Isovector Giant Resonance Research Articles

Manipulation of Giant Multipole Resonances via Vortex γ Photons.

Traditional photonuclear reactions primarily excite giant dipole resonances, making the measurement of isovector giant resonances with higher multipolarities a great challenge. In this Letter, the manipulation of collective excitations of different multipole transitions in even-even nuclei via vortex γ photons is investigated. We develop the calculation method for photonuclear cross sections induced by the vortex γ photon beam using the fully self-consistent random-phase approximation plus particle-vibration coupling (RPA+PVC) model based on Skyrme density functional. We find that the electromagnetic transitions with multipolarity J<|m_{γ}| are forbidden for vortex γ photons due to the angular momentum conservation, with m_{γ} being the projection of total angular momentum of γ photon on its propagation direction. For instance, this allows for probing the isovector giant quadrupole resonance without interference from dipole transitions using vortex γ photons with m_{γ}=2. Furthermore, the electromagnetic transition with J=|m_{γ}|+1 vanishes at a specific polar angle. Therefore, the giant resonances with specific multipolarity can be extracted via vortex γ photons. Moreover, the vortex properties of γ photons can be meticulously diagnosed by measuring the nuclear photon-absorption cross section. Our method opens new avenues for photonuclear excitations, generation of coherent γ photon laser and precise detection of vortex particles, and consequently, has significant impact on nuclear physics, nuclear astrophysics and strong laser physics.

Isoscalar and Isovector Giant Resonances in Closed Shells Nuclei and Bulk Properties of Nuclear Matter

Centroid energies, $$E_{\textrm{CEN}}$$ , of the isoscalar ( $$T=0$$ ) and isovector ( $$T=1$$ ) giant resonances of multipolarities $$L_{m}=0$$ –3 in $${}^{\mathrm{40,48}}$$ Ca, $${}^{\mathrm{68}}$$ Ni, $${}^{\mathrm{90}}$$ Zr, $${}^{\mathrm{116}}$$ Sn, $${}^{\mathrm{144}}$$ Sm, and $${}^{\mathrm{208}}$$ Pb, were calculated within the fully self-consistent spherical Hartree–Fock (HF)-based random-phase approximation (RPA) theory, using 33 different energy density functionals associated with Skyrme-type effective nucleon–nucleon interactions of the standard form commonly employed in the literature. We also calculate the Pearson linear correlation coefficients between each $$E_{\mathrm{CEN}}$$ and each bulk property of nuclear matter (NM), associated with the Skyrme interactions used in the calculations, and determine the sensitivity of $$E_{\mathrm{CEN}}$$ to bulk properties of NM. By comparing the calculated values of $$E_{\textrm{CEN}}$$ to the experimental data, we constrain the values of the bulk NM properties. We find that interactions associated with the values of the NM effective mass, $$m^{\ast}/m=0.70$$ –0.90, incompressibility coefficient, $$K_{\textrm{NM}}=210$$ –240 MeV, and the enhancement coefficient of the energy-weighted sum rule of the isovector giant dipole resonance, $$\kappa=0.25$$ –0.70, best reproduce the experimental data.

Isoscalar and isovector giant resonances in 92,94,96,98,100Mo and 90,92,94Zr

The response functions, S(E), and centroid energies, ECEN, of the isoscalar and isovector giant resonances of multipolarity L = 0–3 in 92,94,96,98,100Mo and 90,92,94Zr have been calculated employing the spherical Hartree-Fock (HF) based random phase approximation (RPA) method, using the Skyrme-type effective nucleon-nucleon interaction. In particular, we investigate the recent experimental results of the uncharacteristic behavior of ECEN for the isoscalar giant resonances, by extending our HF-based RPA calculations with only the KDE0v1 interaction to 32 additional Skyrme interactions. The main result of our investigation is that we find significant disagreements between the theoretical and the experimental values of ECEN for isoscalar giant monopole and dipole resonance of some nuclei. For the isoscalar giant octupole resonance we find the theoretical values of the ECEN to be well above the experimental results. We also study the sensitivity of ECEN to nuclear matter (NM) properties, including the effective mass, the incompressibility coefficient and the symmetry energy at saturation density, by determining the Pearson linear correlation coefficient between the calculated values of ECEN and the various nuclear matter properties of each Skyrme parametrization. For the isovector giant dipole resonance we find good agreement between the experimental and theoretical values of ECEN for Skyrme interactions with values of the enhancement coefficient of the energy weighted sum rule, κ, in the range between 0.25–0.70.

Observation of the Isovector Giant Monopole Resonance via the ^{28}Si(^{10}Be,^{10}B^{*}[1.74 MeV]) Reaction at 100 AMeV.

The (^{10}Be,^{10}B^{*}[1.74 MeV]) charge-exchange reaction at 100 AMeV is presented as a new probe for isolating the isovector (ΔT=1) nonspin-transfer (ΔS=0) response of nuclei, with ^{28}Si being the first nucleus studied. By using a secondary ^{10}Be beam produced by fast fragmentation of ^{18}O nuclei at the NSCL Coupled Cyclotron Facility, applying the dispersion-matching technique with the S800 magnetic spectrometer to determine the excitation energy in ^{28}Al, and performing high-resolution γ-ray tracking with the Gamma-Ray Energy Tracking In-beam Nuclear Array (GRETINA) to identify the 1022-keV γ ray associated with the decay from the 1.74-MeV T=1 isobaric analog state in ^{10}B, a ΔS=0 excitation-energy spectrum in ^{28}Al was extracted. Monopole and dipole contributions were determined through a multipole-decomposition analysis, and the isovector giant dipole resonance and isovector giant monopole resonance (IVGMR) were identified. The results show that this probe is a powerful tool for studying the elusive IVGMR, which is of interest for performing stringent tests of modern density functional theories at high excitation energies and for constraining the bulk properties of nuclei and nuclear matter. The extracted distributions were compared with theoretical calculations based on the normal-modes formalism and the proton-neutron relativistic time-blocking approximation. Calculated cross sections based on these strengths underestimate the data by about a factor of 2, which likely indicates deficiencies in the reaction calculations based on the distorted wave Born approximation.

Separation of contributions of isovector E2 and E1 giant resonances in direct and inverse reactions with real and virtual photons

A brief overview of the methods for separating the contributions of isovector electric quadrupole (E2) and dominant dipole (E1) giant resonances in atomic nuclei, which are excited in direct and inverse reactions with photons (real and virtual), is given. The basic parameters of isovector giant resonance E2, which were declared to date by applying some of these methods to the results with 208Pb, are also presented.

Isoscalar and isovector giant resonances in a self-consistent phonon coupling approach

We present fully self-consistent calculations of isoscalar giant monopole and quadrupole as well as isovector giant dipole resonances in heavy and light nuclei. The description is based on Skyrme energy-density functionals determining the static Hartree–Fock ground state and the excitation spectra within random-phase approximation (RPA) and RPA extended by including the quasiparticle-phonon coupling at the level of the time-blocking approximation (TBA). All matrix elements were derived consistently from the given energy-density functional and calculated without any approximation. As a new feature in these calculations, the single-particle continuum was included thus avoiding the artificial discretization usually implied in RPA and TBA. The step to include phonon coupling in TBA leads to small, but systematic, down shifts of the centroid energies of the giant resonances. These shifts are similar in size for all Skyrme parametrizations investigated here. After all, we demonstrate that one can find Skyrme parametrizations which deliver a good simultaneous reproduction of all three giant resonances within TBA.

Isoscalar and Isovector Giant Monopole Resonances from a Continuum Hartree-Fock Method

We motivate and summarise some recent results in the application of formally exact boundary conditions in nuclear time-dependent Hartree-Fock calculations, mak- ing use of Laplace transformations to calculate the values of the wave functions at the boundaries. We have realised the method in the case of giant monopole resonances of spherically-symmetric nuclei, and present strength functions of 16 O and 40 Ca using a sim- plified version of the Skyrme force, showing that no artefacts from discretisation occur as contaminatnts

Giant Resonances in 40Ca and 48Ca

We present results of fully self-consistent Hartree-Fock based random phase approximation calculations of the centroid energies ECEN of isoscalar (T = 0) and isovector (T = 1) giant resonances of multipolarities L = 0 – 3 in 40Ca and 48Ca, using a wide range of commonly employed Skyrme type nucleon-nucleon effective interactions. We determined the sensitivities of ECEN and of the isotopic differences ECEN(48Ca) – ECEN(40Ca) to physical quantities, such as nuclear matter incompressibility coefficient, symmetry energy density and effective mass, associated with the Skyrme interactions. We compare the results with the available experimental data, in particular, with the recent data indicating that the ISGMR centroid energy in 48Ca is somewhat higher than in 40Ca.

Giant resonances in40Ca and48Ca

We present results of fully self-consistent Hartree-Fock-based random phase approximation calculations of the strength functions $S$($E$) and centroid energies ${E}_{\mathrm{CEN}}$ of isoscalar ($T=0$) and isovector ($T=1$) giant resonances of multipolarities $L=$ 0--3 in ${}^{40}$Ca and ${}^{48}$Ca, using a wide range of commonly employed 18 Skyrme-type nucleon-nucleon effective interactions. We determined the sensitivities of ${E}_{\mathrm{CEN}}$ and of the isotopic differences ${E}_{\mathrm{CEN}}$(${}^{48}$Ca) -- ${E}_{\mathrm{CEN}}$(${}^{40}$Ca) to physical quantities, such as nuclear matter incompressibility coefficient, symmetry energy density, and effective mass, associated with the Skyrme interactions and compare the results with the available experimental data.

Isovector giant E2 resonance and overtone of the isovector giant E1 resonance in photonucleon reactions

The effect of the isovector giant quadrupole resonance and an overtone of the isovector giant dipole resonance on the photodisintegration of medium-mass and heavy nuclei in the energy range of 20–40 MeV is studied on the basis of a combined model of photonucleon reactions. Basic features of these resonances are evaluated within a semimicroscopic approach. Their deformation spitting is described on the basis of the model of polarization vibrations of a two-component nuclear liquid.

Experimental study of the electric dipole strength in the even Mo nuclei and its deformation dependence

Two methods based on bremsstrahlung were applied to the stable even Mo isotopes for the experimental determination of the photon strength function covering the high excitation energy range above 4 MeV with its increasing level density. Photon scattering was used up to the neutron separation energies ${S}_{n}$ and data up to the maximum of the isovector giant resonance (GDR) were obtained by photoactivation. After a proper correction for multistep processes the observed quasicontinuous spectra of scattered photons show a remarkably good match to the photon strengths derived from nuclear photoeffect data obtained previously by neutron detection and corrected in absolute scale by using the new activation results. The combined data form an excellent basis to derive a shape dependence of the $E1$ strength in the even Mo isotopes with increasing deviation from the $N=50$ neutron shell (i.e., with the impact of quadrupole deformation and triaxiality). The wide energy coverage of the data allows for a stringent assessment of the dipole sum rule and a test of a novel parametrization developed previously which is based on it. This parametrization for the electric dipole strength function in nuclei with $A&gt;80$ deviates significantly from prescriptions generally used previously. In astrophysical network calculations it may help to quantify the role the p-process plays in cosmic nucleosynthesis. It also has impact on the accurate analysis of neutron capture data of importance for future nuclear energy systems and waste transmutation.

Semimicroscopic description of the spreading width of isobaric analog resonances in spherical nuclei

A method for semimicroscopic description of the spreading width of isobaric analog resonances (IARs) is applied to the IAR built on the 91Zr, 208,209Pb ground state. This width is expressed in terms of the mean Coulomb field and the imaginary part of the effective optical potential used to describe the total width of the IAR overtone—isovector giant monopole resonance.

Coherent Contributions to Isospin Mixing in the Mirror PairAs67andSe67

Isospin symmetry breaking has been investigated in mass A=67 mirror nuclei through the experimental determination of the E1 strengths of analog electromagnetic transitions. Lifetimes of excited states have been measured in (67)Se and (67)As with the centroid shift method. Through the comparison of the B(E1) strengths of the mirror 9/2(+)-->7/2(-) transitions, the isovector and the isoscalar components of the electromagnetic transition amplitude were extracted. The presence of a large isoscalar component provides evidence for coherent contributions to isospin mixing, probably involving the isovector giant monopole resonance.

Adependence of the enhancement factor in energy-weighted sums for isovector giant resonances

We consider the energy weighted sums (EWS) for isovector giant dipole resonances (IVGDR) in finite nuclei within Landau kinetic theory. The dependence of both IVGDR energy, ${E}_{\mathrm{IVGDR}}$, and the EWS enhancement factor, $\ensuremath{\kappa}(A)$, on the mass number $A$ occurs because of the boundary condition on the moving nuclear surface. The values of ${E}_{\mathrm{IVGDR}}{A}^{1/3}$ and $\ensuremath{\kappa}(A)$ increase with $A$. The obtained value of the enhancement factor is about $10%$ for light nuclei and reaches approximately $20%$ for heavy nuclei. A fit of the enhancement factor to the proper experimental data provides a value for the isovector Landau amplitude of ${F}_{1}^{'}\ensuremath{\simeq}1.1$.

Direct-decay properties of giant resonances

A semi-microscopic approach, based on the continuum-RPA method and a phenomenological treatment of the spreading effect, is outlined and applied to describe direct-decay properties of a few isovector giant resonances. The ability of the approach to describe giant-resonance gross properties is also checked.

Neutron skin and giant resonances in nuclei

Abstract Some aspects, both experimental and theoretical, of extracting the neutron skin Δ R from properties of isovector giant resonances are discussed. Existing proposals are critically reviewed. The method relying on the energy difference between the Gamow–Teller resonance and the isobaric analogue state is shown to lack sensitivity to Δ R . A simple explanation of the linear relation between the symmetry energy and the neutron skin is also presented.

Relativistic Hartree–Bogoliubov theory and the isospin dependence of the effective nuclear force

The role of the isospin dependence of the effective Lagrangian of the Relativistic Mean Field Theory (RMF) is investigated. The standard RMF density functional based on non-linear meson-exchange terms is extended to include density dependent meson–nucleon coupling constants. The new variation of the RMF, in which the isospin dependence enters in both isoscalar and isovector channels, provides improved description of the equation of state of neutron matter, better isovector properties of finite nuclei such as neutron skin thickness, more accurate nuclear masses and a consistent description of both isovector and isoscalar giant resonances.

Relativistic Mean Field theory with density dependent meson couplings

Relativistic mean field theory with density dependent coupling constants is used to investigate ground state properties and properties of excited configurations in nuclei far from stability. It turns out that it is very important to consider also a density dependence the isovector channel. This provides not only an improved description of the equation of state for neutron matter and asymmetric nuclear matter but also for isovector properties of finite nuclei far from stability such as the neutron skin thickness, and the isoscalar and isovector giant resonances.

Effects of self-consistency violation in Hartree-Fock RPA calculations for nuclear giant resonances revisited

We provide accurate assessments of the consequences of violations of self-consistency in Hartree-Fock (HF) based random phase approximation (RPA) calculations of the centroid energy $E_{cen}$ of isoscalar and isovector giant resonances of multi-polarities $L=0-3$ in a wide range of nuclei. This is done by carrying out highly accurate HF-RPA calculations neglecting the particle-hole (ph) spin-orbit or Coulomb interaction in the RPA and comparing with the fully self-consistent HF-RPA results. We find that the shifts in the value of $E_{cen}$ due to self-consistency violation associated with the spin-orbit and Coulomb interactions are comparable or larger than the current experimental errors in $E_{cen}$.

Isovector giant monopole resonances in spherical nuclei

We study isovector giant monopole resonances in spherical nuclei using the time-dependent Hartree–Fock method with different Skyrme forces. The aim of the investigation is to study how, for example, mass number and neutron excess influence the isovector giant monopole resonances and their mixing with isoscalar modes. We also study how nuclear matter properties, such as the asymmetry energy, of different Skyrme forces affect the behaviour of giant resonances. We find isovector and isoscalar modes to be strongly mixed and that there is little correlation between asymmetry energy and isovector resonance energy.