V. Yu. Ponomarev

Complete electric dipole response and the neutron skin in 208Pb

A benchmark experiment on (208)Pb shows that polarized proton inelastic scattering at very forward angles including 0° is a powerful tool for high-resolution studies of electric dipole (E1) and spin magnetic dipole (M1) modes in nuclei over a broad excitation energy range to test up-to-date nuclear models. The extracted E1 polarizability leads to a neutron skin thickness r(skin) = 0.156(-0.021)(+0.025) fm in (208)Pb derived within a mean-field model [Phys. Rev. C 81, 051303 (2010)], thereby constraining the symmetry energy and its density dependence relevant to the description of neutron stars.

Microscopic studies on two-phonon giant resonances

Abstract A new class of giant resonances in nuclei, namely double-giant resonances, is discussed. They are giant resonances built on top of other giant resonances. Investigation on their properties, together with similar studies on low-lying two-phonon states, should give an answer on how far the harmonic picture of boson-type excitations holds in the finite fermion systems like atomic nuclei. The main attention in this review is paid to double-giant dipole resonances (DGDR) which are observed in relativistic heavy ion collisions with very large cross sections. A great experimental and theoretical effort is underway to understand the reaction mechanism which leads to the excitation of these states in nuclei, as well as the better microscopic understanding of their properties. The Coulomb mechanism of the excitation of single- and double-giant resonances in heavy ion collision at different projectile energies is discussed in detail. A contribution of the nuclear excitation to the total cross section of the reaction is also considered. The Coulomb excitation of double resonances is described within both, the second-order perturbation theory approach and in coupled-channels calculation. The properties of single and double resonances are considered within the phenomenologic harmonic vibrator model and microscopic quasiparticle-RPA approach. For the last we use the quasiparticle-phonon model (QPM) the basic ideas and formalism of which are presented. The QPM predictions of the DGDR properties (energy centroids, widths, strength distributions, anharmonicities and excitation cross sections) are compared to predictions of harmonic vibrator model, results of other microscopic calculations and experimental data available.

Boson forbidden low-energy E1-transitions in spherical nuclei

Low-energy E1-transitions in spherical nuclei forbidden in the ideal boson picture are considered. For that the internal fermion structure of nuclear excitations is taken into account. Several examples of such transitions calculated within the Quasiparticle Phonon Model are considered and the role of dipole core polarization is discussed. It is shown that transition probabilities of an order of 10 3 W.u. observed experimentally are well described by this model. c 1998 Elsevier Science B.V.

Magnetic quadrupole resonance in spherical nuclei

Abstract The distribution of the M2 strength in spherical nuclei is studied within the quasiparticle-phonon nuclear model. It is shown that the interaction of the one- and two-phonon states affects strongly this distribution at the excitation energies E x > 15 MeV. In all the nuclei the strength of the M2 transitions is concentrated in the excitation energy region of 6–12 MeV. At these energies the calculated total value of B (M2)↑ is in good agreement with the experimental data in 90 Zr and 208 Pb. The calculations show that a group of states observed in 58 Ni at an energy of about 7 MeV in the (e, e′) experiments is a part of the M2 resonance.

Pygmy dipole resonance in 208Pb

Scattering of protons of several hundred MeV is a promising new spectroscopic tool for the study of electric dipole strength in nuclei. A case study of 208 Pb shows that, at very forward angles, J π = 1 − states are strongly populated via Coulomb excitation. A separation from nuclear excitation of other modes is achieved by a multipole decomposition analysis of the experimental cross sections based on theoretical angular distributions calculated within the quasiparticle-phonon model. The B(E1) transition strength distribution is extracted for excitation energies up to 9 MeV; that is, in the region of the so-called pygmy dipole resonance (PDR). The Coulomb-nuclear interference shows sensitivity to the underlying structure of the E1 transitions, which allows for the first time an experimental extraction of the electromagnetic transition strength and the energy centroid of the PDR.

Observation of a 1− two phonon 2+ ⊗ 3− excitation in 116Sn and 124Sn

Abstract In nuclear resonance fluorescence experiments with unpolarized and linearly polarized bremsstrahlung we have observed for the first time very strong electric dipole transitions at about 3.5 MeV in the spherical, semi magic 116 Sn and 124 Sn nuclei. These transitions can be attributed to possible two phonon 2 + ⊗ 3 − excitations. The measured transition strengths are compared to the results of a QRPA calculation.

Interplay between single-particle and collective degrees of freedom in the excitation of the low-lying states in 142Nd

Abstract The low-lying excited states in 142 Nd were investigated by inelastic electron scattering. The momentum transfer range covered was 0.5–2.8 fm −1 . Transition charge densities were extracted for natural-parity states from 0 + up to 9 − and up to an excitation energy of 3.5 MeV. For several new excited states spin and parity assignments have been suggested. The experimental transition charge densities have been interpreted with the aid of the quasiparticle-phonon model (QPM). The QPM is well-suited to investigate the contribution of collective and single-particle degrees of freedom to excited states in spherical nuclei. On the basis of the QPM calculations it is shown that in 142 Nd both degrees of freedom play an important role, as well as the interplay between them. Both the strength distribution and the structure of the transition charge densities of the low-lying excited states are well described by the calculations. The origin of the structure in the nuclear interior usually predicted by microscopic calculations but not observed experimentally is explained. An argument for the proton number dependence of the excitation energy of the 3 1 − state in the N = 82 isotones is given.

Dipole polarizability of 120 Sn and nuclear energy density functionals

The electric dipole strength distribution in 120Sn between 5 and 22 MeV has been determined at RCNP Osaka from a polarization transfer analysis of proton inelastic scattering at E_0 = 295 MeV and forward angles including 0{\deg}. Combined with photoabsorption data an electric dipole polarizability \alpha_D(120Sn) = 8.93(36) fm^3 is extracted. The dipole polarizability as isovector observable par excellence carries direct information on the nuclear symmetry energy and its density dependence. The correlation of the new value with the well established \alpha_D(208Pb) serves as a test of its prediction by nuclear energy density functionals (EDFs). Models based on modern Skyrme interactions describe the data fairly well while most calculations based on relativistic Hamiltonians cannot.

Gamow-Teller strength distributions at finite temperatures and electron capture in stellar environments

We propose a new method to calculate stellar weak-interaction rates. It is based on the thermofield dynamics formalism and allows calculation of the weak-interaction response of nuclei at finite temperatures. The thermal evolution of the GT{sub +} distributions is presented for the sample nuclei {sup 54,56}Fe and {sup 76,78,80}Ge. For Ge we also calculate the strength distribution of first-forbidden transitions. We show that thermal effects shift the GT{sub +} centroid to lower excitation energies and make possible negative- and low-energy transitions. In our model we demonstrate that the unblocking effect for GT{sub +} transitions in neutron-rich nuclei is sensitive to increasing temperature. The results are used to calculate electron capture rates and are compared to those obtained from the shell model.

Low-energy nuclear spectroscopy in a microscopic multiphonon approach

The low-lying spectra of heavy nuclei are investigated within the quasiparticle– phonon model. This microscopic approach goes beyond the quasiparticle random-phase approximation by treating a Hamiltonian of separable form in a microscopic multiphonon basis. It is therefore able to describe the anharmonic features of collective modes as well as the multiphonon states, whose experimental evidence is continuously growing. The method can be put in close correspondence with the proton–neutron interacting boson model. By associating the microscopic isoscalar and isovector quadrupole phonons with proton–neutron symmetric and mixed-symmetry quadrupole bosons, respectively, the microscopic states can be classified, just as in the algebraic model, according to their phonon content and their symmetry. In addition, these states disclose the nuclear properties which are to be ascribed to genuine shell effects, not included in the algebraic approach. Due to its flexibility, the method can be implemented numerically for systematic studies of spectroscopic properties throughout entire regions of vibrational nuclei. The spectra and multipole transition strengths so computed are in overall good agreement with the experimental data. By exploiting the correspondence of the method with the interacting boson model, it is possible to embed the microscopic states into this algebraic frame and, therefore, face the study of nuclei far from shell closures, not directly accessible to merely microscopic approaches. Here, it is shown how this task is accomplished through systematic investigations of magnetic dipole and, especially, electric dipole modes along paths moving from the vibrational to the transitional regions. The method is very well suited to the study of well-deformed nuclei. It provides reliable descriptions of low-lying