Narrow Results

We suggest that the three-body force due to the Delta-hole excitation plays a crucial role in determining the neutron drip line of oxygen isotopes. This is one of the consequences of the repulsive effective T=1 force produced by the Delta-hole excitation on the shell evolution. The same mechanism makes effective T=1 interaction more repulsive in general.

The shell evolution of even–even drip line argon isotopes ${}^{46\text{–}54}\mathrm{\text{Ar}}$ has been investigated via the shell model calculations using SDPF-U and SDPF-NR two-body effective interactions in two different shell model spaces $\pi (sd)-\nu (fp)$ and $\pi (sd)-\nu (f_{7/2}p)$. In this work, the energy of first $2^{+}$, reduced transition probability $B(E2)$, excitation energy levels as well as how the proton shells evolve with $N$ have been studied. Excellent agreements were obtained for the first $2^{+}$ level along the investigated isotopes within $\pi (sd)-\nu (fp)$ and $\pi (sd)-\nu (f_{7/2}p)$ model spaces.

The systematic search of axial ground state and the emergence of new gaps in neutron-rich ${}^{44-52}$Ar isotopes are the challenging of current calculations of relativistic and nonrelativistic mean field models. For this purpose, we have employed the model based on Relativistic Hartree–Bogoliubov (RHB) with the density-dependent effective interaction of point coupling DD-PCX type. However, the nonrelativistic mean field calculations based on Hartree-Fock model were carried out with Skyrme SV-min parametrization. All the present calculations were performed within axial symmetries. The results provide that most of isotopes show an oblate-deformed ground state. In particular, ${}^{50}$Ar and ${}^{52}$Ar display a spherical shape in their ground states with SV-Min calculations. There is a notable signature of the sub-shell closure at neutron number $N=32$, whereas $N=34$ is expected to be nearly quenched in the spherical ground. Furthermore, coexistence features are predicted in the neutron-rich ${}^{44}$Ar isotope. We have also studied the neutron axial densities and its central depletion in the well-deformed grounds for the first time. In addition to these, the proton single-particle evolution and bubble-like structure at ${}^{46}$Ar are very well reproduced with DD-PCX calculations.

In this talk, I will overview recent studies on the evolution of the shell structure in stable and exotic nuclei, and will show results of related shell-model calculations. This shell evolution is primarily due to the tensor force. The robust mechanism and some examples will be presented. Such examples include the disappearance of existing magic numbers and the appearance of new ones. The nuclear magic numbers have been believed, since Mayer and Jensen, to be constants as 2, 8, 20, 28, 50, … This turned out to be changed, once we entered the regime of exotic nuclei. This shell evolution develops at many places on the nuclear chart in various forms. For instance, the Island of Inversion picture has been changed considerably.

The present work aims at experimentally clarifying the structure change of neutron-rich Mg isotopes as a function of the neutron number by firmly establishing the level schemes of the Mg isotopes in a wide excitation energy range. We use our unique method of β-decay spectroscopy which takes advantage of anisotropic β decay of spin-polarized Na isotopes. The asymmetry parameters of the β decay are the effective tool to unambiguously assign the spin-parity of levels in the daughter nucleus. As the first step of the systematic studies on neutron-rich Mg isotopes from lighter region outside of the N = 20 “island of inversion” to heavier one across through the island, very detailed level scheme of ²⁸Mg and ²⁹Mg have been successfully established at TRIUMF, where the world-highest polarized alkali beams are available.

Characterizing the nature of single-particle states outside of double shell closures is essential to a fundamental understanding of nuclear structure. This is especially true for those doubly magic nuclei that lie far from stability and where the shell closures influence nucleosynthetic pathways. The region around ¹⁰⁰Sn is one of the most important due to the proximity of the N=Z=50 magic numbers, the proton-drip line, and the end of the rp-process. However, owing to the low production rates, there is a lack of spectroscopic information and no firm spin-parity assignment for ground states of odd-A isotopes close to ¹⁰⁰Sn. Neutron knockout reaction experiments on beams of ^108,106Sn and ¹⁰⁶Cd have been performed at the NSCL. By measuring gamma rays and momentum distributions from reaction residues, the spin of ground state and first excited state for ^107,105Sn have been established. The results also show a degree of mixing in the ground states of the isotopes ^108,106Sn between the d_5/2 and g_7/2 single particle-states and they are compared to reaction calculations. Single-, double-, and triple-neutron knockout reactions on the ¹⁰⁶Cd beam have been observed. The spin-parity of ¹⁰⁵Cd is already known, therefore, the measurement of the momentum distributions of the ground and first excited states of this residue is an important validation of the technique used for the light tin isotopes.

I present one of the possible paradigm shifts with exotic nuclei. This is the shell evolution due to nuclear forces, such as tensor, central and three-nucleon forces. I shall present major points with the N=34 magic number confirmed in ⁵⁴Ca by RIBF of RIKEN very recently, after the theoretical prediction made in 2001. The shell evolution has been generalized to phenomena caused by massive particle-hole excitations, being referred to as Type II Shell Evolution. This can be found in ^68,70Ni. In particular, the shape coexistence of spherical, oblate and prolate shapes is suggested theoretically. Thus, the perspectives of physics with exotic nuclei is being expanded further from single-particle aspects to shapes/deformation, changing the landscape of nuclear structure.