Jan Ryckebusch

A shell-model description of 0+ intruder states in even-even nuclei

Abstract Starting from the nuclear shell structure in medium-heavy and heavy nuclei, the excitation energy for low-lying 0 + intruder states is studied. Taking as a simplified model two particle-two hole (2p-2h) excitations across closed shells, the effects of the pairing and the proton-neutron (monopole and quadrupole component) residual interaction on the unperturbed energies are calculated. Application to major closed-shell ( fZ = 50, Z = 82) and to subshell ( Z = 40, Z = 64) regions is performed. We especially concentrate on 0 + intruder states in the even-even Pb nuclei.

REARRANGEMENT EFFECTS IN SHELL-MODEL CALCULATIONS USING DENSITY-DEPENDENT INTERACTIONS

Abstract A general method is developed to treat density-dependent effective nucleon-nucleon interactions in shell-model calculations. A trial function is introduced, that can generate any excitation mode of the nucleus. The method is based on the evaluation of the expectation value of the full Hamiltonian with respect to this generating Slater determinant wave function. Rearrangement effects, due to the density-dependence of the interaction are systematically included. The proposed formalism is applied in first instance to simplified excitation modes, such as two-quasiparticle and particle-hole configurations. A detailed study on the importance of the various types of rearrngement terms is carried out. A more specific application of the general formalism is presented in carrying out an extended shell-model calculation on 116Sn, including, apart from the neutron 2qp-excitations, proton 1p-1h excitations and coupled excitations of the type (1p-1h) ⊗ (2qp).

Electroinduced two-nucleon knockout and correlations in nuclei

Abstract A model to calculate cross sections for electroinduced two-nucleon emission from finite nuclei is presented. Short-range correlations in the wave functions and meson-exchange contributions to the photoabsorption process are implemented. Effects of the short-range correlations are studied with the aid of a perturbation expansion method with various choices of the Jastrow correlation function. The model is used to investigate the relative importance of the different reaction mechanisms contributing to the A(e,e′pn) and A(e,e′pp) process. Representative examples for the target nuclei 12C and 16O and for kinematical conditions accessible with contemporary high-duty cycle electron accelerators are presented. A procedure is outlined to calculate the two-nucleon knockout contribution to the semi-exclusive (e,e′p) cross section. Using this technique we investigate to what extent far semi-exclusive (e,e′p) reactions can be used to detect high-momentum components in the nuclear spectral function.

An RPA model for the description of one-nucleon emission processes and application to 16O(γ, N) reactions

Abstract A coordinate space RPA formalism is proposed to handle one-nucleon emission processes in a self-consistent way. A method is outlined to solve the RPA equations so obtained for the whole class of rotationally invariant effective interactions, including those having a velocity dependence. The method is used to study 16O(γ, N) reactions with Skyrme-type effective interactions. The use of the long-wavelength approximation (LWA) when handling reactions of this type at medium energies, is criticized. Different alternative forms for the electric transition operators are used, applying Siegerts theorem or not, and it is shown that at higher momentum transfer, the obtained results are strongly sensitive to it. We emphasize that especially beyond energy regions where the LWA is valid, the use of Siegerts theorem should be carefully examined.

Effects of the final-state interaction in (γ, pn) and (γ, pp) processes

A model is presented to describe electromagnetically induced two-nucleon emission processes in a shell-model picture. Distortions in the outgoing nucleon waves are accounted for by performing a partial-wave expansion in a real mean-field potential. The antisymmetry condition for the A-body wave functions is shown to be naturally preserved. The model is used to calculate (γ, pn) and (γ, pp) cross sections off the target nuclei 16O and 12C for photon energies ranging from 50 MeV up to the Δ(1232) isobar threshold. Effects due to the pionic currents and intermediate Δ creation are implemented. The impact of the distortions due to the interaction of the outgoing nucleon waves with the A — 2 core is examined. Hadronic form factors are introduced to regularize the πNN vertices and the sensitivity of the cross section to the pion cutoff mass is examined. The relative contribution of the (γ, pp) and the (γ, pn) channel to the total photoabsorption strength is discussed. Further, the photon energy dependence of the (γ, pp)/(γ, pn) ratio is investigated.

Intruder states in odd-mass and odd-odd nuclei

Abstract A shell-model description for intruder states in both odd-mass and odd-odd nuclei is given starting from 1p1h excitations across closed shells. Combining these results with the behaviour of 0+ intruder states in even-even nuclei, evidence for a scaling of the energy with the number of particle and hole pairs formed in exciting the intruder configuration is obtained.

A genetic algorithm analysis of N∗ resonances in p(γ,K+)Λ reactions

The problem of extracting information on new and known

The role of hyperon resonances in p(γ, K+)Λ processes

N^{*}

Helicity amplitudes and electromagnetic decays of hyperon resonances

resonances by fitting isobar models to photonuclear data is addressed. A new fitting strategy, incorporating a genetic algorithm, is outlined. As an example, the method is applied to a typical tree-level analysis of published

Evidence for short-range correlations in O-16

p(\gamma,K^{+})\Lambda