Dominance of low-l component in weakly bound deformed single-neutron orbits

Calculating single-particle (Nilsson) levels in axially symmetric quadrupole-deformed potentials in coordinate space, the structure of weakly bound neutron orbits is studied in the absence of pair correlation. It is shown that in the wave functions of Omega(pi)=1/2(+) orbits, where Omega expresses the projection of the particle angular momentum along the symmetry axis, the l=0 (s(1/2)) component becomes overwhelmingly dominant as the binding energy of the orbits approaches zero, irrespective of the size of the deformation and the kind of Nilsson orbits. Consequently, all Omega(pi)=1/2(+) levels become practically unavailable for both deformation and many-body pair correlation, when the levels approach continuum or lie in the continuum

Wobbling excitations in odd-A nuclei with high-j aligned particles

Using the particle-rotor model in which one high-j quasiparticle is coupled to the core of triaxial shape, wobbling excitations are studied. The family of wobbling phonon excitations can be characterized by: (a) very similar intrinsic structure while collective rotation shows the wobbling feature; (b) strong B(E2;I-->I-1) values for Deltan(w)=1 transitions where n(w) expresses the number of wobbling phonons. For the Fermi level lying below the high-j shell with the most favorable triaxiality gammaapproximate to+20degrees, the wobbling phonon excitations may be more easily identified close to the yrast line, compared with the Fermi level lying around the middle of the shell with gammaapproximate to-30degrees. The spectroscopic study of the yrast states for the triaxial shape with -60degrees<γ<0 are illustrated by taking a representative example with gamma=-30degrees, in which a quantum number related with the special symmetry is introduced to help the physics understanding

One-particle resonant levels in a deformed potential

(S)olving the Schrodinger equation in coordinate space with the appropriate asymptotic boundary conditions, neutron one-particle resonant levels in Y-20-deformed Woods-Saxon potentials are studied. These resonance levels are the natural extension of one-particle bound levels to the continuum and are defined in terms of eigenphase. For one-particle bound levels with Omega(pi)not equal 1/2(+) the corresponding one-particle resonant levels can be always found for small positive energies. For one-particle bound levels with Omega(pi)=1/2(+) the corresponding one-particle resonant levels are either absent or disappearing quickly as energy increases, when we use well-deformed potentials with a realistic size of diffuseness. The possible presence of Omega(pi)=1/2(+) one-particle resonant levels, in which epsilon=0 components in the wave functions play a crucial role, is further studied using a simplified model without spin-orbit potential

Polarization charge of particles near threshold due to the coupling to shape oscillations

It is shown that the isoscalar strength in the energy region just above the low energy threshold, which is created by exciting particles to the continuum in nuclei far from β-stability lines, can be reduced by the attractive coupling to isoscalar shape oscillations. This is in contrast to the well known fact that in β-stable nuclei low lying isoscalar particle hole strengths are always increased by the attractive isoscalar coupling, which leads to an appreciable amount of positive polarization charge. On the other hand, the core polarization effect on bound particles with l=0 and 1 is always positive, but vanishes as the binding energy approaches to zer

Positive-energy one-particle levels in quadrupole-deformed Woods-Saxon potentials

Positive-energy one-particle levels for neutrons in Y20 deformed Woods-Saxon potentials are examined using the eigenphase representation. Taking the example of =1/2+ levels, not only one-particle resonant levels but also all solutions in the eigenphase representation within a model space are studied. It is shown that a particular eigenphase solution among an infinite number of eigenphase solutions at a given energy plays a crucial role in producing low-lying one-particle resonance, whereas for the excitation energy lower than a few MeV the eigenphase sum is almost equal to the particular eigenphase when the sum is expressed by the value mod n. Some one-particle resonant levels defined in terms of eigenphase, which have no correspondence to any bound one-particle levels, are found and discussed. It is shown that the relative probability of the s1/2 component estimated using the probabilities inside the Woods-Saxon potentials is a decisive factor for obtaining one-particle resonant levels as a continuation of weakly bound =1/2+ levels

Shell-structure and pair-correlation in nuclei close to the neutron drip line

First, in the absence of pair correlation the properties of one-particle orbits of neutrons in nuclei close to the neutron drip line are briefly reviewed for both weakly bound neutrons and the neutrons in the continuum. The low-lying states in the continuum, which are close to the Fermi level, are of immediate importance for pair correlation and deformations of those nuclei. Then, the Hartree-Fock-Bogoliubov (HFB) equation in a simplified model is solved in coordinate space with the correct asymptotic boundary conditions, in order to provide quantitative characterization of both the role of weakly bound neutrons and the effect of the resonances in the continuum on pairing correlations

Effect of weakly-bound neutrons on pair-correlation and deformation

The unique role of weakly-bound low-angular-momentum neutrons in the structure of neutron-drip-line nuclei is presented, studying both the many-body pair-correlation in spherical nuclei and the one-particle orbits in the deformed Woods-Saxon potential. Both the HFB equations in the former case and the Schrodinger equation in the latter are solved in coordinate space with correct asymptotic boundary conditions. Combining the results of those two cases, it is concluded, for example, that all Ωx=1/2+ one-particle levels become practically unavailable for both deformation and many-body pair-correlation, when the HF one-particle levels approach continuum or lie in the continuu

Effective pair-gap of weakly-bound neutrons in deformed nuclei

The dependence of effective pair gap on weakly bound neutron orbits is studied in deformed nuclei in comparison with that in spherical nuclei, solving the Hartree-Fock-Bogoliubov equation in a simplified model in coordinate space with the correct asymptotic boundary conditions. In spherical nuclei the effective pair gap of s1/2 neutrons decreases to zero in the limit that the corresponding Hartree-Fock one-particle energy approaches zero. In the same limit, the effective pair gap of =1/2+ neutrons in deformed nuclei becomes very small when the wave functions of =1/2+ orbits contain an appreciable amount of s1/2 components, even if a considerable amount of larger-components remains in the wave functions. Then, the one-particle excitation spectra of deformed even-even neutron-drip-line nuclei, in which an =1/2+ level is weakly bound, can start at much lower energy than twice the average pair gap in the presence of many-body pair correlation

Examining possible neutron-halo nuclei heavier than Mg 37

The even-Z odd-N neutron-halo nuclei, which are the lightest neutron-halo nuclei heavier than Mg37, are explored by studying the shell structure unique in weakly bound neutrons for spherical or deformed shapes. It is pointed out that due to the narrowed N=50 spherical energy gap and a few resulting close-lying neutron one-particle levels, 1g9/2, 3s1/2, and 2d5/2, for spherical shape, nuclei with some weakly bound neutrons filling in those levels may be deformed and have a good chance to show deformed s-wave halos. Promising candidates are Cr472471, Cr492473, Cr512475, and Fe512677 in the case that those nuclei lie inside the neutron drip line. An interesting possibility of the deformed p-wave or s-wave halos is suggested also for the nucleus Ar351853