Specific features of the nuclear drip line in the region of light nuclei

The problem of the drip line in the region of the even isotopes ^4–12He, ^14–44O, and ^38–80Ca with respect to the emission of one or two neutrons is studied in the Hartree-Fock approximation by using the SLy4 and Ska effective Skyrme forces. The “stability peninsula” in the vicinity of ⁴⁰O is found.     

Shell structure of new magic nuclei: Systematics of features

Investigation of the structure of the upper shells of new magic nuclei revealed an empirical regularity with the following characteristic feature: closed proton and neutron subshells, with identically large total angular momenta (j = j coupling), are located near the Fermi energy and there is a closed subshell with j = 1/2 above one of them. The properties of the nuclei exhibiting this feature of closed upper proton and neutron subshells have been investigated in detail. Quantitative manifestation of the new magicity effects, depending on the occupancy of the corresponding subshells with nucleons, has been analyzed. Several nuclei have been found, which, obviously, also have magic properties, and all classical, new magic, and nonmagic oxygen isotopes ^14–48O have been considered from the new point of view.     

Special non-central forces for sd-shell nuclei

A special non-central component ALS of the residual interaction influences significantly the spectra of sd-shell nuclei. Calculations performed for A = 18, 19, 20, 24 show that the spectra calculated with (sd)ⁿ configurations can be essentially improved by choosing a suitable strength for this part of the interaction.     

Microscopical structure of the states of deformed nuclei in the neighborhood of the yrast line

A simple model is derived which allows one to study the structure of the nuclear states in the neighborhood of the “yrast” band. In the present scheme the precession motion plays a role of one of the normal modes of oscillations. (The structure of the dispersion equation for this mode corresponds to the well known classical formula.) Vibrational states associated with quadrupole oscillations of the nuclear shape are determined from a general equation. At slow rotation this equation breaks up into the known equations for β-, Δ- and γ-vibrations and non-collectivized K^π = 1⁺ excitations.     

Hartree-Fock-Bogolyubov description of nuclei near the neutron-drip line

We consider the Hartree-Fock-Bogolyubov theory of nuclei in the coordinate representation and derive and solve the HFB equation for the Skyrme effective interaction. Ground-state wave functions and energies of the tin isotopes with 100 ? A ? 176 have been determined and the results have been compared with the predictions of the HF+BCS and macroscopic-microscopic models. The lightest tin isotope which is unstable with respect to a neutron emission is predicted by the HFB method to be ¹⁵³Sn. In the region of nuclei where experimental data are not available the macroscopic-microscopic and self-consistent approximations give substantially different results.     

Molecular structure of nuclei

Cluster structures of nuclei are discussed, with emphasis on nuclear clustering in unstable nuclei. The subjects we discuss are alpha condensed states, clustering in Be and B isotopes, and clustering in ³²Mg and ³⁰Ne. The subject of alpha cluster condensation comes from the clustering nature of dilute nuclear matter. We discuss that recent heavy-ion central collision experiments give us nice evidence of the clustering in dilute nuclear matter. We then present a new prediction of the existence of the “alpha cluster condensed states” in the self-conjugate 4n nuclei around the breakup threshold energy into n alpha-particles. As for the clustering in neutron-rich Be, we discuss the comparison between the antisymmetrized molecular-dynamics results and the recent experimental data, which shows that the clustering feature manifests itself very clearly in neutron-rich Be isotopes both in the ground and excited states. Clustering in Be isotopes near neutron dripline is intimately related to the breaking of the neutron magic number N = 8. We report our recent study about the possible relationship between the clustering and the breaking of the neutron magic number N = 20 in ³²Mg and ³⁰Ne. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: horiuchi@ruby.scphys.kyoto-u.ac.jp     

Shell structure in nuclei

A review of shell structure for spherical and a variety of deformed nuclei is presented. The microscopic-macroscopic method of Strutinsky is used to calculate potential energy surfaces with the pure harmonic oscillator and the modified harmonic oscillator. New sets of “magic numbers” for a variety of different prolate, oblate and axially asymmetric shapes are generated. Experimental evidence for the special stability caused by these shell effects is presented with special emphasis on the lightest and heaviest nuclei where the effects are most pronounced. The radial diffuseness parameter is treated as a Strutinsky variable and its significance in extrapolating into the superheavy region considered. The calculation of shell effects for high spin states is also reviewed.     

Pairing and the structure of the -shell nuclei

The influence of the isoscalar and isovector pairing components of the effective nucleon-nucleon interaction is evaluated for several isobaric chains, in the framework of full shell model calculations. We show that the combined effect of both isospin channels of the pairing force is responsible for the appearance of ground states in odd-odd nuclei. However, no evidence is found relating them to the Wigner energy. We study the dependence of their contributions to the total energy on the rotational frequency in the deformed nucleus Cr. Both decrease with increasing angular momentum and go to zero at the band termination. Below the backbending their net effect is a reduction of the moment of inertia, more than half of which comes from the proton-neutron channel.     

Energy features of fissile nuclei at the scission point for various fission modes

Mass-spectroscopicdata on the yields of heavy products originating from the thermal-neutron-induced fission of ²³⁵U nuclei are presented over broad ranges of mass numbers A (125 ? A ? 155), kinetic energies E _k (40 ? E _k ? 80 MeV), and effective ion charges z* (18 ? z* ? 29). The potential energy of a fissile system at the scission point is analyzed with allowance for the positions of the minima that correspond to the most probable ways of separation of the system for the standard (S2) and superasymmetric (S3) fission modes.     

On the quark structure of the lightest nuclei

It is shown that in the 9q- and 12q-systems, i.e.,³He-³He and⁴He nuclei, the most energetically favourable colour magnetic types of symmetry are [f]_cs=[621] and [f]_cs=[82²], which corresponds to the orbital configurationsS ⁵ P ⁴ [52²] _X andS ⁶ P ⁶ [62³] _X . The colour magnetic nucleon attraction associated with these configurations increases as the square of the nucleon number in the sequence²H,³He,⁴He, becoming quite strong in⁴He. This may provide a basis for explaining the “mysterious properties” of³He-³He and⁴He. However, with the further increase in A, the above effect drastically decreases. A qualitative calculation on the quark excitation spectrum for the³H-³He and⁴He nuclei is presented. The⁴He nucleus seems to possess some properties of a “quark magic nucleus”. The experimental situation is discussed.     

Prescriptions for the scaling variable of the nucleon structure function in nuclei

We tested several choices of the in-medium value of the Bjorken scaling variable assuming the nucleon structure function in a nucleus to be the same as that of a free nucleon. The results unambiguously show that it is different. The text was submitted by the authors in English.     

Allowance for the shell structure of colliding nuclei in the fusion-fission process

The motion of two nuclei toward each other in fusion-fission reactions is considered. The state of the system of interacting nuclei is specified in terms of three collective coordinates (parameters). These are the distance between the centers of mass of the nuclei and the deformation parameter for each of them (the nose-to-nose orientation of the nuclei is assumed). The evolution of collective degrees of freedom of the system is described by Langevin equations. The energies of the Coulomb and nuclear (Gross-Kalinovsky potential) interactions of nuclei are taken into account in the potential energy of the system along with the deformation energy of each nucleus with allowance for shell effects. The motion of nuclei toward each other are calculated for two reaction types: reactions involving nuclei that are deformed (₄₂¹⁰⁰Mo + ₄₂¹⁰⁰Mo → ₈₄²⁰⁰Po) and those that are spherical (₈₂²⁰⁸Pb + ₈¹⁸O → ₉₀²²⁶Th) in the ground state. It is shown that the shell structure of interacting nuclei affects not only the fusion process as a whole (fusionbarrier height and initial-reaction-energy dependence of the probability that the nuclei involved touch each other) but also the processes occurring in each nucleus individually (shape of the nuclei and their excitation energies at the point of touching).     

Tensor forces and the ground-state structure of nuclei

Two-nucleon momentum distributions are calculated for the ground states of nuclei with mass number A< or =8, using variational Monte Carlo wave functions derived from a realistic Hamiltonian with two- and three-nucleon potentials. The momentum distribution of np pairs is found to be much larger than that of pp pairs for values of the relative momentum in the range (300-600) MeV/c and vanishing total momentum. This order of magnitude difference is seen in all nuclei considered and has a universal character originating from the tensor components present in any realistic nucleon-nucleon potential. The correlations induced by the tensor force strongly influence the structure of np pairs, which are predominantly in deuteronlike states, while they are ineffective for pp pairs, which are mostly in 1S0 states. These features should be easily observable in two-nucleon knockout processes, such as A(e,e'np) and A(e,e'pp).     

Structure of unstable nuclei near proton drip line

Gamov-Teller (GT) states in nuclei near the proton drip line are studied by using a microscopic Hartree-Fock (H-F) + Tamm-Dancoff approximation (TDA) (or random phase approximation (RPA)). The calculations predict that giant Gamow-Teller (GT) β-decays are possible for N=Z nuclei heavier than ₂₈⁵⁶Ni₂₈, carrying most of the sum rule strength. The amplitude of isospin T=1 admixed to the T=0 ground state in N=Z nuclei is also discussed in relation with Fermi β-decay sum rule. Finally, the shapes of unstable nuclei near the proton drip line are studied by using the finite-range droplet model (FRDM).     

Study of drip line nuclei through two-step fragmentation

We have studied the structure of light neutron-rich nuclei around N = 16 by employing the in-beam -ray spectroscopy technique using the fragmentation of secondary beams of ^25,26Ne, ^27,28Na and ^29,30Mg isotopes. This secondary-beam cocktail was obtained by the fragmentation of a ³⁶S beam at 77.5 MeV by the SISSI/GANIL facility. By a second-step fragmentation, we have measured -ray-residue coincidences in ^17-20C and ^23,24O and described the obtained levels in the framework of the shell model.Received: 31 October 2002, Published online: 24 February 2004PACS: 23.20.Lv transitions and level energies - 21.60.Cs Shell model - 27.20. + n - 27.30. + t