Isoscalar Giant Dipole Resonance Research Articles

THE ESCAPE WIDTHS AND STRENGTH OF ISOSCALAR GIANT DIPOLE RESONANCE IN 208Pb

The structure of the isoscalar giant dipole resonance (ISDGR) in 208Pb has been investigated by using the continuum Randon Phase Aproximation (RPA). The main properties (strength function and escape widths) of the ISDGR are calculated and compared with recent available experimental data.

Evidence for direct neutron decay of the isoscalar giant dipole resonances inZr90,Sn116, andPb208

The direct and statistical neutron decay of the isoscalar giant dipole resonance was studied in Zr-90, Sn-116, and Pb-208 using the (alpha,alpha(')n) reaction at a bombarding energy of 200 MeV. The spectra of fast decay neutrons populating valence hole-states of the Z,N-1 nuclei were analyzed, and estimates for the branching ratios were determined. The results were compared to recent continuum random-phase approximation calculations.

NUCLEAR MATTER PROPERTIES FROM COLLECTIVE MODES IN NUCLEI

Bulk properties of nuclear matter associated with the equation of state, the nuclear matter incompressibility coefficient, Knm, in particular, are very important physical quantities required in the study of properties of nuclei, supernova collapse, neutron stars and heavy-ion collisions. Here we review the current status of Knm and the experimental and theoretical methods used to determine the value of Knm from the excitation cross-sections and the transition strength distributions of compression modes in nuclei, mainly the the isoscalar giant monopole resonance and the isoscalar giant dipole resonance.

MICROSCOPIC STRUCTURE OF THE DIPOLE COMPRESSION MODE IN HEAVY NUCLEI

Neutron decay of the isoscalar giant dipole resonance (ISGDR) has been studied in90Zr ,116Sn and208Pb using the (α,α′) reaction at a bombarding energy of 200 MeV. The same reaction was used to study proton decay of the ISGDR in58Ni and208Pb . Neutron and proton decay of the ISGDR to low-lying neutron and proton hole states in the daughter nuclei, respectively, allowed an experimental determination of the ISGDR microscopic structures. The determined branching ratios are compared to recent continuum random-phase-approximation (CRPA) calculations. Reasonable agreement was observed for208Pb and58Ni , whereas strong differences for90Zr and116Sn were found.

Deducing the nuclear-matter incompressibility coefficient from data on isoscalar compression modes

Accurate assessment of the value of the incompressibility coefficient, K, of symmetric nuclear matter, which is directly related to the curvature of the equation of state (EOS), is needed to extend our knowledge of the EOS in the vicinity of the saturation point. We review the current status of K as determined from experimental data on isoscalar giant monopole and dipole resonances (compression modes) in nuclei, by employing the microscopic theory based on the random-phase approximation (RPA).

“Bi-modal” isoscalar giant dipole strength in 58Ni

The strength distribution of the isoscalar giant dipole resonance (ISGDR) in 58Ni has been obtained over the energy range 10.5–49.5 MeV via extreme forward angle scattering (including 0°) of 386 MeV α particles. We observe a “bi-modal” E1 strength distribution for the first time in an A<90 nucleus. The observed ISGDR strength distribution is in reasonable agreement with the predictions of a recent RPA calculation.

Exploring the extended density-dependent Skyrme effective forces for normal and isospin-rich nuclei to neutron stars

We parameterize the recently proposed generalized Skyrme effective force (GSEF) containing extended density dependence. The parameters of the GSEF are determined by the fit to several properties of the normal and isospin-rich nuclei. We also include in our fit a realistic equation of state for the pure neutron matter up to high densities so that the resulting Skyrme parameters can be suitably used to model the neutron star with the "canonical" mass ($\sim 1.4 M_\odot$). For the appropriate comparison we generate a parameter set for the standard Skyrme effective force (SSEF) using exactly the same set of the data as employed to determine the parameters of the GSEF. We find that the GSEF yields larger values for the neutron skin thickness which are closer to the recent predictions based on the isospin diffusion data. The Skyrme parameters so obtained are employed to compute the strength function for the isoscalar giant monopole, dipole and quadrupole resonances. It is found that in the case of GSEF, due to the the larger value of the nucleon effective mass the values of centroid energies for the isoscalar giant resonances are in better agreement with the corresponding experimental data in comparison to those obtained using the SSEF. We also present results for some of the key properties associated with the neutron star of "canonical" mass and for the one with the maximum mass.

Interplay of compressional and vortical nuclear currents in overtones of the isoscalar giant dipole resonance

Within a semiclassical nuclear Fermi-fluid dynamic approach, the properties of the isoscalar giant dipole resonance (ISGDR) and the structure of various electromagnetic characteristics associated with the most important states building this resonance are investigated. The apparent puzzling outcome of microscopic predictions that the ISGDR distribution is split into two main broad structures is confirmed within the presented macroscopic approach by the occurrence of a ``low-lying'' and a ``high-lying'' state, as the first two overtones of the same resonance. Macroscopicaly, they are pictured as a combination of compressional and vortical nuclear flows. The second part of the paper analyzes the electromagnetic structure of the ISGDR relevant to reactions with inelasticly scattered electrons and the relation between the vorticity and the toroidal dipole moment. The relative strengths of the compressional and vortical collective currents are evaluated by means of electron-scattering sum rules.

Relativistic and nonrelativistic calculations of the isoscalar monopole and dipole states

The study of the isoscalar giant monopole resonance (ISGMR) should allow extracting a value for the nuclear incompressibility coefficient K∞. In this contribution, we review the most recent attempts along this line. While the nonrelativistic (Skyrme, Gogny) models predict K∞ to be around 220–235 MeV, the values obtained from the relativistic calculations are significantly larger (250–270 MeV). We argue that the most plausible reason for this discrepancy lies in the different behavior of the symmetry energy in the two classes of models. We also discuss the role of the isoscalar giant dipole resonance (ISGDR). We conclude that a number of experimental ambiguities still prevent us from deducing K∞ from the ISGDR with a comparable accuracy as from the ISGMR.

Overtones of isoscalar giant resonances in medium-heavy and heavy nuclei

A semimicroscopic approach based on both the continuum random-phase-approximation method and a phenomenological treatment of the spreading effect is extended and applied to describe the main properties (particle-hole strength distribution, energy-dependent transition density, partial direct-nucleon-decay branching ratios) of the isoscalar giant dipole, second monopole, and second quadrupole resonances. The abilities of the approach are checked by description of the gross properties of the main-tone resonances. Calculation results obtained for the resonances in a few singly- and doubly-closed-shell nuclei are compared with available experimental data.

Systematics of the bimodal isoscalar giant dipole resonance

The systematic behavior of the isoscalar giant dipole resonance (ISGDR) in $^{90}\mathrm{Zr}$, $^{116}\mathrm{Sn}$, $^{144}\mathrm{Sm}$, and $^{208}\mathrm{Pb}$ is studied with inelastic $\ensuremath{\alpha}$ scattering at ${E}_{\ensuremath{\alpha}}=386\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$. Multipole-decomposition analysis is applied to extract the excitation strengths of giant resonances from the $(\ensuremath{\alpha},{\ensuremath{\alpha}}^{\ensuremath{'}})$ differential cross sections at ${\ensuremath{\theta}}_{\mathrm{lab}}=0.64\ifmmode^\circ\else\textdegree\fi{}--13.5\ifmmode^\circ\else\textdegree\fi{}$. The bimodal structure of the ISGDR is discussed and compared with recent theoretical results from $\text{Hartree}\text{\penalty1000-\hskip0pt}\text{Fock}+\text{random}\text{\penalty1000-\hskip0pt}\text{phase}\text{\penalty1000-\hskip0pt}\text{approximation}$ calculations.

The effect of deformation in the isoscalar giant dipole resonance

Background-free inelastic scattering spectra have been obtained for144,148,150,152,154Sm with 386 MeV α particles at forward angles (including 0°) to investigate the effect of deformation on the isoscalar giant dipole resonance. The strength distributions for isoscalar giant resonances have been extracted via a multipole decomposition analysis using angular distributions calculated in the framework of the density-dependent single-folding model. The effects of deformation are found to be different for the low- and high-excitation energy components of the isoscalar giant dipole resonance. Evidence has been obtained for the theoretically-predicted mixing between the isoscalar giant dipole resonance and the high energy octupole resonance.

The effect of deformation in the isoscalar giant dipole resonance

Background-free inelastic scattering spectra have been obtained for 144,148,150,152,154 Sm with 386 MeV α particles at forward angles (including 0°) to investigate the effect of deformation on the isoscalar giant dipole resonance. The strength distributions for isoscalar giant resonances have been extracted via a multipole decomposition analysis using angular distributions calculated in the framework of the density-dependent single-folding model. The effects of deformation are found to be different for the low- and high-excitation energy components of the isoscalar giant dipole resonance. Evidence has been obtained for the theoretically-predicted mixing between the isoscalar giant dipole resonance and the high energy octupole resonance.

The isoscalar giant dipole resonance: A status report

The current status of the experimental work on the Isoscalar Giant Dipole Resonance (ISGDR) is reviewed. Results are presented from recent measurements at RCNP, Osaka, using small-angle inelastic scattering of 400 MeV α particles, and the implications of these results on the determination of nuclear incompressibility are discussed.

The isoscalar giant dipole resonance: A status report

The current status of the experimental work on the Isoscalar Giant Dipole Resonance (ISGDR) is reviewed. Results are presented from recent measurements at RCNP, Osaka, using small-angle inelastic scattering of 400 MeV α particles, and the implications of these results on the determination of nuclear incompressibility are discussed.

Isoscalar giant dipole resonance for several nuclei with A ≥ 90

The giant resonance region in several nuclei in A ≥ 90 have been re-measured using 240 MeV α particle scattering with a detector provides that both horizontal and vertical angles. Slice analyses with multipole fits were performed and strength due to both low and high energy components of the isoscalar giant dipole resonance (ISGDR) was identified. E1 strengths for the high energy component corresponding to 70±15%,68±15% and 67±14% of the isoscalar E1 energy-weighted sum rule were identified, with centroid energies of 26.3±0.6 MeV, 24.7±0.6 MeV and 21.7±0.6 MeV for90Zr,144Sm and208Pb, respectively. The centroid of the high energy component of the strength distribution is in closer agreement with the microscopic prediction generated using compressibilities obtained from the isoscalar giant monopole resonance data.

Particle decay of the isoscalar giant dipole resonance inPb

The direct particle decay of the isoscalar giant dipole resonance (ISGDR) has been studied in208Pb using the (α, α′p) and (α, α′n) reactions at a bombarding energy of 200 MeV. The direct-decay channels of proton and neutron emissions were observed in coincidence with the scattered α-particles in order to study the population of proton-hole states in207T1 and neutron-hole states in207Pb. The results of the partial and total branching ratios allowed us to test recently performed continuum-RPA (CRPA) calculations for the ISGDR and thereby to learn about its microscopic structure.

Theoretical understanding of the nuclear incompressibility: where do we stand?

The status of the theoretical research on the compressional modes of finite nuclei and the incompressibility K ∞ of nuclear matter, is reviewed. It is argued that the recent experimental data on the Isoscalar Giant Monopole Resonance (ISGMR) allow extracting the value of K ∞ with an uncertainity of about ± 12 MeV. Non-relativistic (Skyrme, Goguy) and relativistic mean field models predict for K ∞ values which are significantly different from one another, namely ≈ 220–235 and 250–270 MeV respectively. It is shown that the solution of this puzzle requires a better determination of the symmetry energy at, and around, saturation. The role played by the experimental data of the Isoscalar Giant Dipole Resonance (ISGDR) is also discussed.

Particle decay of the isoscalar giant dipole resonance inPb

The direct particle decay of the isoscalar giant dipole resonance (ISGDR) has been studied in208Pb using the (α, α′p) and (α, α′n) reactions at a bombarding energy of 200 MeV. The direct-decay channels of proton and neutron emissions were observed in coincidence with the scattered α-particles in order to study the population of proton-hole states in207T1 and neutron-hole states in207Pb. The results of the partial and total branching ratios allowed us to test recently performed continuum-RPA (CRPA) calculations for the ISGDR and thereby to learn about its microscopic structure.

Theoretical understanding of the nuclear incompressibility: where do we stand?

The status of the theoretical research on the compressional modes of finite nuclei and the incompressibility K ∞ of nuclear matter, is reviewed. It is argued that the recent experimental data on the Isoscalar Giant Monopole Resonance (ISGMR) allow extracting the value of K ∞ with an uncertainity of about ± 12 MeV. Non-relativistic (Skyrme, Goguy) and relativistic mean field models predict for K ∞ values which are significantly different from one another, namely ≈ 220–235 and 250–270 MeV respectively. It is shown that the solution of this puzzle requires a better determination of the symmetry energy at, and around, saturation. The role played by the experimental data of the Isoscalar Giant Dipole Resonance (ISGDR) is also discussed.