G. Co

Charge response in 12C and 40Ca

Abstract The charge response for 12 C and 40 Ca in the quasi-elastic region is calculated by means of non-relativistic many-body theory. Within the continuum RPA a finite-range interaction is used which gives a proper description of the low-energy spectrum and the giant resonances. 2p2h effects are included in a phenomenological way utilizing information from the empirical optical potential. The results indicate that the main corrections to the free response are given by the mean field non-locality and the 2p2h effects. Comparisons with nuclear matter indicate the importance of finite size effects. Good agreement with experiment is found in 12 C while the theory overestimates the data in 40 Ca.

Effects of two-particle-two-hole correlations on the longitudinal quasielastic response in 12C☆

Abstract In an extended RPA theory which includes 1p1h as well as 2p2h excitations we have calculated the quasielastic charge response in 12C for momentum transfers of 200, 250 and 300 MeV c . Using a realistic G-matrix interaction in local density approximation we find reasonable agreement with the (e, e) data for 200 and 250 MeV c while at 300 MeV c , as a result of shell truncations, the agreement starts to degrade.

Meson-exchange currents in quasi-elastic electron scattering from 12C and 40Ca nuclei

Abstract Quasi-elastic transverse responses in 12 C and 40 Ca have been calculated considering both one- and two-body nuclear currents. In the two-body current we have taken into account, in addition to the contact and pion in flight terms, also diagrams including the contribution of the Δ isobar. In the evaluation of the transverse response both one-particle one-hole (1 p 1 h ) and 2 p 2 h states have been considered. Our calculations show that the contribution of the meson-exchange currents (MEC) to the 1 p 1 h channel reduces the transverse response. This effect is partially compensated in the peak region by the 2 p 2 h response. A study of the relative importance of the various pieces contributing to the response function is presented. Our 1 p 1 h results (including MEC) are in agreement with a Fermi gas calculation performed with an effective value of the Fermi momentum. The comparison with the experimental data is done after the inclusion of the final state interaction

Model calculations of doubly closed shell nuclei in CBF theory (I)

Abstract Correlated basis function theory and Fermi hypernetted chain theory are extended to treat finite Fermi systems. In this first paper, the effects of the scalar nucleon-nucleon correlations are investigated by studying some model N = Z nuclei. Results of calculations performed using central nucleon-nucleon potentials, without tensor components, are presented and are compared with results from other theories.

Model calculations of doubly closed shell nuclei in CBF theory III. jj coupling and isospin dependence

Abstract Correlated basis function theory and Fermi hypernetted chain technique are extended to study medium-heavy, doubly closed shell nuclei in the jj coupling scheme, with different single-particle wave functions for protons and neutrons and isospin-dependent two-body correlations. Central semirealistic interactions are used. Ground-state energies, one-body densities, distribution functions and momentum distributions are calculated for 12 C, 16 O, 40 Ca, 48 Ca and 208 Pb nuclei. The values of the ground-state energies provided by isospin-dependent correlations are significantly lower than those obtained with isospin-independent correlations. In finite nuclear systems, the two-body Euler equations provide correlation functions variationally more effective than those obtained with the same technique in infinite nuclear matter.

A model for particle emission induced by electron scattering

Abstract A model for the description of particle emission after electro-excitation of a nucleus is developed. The coupling between the individual decay channels is treated within the framework of the continuum random-phase approximation. The interference terms between charge and current operators as well as the interference term between the two different transverse components of the current operator are included in the calculation. The model is applied to the proton decay of 16 O. The sensitivity of the angular distribution of the emitted particle to the presence of weak multipole strengths is analysed and signatures for monopole strength are discussed. Coincidence experiments are shown to be a selective tool to excite bound states embedded in the continuum. Quasi-free knock-out is found to start immediately above the giant-dipole-resonance region.

Model calculations of doubly closed shell nuclei in CBF theory: II. One body density matrix and momentum distribution

Abstract The one body density matrix and the momentum distribution of N = Z nuclei are computed using correlated basis function theory and Fermi hypernetted chain theory. The results obtained with central nucleon-nucleon interactions are presented and the effects of the scalar internucleon correlations are enlightened. A version of the local density approximation for the one body density matrix, that takes particular care of the dynamical correlations, is studied. Its effectiveness is checked against correlated basis function calculations in model nuclei. The approximation is used to evaluate the momentum distribution in realistic models of nuclei.

Tensor and tensor-isospin terms in the effective Gogny interaction

We discuss the need of including tensor terms in the effective Gogny interaction used in mean-field calculations. We show in one illustrative case that, with the usual tensor term that is employed in the Skyrme interaction (and that allows us to separate the like-nucleon and the neutron-proton tensor contributions), we can describe the evolution of the N=28 neutron gap in calcium isotopes. We propose to include a tensor and a tensor-isospin term in finite-range interactions of Gogny type. The parameters of the two tensor terms allow us to treat separately the like-nucleon and the neutron-proton contributions. Two parameterizations of the tensor terms have been chosen to reproduce different neutron single-particle properties in the 48Ca nucleus and the energy of the first 0- state in the 16O nucleus. By employing these two parameterizations we analyze the evolution of the N=14, 28, and 90 neutron energy gaps in oxygen, calcium and tin isotopes, respectively. We show that the combination of the parameters governing the like-nucleon contribution is crucial to correctly reproduce the experimental (where available) or shell-model trends for the evolution of the three neutron gaps under study.

Tensor effective interaction in self-consistent random-phase approximation calculations

We present a study of the effects of the tensor-isospin term of the effective interaction in Hartree-Fock and random-phase approximation calculations. We used finite-range forces of Gogny type, and we added to them a tensor-isospin term which behaves, at large internucleonic distances, as the analogous term of the microscopic interactions. The strength of this tensor force has been chosen to reproduce the experimental energy of the lowest 0{sup -} excited state in {sup 16}O, which shows large sensitivity to this term of the interaction. With these finite-range interactions, we have studied the effects of the tensor-isospin force in ground and excited states of carbon, oxygen, calcium, nickel, zirconium, tin, and lead isotopes. Our results show that the tensor force affects mainly the nucleon single-particle energies. However, we found some interesting cases where also bulk nuclear properties are sensitive to the tensor interaction.

Theoretical analysis of proton decay of electro-excited carbon

Abstract A theoretical model for the microscopic description of electron-induced coincidence experiments is developed, which includes final-state interactions within the continuum random-phase approximation. Charge and current interference effects on the angular distributions of the emitted particle are included. The analysis of the recent 12 C(e, e′p) data by Calarco et al. exhibits clear effects due to the presence of monopole strength, even though no concentrated monopole resonance is present in the theory.