Nuclear Giant Resonances Research Articles

In-medium nucleon-nucleon cross sections from characteristics of nuclear giant resonances and nuclear stopping power

In-medium nucleon-nucleon cross sections from characteristics of nuclear giant resonances and nuclear stopping power

Nuclear giant quadruple resonance within a transport approach and its constraint on the nucleon effective mass

We study the nuclear iso-scalar giant quadruple resonance~(ISGQR) based on the Boltzmann-Uehling-Uhlenbeck~(BUU) transport equation. The mean-field part of the BUU equation is described by the Skyrme nucleon-nucleon effective interaction, and its collision term, which embodies the two-particle-two-hole ($2$p-$2$h) correlation, is implemented through the stochastic approach. We find that the width of ISGQR for heavy nuclei is exhausted dominated by collisional damping, which is incorporated into the BUU equation through its collision term, and it can be well reproduced through employing a proper in-medium nucleon-nucleon cross section. Based on further Vlasov and BUU calculations with a number of representative Skyrme interactions, the iso-scalar nucleon effective mass at saturation density is extracted respectively as $m^{*}_{s,0}/m$ $=$ $0.83\pm0.04$ and $m^{*}_{s,0}/m$ $=$ $0.82\pm0.03$ from the measured excitation energy $E_x$ of the ISGQR of $\isotope[208]{Pb}$. The small discrepancy between the two constraints indicates the negligible role of $2$p-$2$h correlation in constraining $m_{s,0}^*$ with the ISGQR excitation energy.

Isoscalar and isovector giant resonances in 44Ca, 54Fe, 64,68Zn and 56,58,60,68Ni

We have studied the uncharacteristic behavior of the measured values of the isoscalar and isovector centroid energies, ECEN, of nuclear giant resonances of multipolarity from L = 0 to L = 3 in 44Ca, 54Fe, 64,68Zn and 56,58,60,68Ni. For this purpose, we carried out calculations of ECEN within the spherical Hartree-Fock (HF)-based random phase approximation (RPA) theory with 33 distinct Skyrme-like effective nucleon-nucleon interactions. We have also determined the Pearson linear correlation coefficients between centroid energies, obtained from the HF-RPA, and the various properties of nuclear matter (NM) of each interaction and determined the sensitivity of ECEN to NM properties. We compared the theoretical values of ECEN obtained from the HF-RPA calculations with experimental data and discuss the results, pointing out significant disagreements between theoretical and experimental values. We note in particular, that we obtain good agreement for the theoretical ECEN of the isovector giant dipole resonance and the available experimental data.

Nuclear Collective Dynamics in Transport Model With the Lattice Hamiltonian Method

We review the recent progress on studying the nuclear collective dynamics by solving the Boltzmann-Uehling-Uhlenbeck (BUU) equation with the lattice Hamiltonian method treating the collision term by the full-ensemble stochastic collision approach. This lattice BUU (LBUU) method has recently been developed and implemented in a GPU parallel computing technique, and achieves a rather stable nuclear ground-state evolution and high accuracy in evaluating the nucleon-nucleon (NN) collision term. This new LBUU method has been applied to investigate the nuclear isoscalar giant monopole resonances and isovector giant dipole resonances. While the calculations with the LBUU method without the NN collision term (i.e., the lattice Hamiltonian Vlasov method) describe reasonably the excitation energies of nuclear giant resonances, the full LBUU calculations can well reproduce the width of the giant dipole resonance of $^{208}$Pb by including a collisional damping from NN scattering. The observed strong correlation between the width of nuclear giant dipole resonance and the NN elastic cross section suggests that the NN elastic scattering plays an important role in nuclear collective dynamics, and the width of nuclear giant dipole resonance provides a good probe of the in-medium NN elastic cross section.

Constraining the in-medium nucleon-nucleon cross section from the width of nuclear giant dipole resonance

We develop a new lattice Hamiltonian method for solving the Boltzmann-Uehling-Uhlenbeck (BUU) equation. Adopting the stochastic approach to treat the collision term and using the GPU parallel computing to carry out the calculations allows for a rather high accuracy in evaluating the collision term, especially its Pauli blocking, leading thus to a new level of precision in solving the BUU equation. Applying this lattice BUU method to study the width of giant dipole resonance (GDR) in nuclei, where the accurate treatment of the collision term is crucial, we find that the obtained GDR width of 208Pb shows a strong dependence on the in-medium nucleon-nucleon cross section σNN⁎. A very large medium reduction of σNN⁎ is needed to reproduce the measured value of the GDR width of 208Pb at the Research Center for Nuclear Physics in Osaka, Japan.

The role of giant resonances in nuclear astrophysics: An overview

The microscopic study of nuclear giant resonances has been a passion of Pier Francesco Bortignon. These resonances play important roles in various astrophysical scenarios. This article summarizes how the improved description of giant resonances has helped to deepen our understanding of the dynamics of core-collapse and electron capture supernovae as well as of the nucleosynthesis associated with these events and also of thermonuclear supernovae. We briefly mention the role of giant resonances for s- and r-process nucleosynthesis and give an overview of the impact of improved supernova modelling and advanced descriptions of neutrino-nucleus reactions on the synthesis of selected nuclei by the neutrino process.

Gross, intermediate and fine structure of nuclear giant resonances: Evidence for doorway states

We review the phenomenon of fine structure of nuclear giant resonances and its relation to different resonance decay mechanisms. Wavelet analysis of the experimental spectra provides quantitative information on the fine structure in terms of characteristic scales. A comparable analysis of resonance strength distributions from microscopic approaches incorporating one or several of the resonance decay mechanisms allows conclusions on the source of the fine structure. For the isoscalar giant quadrupole resonance (ISGQR), spreading through the first step of the doorway mechanism, i.e.\ coupling between one particle-one hole ($1p1h$) and two particle-two hole ($2p2h$) states is identified as the relevant mechanism. In heavy nuclei it is dominated by coupling to low-lying surface vibrations, while in lighter nuclei stochastic coupling becomes increasingly important. The fine structure observed for the isovector giant dipole resonance (IVGDR) arises mainly from the fragmentation of the $1p1h$ strength (Landau damping), although some indications for the relevance of the spreading width are also found.

Finite amplitude method applied to the giant dipole resonance in heavy rare-earth nuclei

Background: The quasiparticle random phase approximation (QRPA), within the framework of nuclear density functional theory (DFT), has been a standard tool to access the collective excitations of atomic nuclei. Recently, the finite amplitude method (FAM) was developed in order to perform the QRPA calculations efficiently without any truncation on the two-quasiparticle model space.Purpose: We discuss the nuclear giant dipole resonance (GDR) in heavy rare-earth isotopes, for which the conventional matrix diagonalization of the QRPA is numerically demanding. A role of the Thomas-Reiche-Kuhn (TRK) sum rule enhancement factor, connected to the isovector effective mass, is also investigated.Methods: The electric dipole photoabsorption cross section was calculated within a parallelized FAM-QRPA scheme. We employed the Skyrme energy density functional self-consistently in the DFT calculation for the ground states and FAM-QRPA calculation for the excitations.Results: The mean GDR frequency and width are mostly reproduced with the FAM-QRPA, when compared to experimental data, although some deficiency is observed with isotopes heavier than erbium. A role of the TRK enhancement factor in actual GDR strength is clearly shown: its increment leads to a shift of the GDR strength to higher-energy region, without a significant change in the transition amplitudes.Conclusions: The newly developed FAM-QRPA scheme shows remarkable efficiency, which enables one to perform systematic analysis of GDR for heavy rare-earth nuclei. The theoretical deficiency of the photoabsorption cross section could not be improved by only adjusting the TRK enhancement factor, suggesting the necessity of an approach beyond self-consistent QRPA and/or a more systematic optimization of the energy density functional (EDF) parameters.

Novel scintillators and silicon photomultipliers for nuclear physics and applications

Until comparatively recently, scintillator detectors were seen as an old-fashioned tool of nuclear physics with more attention being given to areas such as gamma-ray tracking using high-purity germanium detectors. Next-generation scintillator detectors, such as lanthanum bromide, which were developed for the demands of space science and gamma- ray telescopes, are found to have strong applicability to low energy nuclear physics. Their excellent timing resolution makes them very suitable for fast timing measurements and their much improved energy resolution compared to conventional scintillators promises to open up new avenues in nuclear physics research which were presently hard to access. Such "medium-resolution" spectroscopy has broad interest across several areas of contemporary interest such as the study of nuclear giant resonances. In addition to the connections to space science, it is striking that the demands of contemporary medical imaging have strong overlap with those of experimental nuclear physics. An example is the interest in PET-MRI combined imaging which requires putting scintillator detectors in a high magnetic field environment. This has led to strong advances in the area of silicon photomultipliers, a solid-state replacement for photomultiplier tubes, which are insensitive to magnetic fields. Broad application to nuclear physics of this technology may be foreseen.

The configurational splitting of giant dipole resonance

This review considers the history and meaning of the phenomenon of configurational splitting of giant dipole resonance in light nuclei, which was first discovered a quarter of a century ago and induced profound changes in the commonly accepted understanding of the structure of light nuclei and the mechanisms of their high-energy excitations. The experiments that led to the discovery of this phenomenon are described. The present status of the phenomenon of the configurational splitting of nuclear giant dipole resonance is discussed and directions of its future studies are outlined.

Density content of nuclear symmetry energy from nuclear observables

The nuclear symmetry energy at a given density measures the energy transferred in converting symmetric nuclear matter into the pure neutron matter. The density content of nuclear symmetry energy remains poorly constrained. Our recent results for the density content of the nuclear symmetry energy, around the saturation density, extracted using experimental data for accurately known nuclear masses, giant resonances and neutron-skin thickness in heavy nuclei are summarized.

Photo-disintegration of the iron nucleus in fractured magnetite rocks with magnetostriction

There has been considerable interest in recent experiments on iron nuclear disintegrations observed when rocks containing such nuclei are crushed and fractured. The resulting nuclear transmutations are particularly strong for the case of magnetite rocks, i.e. loadstones. We argue that the fission of the iron nucleus is a consequence of photo-disintegration. The electro-strong coupling between electromagnetic fields and nuclear giant dipole resonances are central for producing observed nuclear reactions. The large electron energies produced during the fracture of piezomagnetic rocks are closely analogous to the previously discussed case of the fracture of piezoelectric rocks. In both cases electro-weak interactions can produce neutrons and neutrinos from energetic protons and electrons thus inducing nuclear transmutations. The electro-strong condensed matter coupling discussed herein represents new many body collective nuclear photo-disintegration effects.

Extension of the continuum time-dependent Hartree-Fock method to proton states.

This paper deals with the solution of the spherically symmetric time-dependent Hartree-Fock approximation applied to nuclear giant monopole resonances in the small amplitude regime. The problem is spatially unbounded as the resonance state is in the continuum. The practical requirement to perform the calculation in a finite-sized spatial region yields an artificial boundary, which is not present physically. The question of how to ensure the boundary does not interfere with the internal solution, while keeping the overall calculation time low is studied. Here we propose an absorbing boundary condition scheme to handle the conflict. The derivation, via a Laplace transform method, and implementation is described. An inverse Laplace transform required by the absorbing boundaries is calculated using a method of nonlinear least squares. The accuracy and efficiency of the scheme is tested and results presented to support the case that they are an effective way of handling the artificial boundary.

Continuum time-dependent Hartree-Fock method for giant resonances in spherical nuclei

This paper deals with the solution of the spherically symmetric time-dependent Hartree-Fock approximation applied in the case of nuclear giant monopole resonances in the small and large amplitude regimes. The problem is spatially unbounded as the resonance state is in the continuum. The practical requirement to perform the calculation in a finite-sized spatial region results in a difficulty with the spatial boundary conditions. Here we propose an absorbing boundary condition scheme to handle the conflict. The derivation, via a Laplace transform method, and implementation is described. The accuracy and efficiency of the scheme is tested and the results presented to support the case that they are a effective way of handling the artificial boundary.

Fully Self-Consistency in Relativistic Random Phase Approximation

Fully self-consistent relativistic random phase approximation (RRPA) is built on the relativistic mean field ground state with a non-linear relativistic Lagrangian. The consistency requires that the same effective interaction is adopted to simultaneously describe both the ground states and the excited states of the nucleus. Reliable and accurate numerical results of the nuclear giant resonances obtained in the RRPA require fully consistent calculations. In some excitation modes they are extremely sensitive to consistent treatment, e.g., such as isoscalar giant monopole and dipole resonances (ISGMR and ISGDR). In this work we perform the numerical calculations in the case of ISGDR for 208Pb and check the consistency. The spurious state in the ISGDR vanishes once the full self-consistency is achieved.

Self-consistent RPA calculations with Skyrme-type interactions: The skyrme_rpa program

Random Phase Approximation (RPA) calculations are nowadays an indispensable tool in nuclear physics studies. We present here a complete version implemented with Skyrme-type interactions, with the spherical symmetry assumption, that can be used in cases where the effects of pairing correlations and of deformation can be ignored. The full self-consistency between the Hartree–Fock mean field and the RPA excitations is enforced, and it is numerically controlled by comparison with energy-weighted sum rules. The main limitations are that charge-exchange excitations and transitions involving spin operators are not included in this version. Program summaryProgram title: skyrme_rpa (v 1.00)Catalogue identifier: AENF_v1_0Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AENF_v1_0.htmlProgram obtainable from: CPC Program Library, Queen’s University, Belfast, N. IrelandLicensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.htmlNo. of lines in distributed program, including test data, etc.: 5531No. of bytes in distributed program, including test data, etc.: 39435Distribution format: tar.gzProgramming language: FORTRAN-90/95; easily downgradable to FORTRAN-77.Computer: PC with Intel Celeron, Intel Pentium, AMD Athlon and Intel Core Duo processors.Operating system: Linux, Windows.RAM: From 4 MBytes to 150 MBytes, depending on the size of the nucleus and of the model space for RPA.Word size: The code is written with a prevalent use of double precision orREAL(8) variables; this assures 15 significant digits.Classification: 17.24.Nature of problem: Systematic observations of excitation properties in finite nuclear systems can lead to improved knowledge of the nuclear matter equation of state as well as a better understanding of the effective interaction in the medium. This is the case of the nuclear giant resonances and low-lying collective excitations, which can be described as small amplitude collective motions in the framework of the Random Phase Approximation (RPA). This work provides a tool where one starts from an assumed form of nuclear effective interaction (the Skyrme forces) and builds the self-consistent Hartree–Fock mean field of a given nucleus, and then the RPA multipole excitations of that nucleus.Solution method: The Hartree–Fock (HF) equations are solved in a radial mesh, using a Numerov algorithm. The solutions are iterated until self-consistency is achieved (in practice, when the energy eigenvalues are stable within a desired accuracy). In the obtained mean field, unoccupied states necessary for the RPA calculations are found. For all single-particle states, box boundary conditions are assumed. To solve the RPA problem for a given value of total angular momentum and parity Jπ a coupled basis is constructed and the RPA matrix is diagonalized (protons and neutrons are treated explicitly, and no approximation related to the use of isospin formalism is introduced). The transition amplitudes and transition strengths associated to given external operators are calculated. The HF densities and RPA transition densities are also evaluated.Restrictions: The main restrictions are related to the assumed spherical symmetry and absence of pairing correlations.Running time: The typical running time depends strongly on the nucleus, on the multipolarity, on the choice of the model space and of course on the computer. It can vary from a few minutes to several hours.

Microscopic theory of theγdecay of nuclear giant resonances

In the past decades, the \gamma-decay of giant resonances has been studied using phenomenological models. In keeping with possible future studies performed with exotic beams, microscopically-based frameworks should be envisaged. In the present paper, we introduce a model which is entirely based on Skyrme effective interactions, and treats the ground-state decay within the fully self-consistent Random Phase Approximation (RPA) and the decay to low-lying states at the lowest order beyond RPA. The model is applied to 208Pb and 90Zr, and the results are compared with experimental data.

Nuclear Excitation by a Zeptosecond Multi-MeV Laser Pulse

A zeptosecond multi-MeV laser pulse may either excite a "plasma" of strongly interacting nucleons or a collective mode. We derive the conditions on laser energy and photon number such that either of these scenarios is realized. We use the nuclear giant dipole resonance as a representative example, and a random-matrix description of the fine-structure states and perturbation theory as tools.

Time scales in nuclear giant resonances

We propose a general approach to characterise fluctuations of measured cross sections of nuclear giant resonances. Simulated cross sections are obtained from a particular, yet representative self-energy which contains all information about fragmentations. Using a wavelet analysis, we demonstrate the extraction of time scales of cascading decays into configurations of different complexity of the resonance. We argue that the spreading widths of collective excitations in nuclei are determined by the number of fragmentations as seen in the power spectrum. An analytic treatment of the wavelet analysis using a Fourier expansion of the cross section confirms this principle. A simple rule for the relative life times of states associated with hierarchies of different complexity is given.

Fine tuning in an effective field based relativistic mean field model, and properties of neutron-rich matter

Properties of symmetric nuclear matter and neutron-rich matter predicted by effective field based relativistic mean field parameter sets are compared to those presented in a recent analysis of nuclear giant monopole resonance and isospin diffusion data and a study of the neutron skin of finite nuclei, as well as some selected observational data of neutron stars. Motivated by the fact that none of the published parameter sets of this model is simultaneously consistent with all previously mentioned constraints, an improvement not only in the isovector sector but also in isoscalar sector of this model is proposed. The properties of symmetric nuclear matter and neutron-rich matter, as well as some basic properties of static neutron stars predicted by the proposed parameter set (G2**), are discussed.