Isovector Giant Resonance Research Articles

Investigation of IVGRs via theNi(t,He)Co reaction

A 130 MeV primary tritium beam, newly developed at the superconducting cyclotron AGOR, has been used for the investigation of 1ħω and 2ħω isovector giant resonances in58Co via theτ+ channel. The angular distributions of double-differential cross sections have been extracted betweenEx = 7 and 30 MeV fromθlab = 0° to 6°. A DWBA analysis with normal-mode (NM) wave functions and a multipole-decomposition analysis (MDA) indicate strength for the monopole resonances (dominated by the spin-flip transition) fragmented between 10 and 27 MeV excitation energy, in rather good agreement with preliminary RPA calculations.

Investigation of IVGRs via the 58Ni(t, 3He) 58Co reaction

A 130 MeV primary tritium beam, newly developed at the superconducting cyclotron AGOR, has been used for the investigation of 1ħω and 2ħω isovector giant resonances in 58Co via the τ + channel. The angular distributions of double-differential cross sections have been extracted between E x = 7 and 30 MeV from θ lab = 0° to 6°. A DWBA analysis with normal-mode (NM) wave functions and a multipole-decomposition analysis (MDA) indicate strength for the monopole resonances (dominated by the spin-flip transition) fragmented between 10 and 27 MeV excitation energy, in rather good agreement with preliminary RPA calculations.

Octupole Strength of β-Stable and Drip-Line Nuclei**The project supported by National Natural Science Foundation of China under Grant No. 10047001, the Major State Basic Research Development Program under Grant No. G2000-0774-07 and the CAS Knowledge Innovation Project No. KJCX2-SW-NO2

The octupole excitations of β-stable nucleus , a neutron skin nucleus and a drip-line nucleus are studied by using the self-consistent Hartree–Fock calculation plus the random phase approximation (RPA) with Skyrme interaction. The lowest isoscalar (IS) excitation below threshold for nuclei and and the IS and isovector (IV) giant resonances of nuclei , and can be well described by collective model. For skin nucleus and drip-line nucleus , the low-lying unperturbed neutron octupole strength () of transitions to non-resonant states are nearly unaffected and the transitions to bound states are absorbed into collective states by taking into account the RPA correlation.

Direct proton-decay properties of the isobaric analog and isovector monopole giant resonances

A self-consistent shell-model continuum-random-phase approximation approach is developed to evaluate the partial proton widths of isobaric analog resonances in spherical nuclei from a wide mass interval. The results are found to be in reasonable agreement with the experimental data. An attempt to deduce the single-particle spectroscopic factors from the comparison of the experimental and calculated widths is undertaken. The direct proton-decay branching ratio for the isovector giant monopole resonance in ${}^{208}\mathrm{Bi}$ is estimated in connection with recent experimental data.

Direct and statistical proton decay from isovector giant resonances in 90Nb

Direct and statistical proton decay from isovector giant resonances in 90Nb

Observation of the isovector giant monopole resonances in the ( 3He, t) reaction

Observation of the isovector giant monopole resonances in the ( 3He, t) reaction

Relativistic RPA and Giant Resonances in β Stable and Unstable Nuclei**The project partly supported by National Natural Science Foundation of China and The Science Foundationfor Nuclear Industry of China

The nuclear isoscalar and isovector giant resonances in stable and unstable nuclei are studied in the framework of the relativistic random phase approximation. The classical meson propagators with nonlinear self-interactions are constructed in momentum space from the second variation of the action. The relativistic models with the parameter sets, TM1 and NLSH, which provide good account of static ground state properties, can also well describe the collective states of nuclei, such as giant resonances. The isovector giant dipole resonances in the unstable Ar isotope chain are investigated.

Displacement fields of excited states in stable and neutron drip-line nuclei

We study displacement fields of multipole excitations in a halo nucleus 4 11Be 7 and a skin nucleus 20 60Ca 40 in comparison with those of collective states in a β-stable nucleus 82 208Pb 126. The self-consistent Hartree-Fock calculation plus the random phase approximation with Skyrme interactions is performed in the coordinate space, taking into account the proton and neutron degree of freedom. Transition densities, radial current components and displacement fields of giant resonances in drip-line nuclei are found very similar to those in β stable nuclei. The isovector giant resonances are not properly understood in terms of any available collective models, while the isoscalar giant resonances are in a good approximation expressed by collective models. It is found that the radial velocity of the highest-lying peak of isovector giant multipole resonances vanishes at a radius slightly outside the surface. It is shown that transition densities and current components of the excitations just above the threshold of the drip-line nuclei are extended to far outside the nuclear surface, where the displacement fields show a simple pattern.

Isovector giant resonances in 6He, 12B, 90Y, 120In, and 208Tl observed in the ( 7Li, 7Be) charge-exchange reaction

The ( 7Li, 7Be) and ( 7Li, 7Beγ) reactions have been studied at bombarding energies of 50 A MeV on targets of 6Li, 12C, 90Zr, 120Sn, 208Pb, and polystyrene. The detection of Doppler-shifted γ-rays from excited 7Be permits the identification of the spinflip and non-spinflip characteristics. Transitions to over 20 discrete states and resonances have been observed. Angular distributions for 6Li and 12C are well described by microscopic one-step distorted-waves calculations with shell-model transition amplitudes. The transitions to the ground states of 6He and 12C exhibit significant Gamow-Teller strength, but ∼7% and ∼17%, respectively, of the calculated cross sections at θ ≈ 0° arise from the tensor interaction. A broad resonance at ∼5.6 MeV in the halo nucleus 6He with J π = (2 +,1 −,0 +) seems to correspond to a structure predicted at this excitation energy containing soft modes of excitation with J π = 0 +and/or 2 +. A resonance in 12B at ∼4.5 MeV is described by a superposition of known 2 − and 4 − states, and another one at ∼7.5 MeV is dominated by a transition to a 2 − state with weaker contributions from two 1 − states (spin dipole and giant dipole). Four to six resonances each, mostly spin-dipole resonances, were identified in the final nuclei 90Y, 120In, and 208Tl. The non-resonant quasi-free charge-exchange reaction p bound( 7Li, 7Be)n free was observed for all targets up to effective excitation energies of 50 MeV. The mechanism is related to that on free protons. The effective number of participating protons was determined.

Relativistic calculations of giant resonances with non-linear models

The nuclear isoscalar and isovector giant resonances in stable and unstable nuclei are studied in the framework of the relativistic random phase approximation. The meson propagators with non-linear self-interactions are constructed in momentum space from the second variation of the action. It is found that relativistic models with relatively large values of the compression modulus can nevertheless describe satisfactorily the breathing mode energy in 208Pb. Results for isovector monopole resonances in closed shell nuclei, and for the giant dipole resonance in Argon isotopes are also discussed.

Giant resonances in the relativistic RPA with non-linear interactions

Nuclear isoscalar and isovector giant resonances are studied in the framework of the relativistic random phase approximation starting from effective Lagrangians containing meson self-interaction terms. The meson propagators are worked out in momentum space from the second variation of the action of the system. The N N excitations are not included to be consistent with the relativistic mean field approximation used for these effective Lagrangians. It turns out that the non-linear models such as TM1, NL-SH, etc., which are of great success in describing nuclear ground state properties in mean field approximation can also well reproduce the isoscalar monopole, quadrupole as well as isovector monopole, dipole resonances of spherical nuclei.

Structure of giant quadrupole resonances in neutron drip line nuclei

We study the giant quadrupole resonances in the neutron drip line nucleus, ${}_{8}^{28}{\mathrm{O}}_{20},$ using the self-consistent Hartree-Fock calculation plus the random-phase approximation (RPA) with Skyrme interactions. Including simultaneously both the isoscalar and the isovector correlation, the RPA response function is calculated in the coordinate space so as to take into account properly the continuum effect. Though in neutron drip line nuclei the distribution of the quadrupole strength is much affected by the presence of the low-energy threshold strength, the isoscalar and the isovector correlation structure as well as the transition densities of the isoscalar giant resonance are in good agreement with those given by the collective model. In contrast, the transition density of the isovector giant resonance shows a clear deviation from that of the liquid-drop polarization mode.

The (7Li, 7Be gamma ) reaction and isovector spin strength in 40Ca.

The $^{40}\mathrm{Ca}$${(}^{7}$Li,${\mathrm{}}^{7}$Be\ensuremath{\gamma}) reaction at E${(}^{7}$Li)/A = 70 MeV has been used to investigate isovector giant resonances and to measure the ratio of \ensuremath{\Delta}S=1 to \ensuremath{\Delta}S=0 spin strength in $^{40}\mathrm{Ca}$ at high excitation energy. In the singles spectrum, the analogue of the giant dipole resonance is observed. Another resonance is observed at ${\mathit{E}}_{\mathit{x}}$\ensuremath{\approxeq}15 MeV in the residual nucleus, which we suggest is a collective ${2}^{\mathrm{\ensuremath{-}}}$ state. The isovector spin transfer strength at high excitation for the calcium target is compared to that measured with a carbon target under the same experimental conditions. Although more spin transfer strength at high excitation is observed for the calcium target than for the carbon target, this only accounts in part for an apparent discrepancy between previous $^{40}\mathrm{Ca}$(p\ensuremath{\rightarrow},p${\ensuremath{\rightarrow}}^{\ensuremath{'}}$) and $^{12}\mathrm{C}$${(}^{7}$Li,${\mathrm{}}^{7}$Be) experiments. The general utility of the ${(}^{7}$Li, $^{7}\ensuremath{\gamma}$) reaction is discussed. \textcopyright{} 1996 The American Physical Society.

Orbital response to backward (e, e′) scattering at low and high excitation energies

Abstract The response of orbital excitations with K π = 1 + in heavy deformed nuclei to backward ( e , e ′) scattering is studied in RPA on the example of 154 Sm. The summed M1 and E2 cross sections are of comparable magnitude over wide ranges of transferred momenta: 0.9 q −1 for low excitation energies E x q −1 in the high-energy region 17 E x q , where the M1 response dominates at low energy and the E2 response in the high-energy region. The transition densities of the single strongest RPA states from the two energy regions are similar to that of the scissors state, apart from their smaller amplitudes. The spin M1 responses are similar at low and high energy and dominant for high q . The orbital M1 response is dominant up to high q -values at low energy and only for small q at high energy. The electric response of the low RPA state (mainly C2) is suppressed at a full backward angle, while it persists at high energy for θ ⩾ 165° due to its dominant transverse E2 component. The E2 strength below 9 MeV and between 13 and 17 MeV belongs neither to the isoscalar nor to the isovector giant resonances.

Isovector giant resonances in 6He, 12B, 90B, 120In, and 208Tl

The ( 7Li, 7Be) charge-exchange reaction has been studied at E( 7Li)=350 MeV on targets of 6Li, 12C, 90Zr, 120Sn, and 208Pb. Coincidences with Doppler-shifted γ-rays from the de-excitation in flight of the first-excited state in 7Be were recorded to identify spinflip transitions. Numerous discrete states and giant resonances were observed. Angular distributions were obtained for 6Li and 12C targets. They are well described by microscopic finite-range distorted-waves calculations with theoretical shell-model transition amplitudes and a nucleon-nucleon interaction containing central and tensor components.

Decay of the Isovector Giant Resonances excited in Charge Exchange Reactions

Isovector Resonances are studied through the ( 13C, 13N-n) reaction at 60A.MeV incident energy on 208Pb and 58Ni targets. Neutron decay of the resonances observed in these reactions has been measured in coincidence with the 13N ejectile. The measured neutron multiplicity has been used to deduce the contribution of fast processes like Pick-Up Break-Up and Quasi-Free Charge Exchange to the singles spectrum. The analysis of the neutron decay of the observed resonances revealed a direct branch to well defined final states in the residual nuclei, 207Tl and 57Co. In the case of isovector resonances in 58Co, an angular distribution of this direct branch has been extracted. The strong anisotropy observed for this distribution does not appear compatible with the decay of a pure monopole state.

Isospin mixing in proton-rich N

Estimates of isospin mixing in proton-rich medium-heavy nuclei in the region of mass numbers {ital A}=80 and 100 are made with the use of Hartree-Fock (HF) calculations with Skyrme forces. An isospin mixing probability of 4--5 % is predicted in the ground state of {sup 100}Sn and about 3--4 % in {sup 80}Zr. These are about a factor of 2 larger than the estimates given by Bohr and Mottelson who directly evaluate the isospin mixing with a collective model description of the isovector giant monopole resonance (IVGMR). We show that if we use the energy-weighted sum rule together with the energies of the IVGMR given by Bohr and Mottelson, the predicted isospin admixtures are close to those of HF calculations. More realistic estimates of isospin mixing with random-phase approximation are also presented in the case of {sup 100}Sn.

Isovector Giant Resonances through the (7Li, 7Be) Reaction

Isovector Giant Resonances through the (<SUP>7</SUP>Li, <SUP>7</SUP>Be) Reaction

Collective isospin excitations in nuclear matter droplets.

The isovector giant resonances in heavy nuclei are considered. These excitations correspond to a gapless Goldstone mode which comes along due to the broken symmetry in the isospin space, and can exhibit themselves in a nucleus where the numbers of neutrons and protons are not equal. These modes can be interpreted as weakly damped fluctuations of the transverse isospin components, which propagate through the nuclear matter. The dispersion law of such isospin waves is calculated on the basis of the Fermi-liquid theory as well as by means of the semiclassical variational method. The finite size of the nuclei leads to quantization of the spectra and results in a series of excited states. The number of long-living excited states is strongly affected by the Landau damping. The orthogonality relations and energy-weighted sum rule are formulated. The collective isospin modes of [sup 208]Pb and other heavy nuclei are computed.

Isovector giant resonances studied by the ( 7Li, 7Be) reaction

Spin-flip ( ΔS=1) and non spin-flip ( ΔS=0) excitations are separately observed using the ( 7Li, 7Be-γ) reaction. Such a probe provides direct evidence of both electric and magnetic isovector giant resonances as well as relative strength distributions of ΔS=0 and ΔS=1 excitations. Characteristics of the ( 7Li, 7Be) reaction and prospects of studying the isovector giant resonance at intermediate incident energies of 50∼100 MeV/ A are discussed.