Isovector Giant Resonance Research Articles

Isovector giant resonances in a relativistic mean-field theory

Isovector giant resonances are modeled as collective excitations of a relativistic baryon-meson field theory. Eigenvalue equations for small amplitude isovector motion about the static mean-field solutions are derived. Energy spectra, transition densities, and form factors for inelastic electron scattering are calculated.

Spin-Dependent Isovector Giant Resonances as a Possible Precursor of the Pion-Condensation in Finite Nuclei

The pion-condensation is an instability of the nuclear medium through a softening of the spin-isospin sound mode with high momentum (~1.5PF, PF being the Fermi momentum) due to the attractive OPEP (OnePion-Exchange-PotentiaI).') It means that the precursor of the condensation is a softening of the mode. Here we examine the possiblity that it manifests itself directly in finite nuclei as a softening of spin-dependent isovector giant resonances (M-mode). Following Dabrowski and Haensel;) we adopt an extended version of the SteinwedelJensen (S-]) model) to describe the mode in the simplest way: The neutron (proton) fluid is replaced by the A+(A-) fluid consisting of neutrons with the up (down) spin and protons with the down (up) spin. The restoring force is given by the spin-isospin (S1) symmetry energy Cd7: in place of the isospin symmetry energy C 7: in the S-J model. In this way for the energy of the or-mode of a given multipolarity L and node n, we have CliP = cq';,L) with c=/2cd7:/M and q~L)=z~L)/R. Here, M is the nucleon mass, R the radius of the nucleus and z~L) the n-th zero of the derivative of the spherical Bessel function of order L. The corresponding difference between the local densities of A+ and Afluids with a multipolarity L has a spatial form of

Nuclear Vibrations and Effective Interactions

We derive separable interactions which are equivalent to the zero-range Skyrme forces in the random phase approximation. The isoscalar and the isovector tl-pole-tl-pole forces proposed by Bohr and Mottelson are obtained as a special case of the separable interactions. Spin- and isospin-dependent separable forces are also discussed. 3 ) reproduces very well static properties of the nuclear ground states in the broad region of the mass table. The same interaction has been shown by many authors 4)~6) to explain very well the gross structure of the dynamic properties of nuclei like giant multipole resonances. It is well known that the separable Q-Q force developed by Bohr and Mottelson et al.7),8) has played an important role in comprehensive description of nuclear low-lying collective excitations. The introduction of the separable force makes it possible to perform the calculations of many-body correlation problems in a simple and intuitive manner and to analyze experimental data systematically throughout the periodic table. Moreover, Bohr and Mottelson et a1. 7 ),9) have extended their idea of the separable-force to the study of giant multipole resonances by using the sum-rules and the self-consistent condition between the vibrating potential and density. Their model has been Quite successful in the calculation of the excitation energies of the isoscalar and isovector giant resonances and, also, in the estimate of the core polarization effects. Up till now, some authors10),11J discussed the connection between the separable A-pole-A-pole interaction and the more sophisticated interaction, that is, the Skyrme force related with the isoscalar and the isovector giant resonances. 'l Present address: Division de Physique Theorique, Intitut de Physique Nucleaire, 91406 Orsay-Cedex, France.

Observation of aT>Gamow-Teller State inCa48(p,n)Sc48at 160 MeV

Neutron spectra from the reaction $^{48}\mathrm{Ca}$($p$,$n$) $^{48}\mathrm{Sc}$ at 160 MeV reveal a prominent narrow peak at ${E}_{x}=16.8$ MeV. This state is interpreted as a spin-flip isovector giant resonance; it carries a significant fraction of the ${T}_{g}$ ($T=4$) Gamow-Teller strength. The observed position of this resonance agrees with both a recent shell-model prediction and with the position expected from systematics for the analog of the giant $M1$ resonance in $^{48}\mathrm{Ca}$.

Excitation of isovector giant resonances by inelastic scattering of positive pions on90Zr at 226 MeV

We studied the region of giant resonances with positive pions of 226 MeV scattered inelastically on90Zr. Two groups of resonances were seen: the first structure between 12 and 19 MeV excitation energy is explained as a sum of the isoscalar quadrupole resonance at 14 MeV, the isovector dipole resonance at 16.5 MeV and possibly some E0 strength. The second group between 24 and 34 MeV excitation energy also corresponds to more than a simple multipolarity and may be described as a sum of a monopole and a quadrupole isovector resonance.

Isovector giant monopole resonances: A sum-rule approach

Several useful sum rules associated with isovector giant monopole resonances are calculated for doubly closed shell nuclei. The calculation is based on techniques known from constrained and adiabatic time-dependent Hartree-Fock theories and assume various Skyrme interactions. The results obtained form, together with the compiled literature, the basis for a quantitative description of the RPA strength distribution in terms of energy-weighted moments. These, together with strength distribution properties, are determined by a hierarchy of determinantal relations between moments. The isovector giant monopole resonance turns out to be a rather broad resonance centered at E = 46A −1 10 MeV with an extended width of more than 16 MeV. The consequences regarding isospin impurities in the nuclear ground state are discussed.

Time-dependent Hartree-Fock polarizability and random phase approximation sum rules

Abstract Different isovector and isoscalar giant resonances (dipole, monopole and quadrupole) have been studied by evaluating energy-weighted and cubic energy-weighted sum rules. Calculations have been performed within the RPA using different variants of the Skyrme interaction. The link between the two RPA rum rules and the parameters of a collective Hamiltonian has been explicitly shown. Excitation energies and strengths are compared with experiments and with the predictions of other approaches.

Excitation of collective states in spherical nuclei in high-energy proton interactions

The excitation of collective states in spherical nuclei is investigated in interactions with ≈ 1 GeV protons. Collective states of dipole and quadrupole type in a large energy range, including the isoscalar and isovector giant resonances, are considered. The cross sections for proton-nucleus interactions are found within the single-inelastic-scattering approximation of Glauber theory with the use of semi-microscopic nuclear wave functions. The largest probability of excitation is proved to take place for states of isoscalar type. Isovector states are strongly suppressed. It is shown that 1 GeV proton inelastic scattering can be used for the excitation of giant resonances of isoscalar type. The probability of their excitation is of the same order as that for low-lying collective states.