Dipole excitation and geometry of borromean nuclei

We analyze the Coulomb breakup cross sections of $^{11}$Li and $^6$He nuclei using a three-body model with a density-dependent contact interaction. We show that the concentration of the B(E1) strength near the threshold can be well reproduced with this model. With the help of the calculated B(E1) value, we extract the root-mean-square (rms) distance between the core nucleus and the center of mass of two valence neutrons without resorting to the sum rule, which may suffer from unphysical Pauli forbidden transitions. Together with the empirical rms distance between the neutrons obtained from the matter radius study and also from the three-body correlation study in the break-up reaction, we convert these rms distances to the mean opening angle between the valence neutrons from the core nucleus. We find that the obtained mean opening angles in $^{11}$Li and $^6$He agree with the three-body model predictions.Comment: 4 pages, 4 eps figure

Collisions of Deformed Nuclei and Superheavy-Element Production

A detailed understanding of complete fusion cross sections in heavy-ion collisions requires a consideration of the effects of the deformation of the projectile and target. Our aim here is to show that deformation and orientation of the colliding nuclei have a very significant effect on the fusion-barrier height and on the compactness of the touching configuration. To facilitate discussions of fusion configurations of deformed nuclei, we develop a classification scheme and introduce a notation convention for these configurations. We discuss particular deformations and orientations that lead to compact touching configurations and to fusion-barrier heights that correspond to fairly low excitation energies of the compound systems. Such configurations should be the most favorable for producing superheavy elements. We analyse a few projectile-target combinations whose deformations allow favorable entrance-channel configurations and whose proton and neutron numbers lead to compound systems in a part of the superheavy region where alpha half-lives are calculated to be observable, that is, longer than 1 microsecond.Comment: 15 pages. LaTeX with iopconf.sty style file. Presented at 2nd RIKEN/INFN Joint Symposium, Wako-shi, Saitama, Japan, May 22-26, 1995. To be published in symposium proceedings by World Scientific, Singapore. Seven figures not included here. PostScript version with figures available at http://t2.lanl.gov/pub/publications/publications.html or at ftp://t2.lanl.gov/pub/publications/riken9

Spin-isospin Response in Finite Nuclei from an Extended Skyrme Interaction

The magnetic dipole (M1) and the Gamow-Teller (GT) excitations of finite nuclei have been studied in a fully self-consistent Hartree-Fock (HF) plus random phase approximation (RPA) approach by using a Skyrme energy density functional with spin and spin-isospin densities. To this end, we adopt the extended SLy5st interaction which includes spin-density dependent terms and stabilize nuclear matter with respect to spin instabilities. The effect of the spin-density dependent terms is examined in both the mean field and the spin-flip excited state calculations. The numerical results show that those terms give appreciable repulsive contributions to the M1 and GT response functions of finite nuclei.Comment: 6 pages, 2 figure

Charge radius and dipole response of 11^{11}Li

We investigate the consistency of the measured charge radius and dipole response of $^{11}$Li within a three-body model. We show how these observables are related to the mean square distance between the $^9$Li core and the center of mass of the two valence neutrons. In this representation we find by considering the effect of smaller corrections that the discrepancy between the results of the two measurements is of the order of 1.5$\sigma$. We also investigate the sensitivity to the three-body structure of $^{11}$Li and find that the charge radius measurement favors a model with a 50% s-wave component in the ground state of the two-neutron halo, whereas the dipole response is consistent with a smaller s-wave component of about 25% value.Comment: 6 pages, 3 figure

Generalized Jarzynski Equality under Nonequilibrium Feedback Control

The Jarzynski equality is generalized to situations in which nonequilibrium systems are subject to a feedback control. The new terms that arise as a consequence of the feedback describe the mutual information content obtained by measurement and the efficacy of the feedback control. Our results lead to a generalized fluctuation-dissipation theorem that reflects the readout information, and can be experimentally tested using small thermodynamic systems. We illustrate our general results by an introducing "information ratchet," which can transport a Brownian particle in one direction and extract a positive work from the particle

BCS-BEC crossover of neutron pairs in symmetric and asymmetric nuclear matter

We propose new types of density dependent contact pairing interaction which reproduce the pairing gaps in symmetric and neutron matter obtained by a microscopic treatment based on the nucleon-nucleon interaction. These interactions are able to simulate the pairing gaps of either the bare interaction or the interaction screened by the medium polarization effects. It is shown that the medium polarization effects cannot be cast into the density power law function usually introduced together with the contact interaction and require the introduction of another isoscalar term. The BCS-BEC crossover of neutrons pairs in symmetric and symmetric nuclear matter is studied by using these contact interactions. It is shown that the bare and screened pairing interactions lead to different features of the BCS-BEC crossover in symmetric nuclear matter. For the screened pairing interaction, a two-neutron BEC state is formed in symmetric matter at $k_{Fn}\sim 0.2$ fm$^{-1}$ (neutron density $\rho_n/\rho_0\sim 10^{-3}$). Contrary the bare interaction does not form the BEC state at any neutron density

Effect of pairing correlations on incompressibility and symmetry energy in nuclear matter and finite nuclei

The role of superfluidity in the incompressibility and in the symmetry energy is studied in nuclear matter and finite nuclei. Several pairing interactions are used: surface, mixed and isovector dependent. Pairing has a small effect on the nuclear matter incompressibility at saturation density, but the effects are significant at lower densities. The pairing effect on the centroid energy of the isoscalar Giant Monopole Resonance (GMR) is also evaluated for Pb and Sn isotopes by using a microscopic constrained-HFB approach, and found to change at most by 10% the nucleus incompressibility $K_A$. It is shown by using the Local Density Approximation (LDA) that most of the pairing effect on the GMR centroid come from the low-density nuclear surface.Comment: 9 pages, 6 figure

Effective pairing interactions with isospin density dependence

We perform Hartree-Fock-Bogoliubov (HFB) calculations for semi-magic Calcium, Nickel, Tin and Lead isotopes and $N$=20, 28, 50 and 82 isotones using density-dependent pairing interactions recently derived from a microscopic nucleon-nucleon interaction. These interactions have an isovector component so that the pairing gaps in symmetric and neutron matter are reproduced. Our calculations well account for the experimental data for the neutron number dependence of binding energy, two neutrons separation energy, and odd-even mass staggering of these isotopes. This result suggests that by introducing the isovector term in the pairing interaction, one can construct a global effective pairing interaction which is applicable to nuclei in a wide range of the nuclear chart. It is also shown with the local density approximation (LDA) that the pairing field deduced from the pairing gaps in infinite matter reproduces qualitatively well the pairing field for finite nuclei obtained with the HFB method

Microscopic Study of the Isoscalar Giant Monopole Resonance in Cd, Sn and Pb Isotopes

The isoscalar giant monopole resonance (ISGMR) in Cd, Sn and Pb isotopes has been studied within the self-consistent Skyrme Hartree-Fock+BCS and quasi-particle random phase approximation (QRPA). Three Skyrme parameter sets are used in the calculations, i.e., SLy5, SkM* and SkP, since they are characterized by different values of the compression modulus in symmetric nuclear matter, namely K=230, 217, and 202 MeV, respectively. We also investigate the effect of different types of pairing forces on the ISGMR in Cd, Sn and Pb isotopes. The calculated peak energies and the strength distributions of ISGMR are compared with available experimental data. We find that SkP fails completely to describe the ISGMR strength distribution for all isotopes due to its low value of the nuclear matter incompressibility, namely K=202 MeV. On the other hand, the SLy5 parameter set, supplemented by an appropriate pairing interaction, gives a reasonable description of the ISGMR in Cd and Pb isotopes. A better description of ISGMR in Sn isotopes is achieved by the SkM* interaction, that has a somewhat softer value of the nuclear incompressibility.Comment: Submitted to Phys. Rev.