. The case of 90Zr is taken as an illustration.