We present a new effective interaction for shell-model calculations in the model space consisting of the single-particle orbits 1p 3/2 , 0f 5/2 , 1p 1/2 , and 0g 9/2 . Starting with a realistic interaction based on the Bonn-C potential, 133 two-body matrix elements and four single-particle energies are modified empirically so as to fit 400 experimental energy data out of 69 nuclei with mass numbers A = 63 ∼ 96. The systematics of binding energies, electromagnetic moments and transitions, and low-lying energy levels are described. The soft Z = 28 closed core is observed, in contrast to the stable N = 50 shell closure. The new interaction is applied to systematic studies of three different chains of nuclei, Ge isotopes around N = 40, N = Z nuclei with A = 64 ∼ 70, and N = 49 odd-odd nuclei, focusing especially on the role of the g 9/2 orbit. The irregular behavior of the 0 + 2 state in Ge isotopes is understood as a result of detailed balance between the N = 40 single-particle energy gap and the collective effects. The development of the band structure in N = Z nuclei is interpreted in terms of successive excitations of nucleons into the g 9/2 orbit. The triaxial/γ -soft structure in 64 Ge and the prolate/oblate shape coexistence in 68 Se are predicted, showing a good correspondence with the experimental data. The isomeric states in 66 As and 70 Br are obtained with the structure of an aligned proton-neutron pair in the g 9/2 orbit. Low-lying energy levels in N = 49 odd-odd nuclei can be classified as proton-neutron pair multiplets, implying that the obtained single-particle structure in this neutron-rich region appears to be appropriate. These results demonstrate that, in spite of the modest model space, the new interaction turns out to describe rather well properties related to the g 9/2 orbit in various cases, including moderately deformed nuclei.

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The cross sections for single-neutron removal from the very neutron-rich nucleus 31Ne on Pb and C targets have been measured at 230 MeV/nucleon using the RIBF facility at RIKEN. The deduced large Coulomb breakup cross section of 540(70) mb is indicative of a soft E1 excitation. Comparison with direct-breakup model calculations suggests that the valence neutron of 31Ne occupies a low-l orbital (most probably 2p(3/2)) with a small separation energy (S(n) approximately < 0.8 MeV), instead of being predominantly in the 1f(7/2) orbital as expected from the conventional shell ordering. These findings suggest that 31Ne is the heaviest halo system known.

Unambiguous values of the spin and magnetic moment of 31 Mg are obtained by combining the results of a hyperfine-structure measurement and a -NMR measurement, both performed with an optically polarized ion beam. With a measured nuclear g factor and spin I 1=2, the magnetic moment 31 Mg ÿ0:8835515 N is deduced. A revised level scheme of 31 Mg (Z 12, N 19) with ground state spin/parity I 1=2 is presented, revealing the coexistence of 1p-1h and 2p-2h intruder states below 500 keV. Advanced shell-model calculations and the Nilsson model suggest that the I 1=2 ground state is a strongly prolate deformed intruder state. This result plays a key role for the understanding of nuclear structure changes due to the disappearance of the N 20 shell gap in neutron-rich nuclei. Since Mayer and Jensen established the concept of shell structure in atomic nuclei, magic nucleon numbers have played a decisive role in describing the nuclear system [1]. About a quarter century later, the discovery of the anomalous ground state properties of 31 Na [2,3] suggested that the magic shell structure can be broken. Shell-model calculations allowing particle-hole (p-h) excitations across the N 20 shell gap proposed that a group of nuclei with deformed ground states appears between Z 10-12 and N 20-22. Because the p-h excited intruder states come lower in energy than the normal shell-model states, this region has been called the ''island of inversion '' [4]. In fact, -decay experiments [5,6] It has been suggested that the N 20 shell gap is changing from one nucleus to another [11,12] due to changes in the proton-neutron interaction. The boundary of the island of inversion can thus be shifted or smeared out, and intruder ground states might appear outside the earlier defined boundaries. Since the size of the shell gap is related to the single-particle energies [determined mainly by the monopole part of the nucleon-nucleon (NN) interaction], the mapping of the boundary is linked to one of the most basic and unanswered questions in present day nuclear structure physics: the microscopic mechanism to determine the monopole part of the NN interaction.We present in this Letter a measurement of the ground state spin and magnetic moment of the exotic even-odd nucleus 31 Mg (Z 12, N 19). The earlier observed anomalous lifetime and the branching intensities in its decay have never been explained [5,13], although the high level density suggested the presence of intruder states at low excitation energy [14]. However, unambiguous spin/ parity assignments are needed in order to establish the coexistence of normal sd-shell states with 1p-1h and 2p-2h intruder states. In addition to the ground state spin and parity, the magnetic moment value and sign provides direct information on the odd-neutron configuration.The spin and magnetic moment of 31 Mg are measured by combining the results from two experimental techniques, based on the atomic hyperfine structure and on the nuclear interaction with external magnetic fields. Both methods rely on an optically polarized...

We first review the shell evolution in exotic nuclei driven by nuclear forces. We then demonstrate that the underlying mechanism played by the balance of the tensor and central components in the effective nucleon-nucleon interaction is crucial when describing shape coexistence. This effect will be referred to as type II shell evolution, while the shell evolution passing through a series of isotopes or isotones is denoted as type I. We describe type II shell evolution in some detail for the case of the 68 Ni nucleus as an example. We present how the fission dynamics can be related to enhanced deformation triggered by type II shell evolution, at its initial stage. It is suggested that the island of stability may be related to the suppression of this mechanism.

High-resolution study of Gamow-Teller excitations in thê {42}Ca(^{3}He,t)^{42}Sc reaction and the observation of a "low-energy super-Gamow-Teller state"Phys. Rev. C 91, 064316

The breakdown of the N = 20 magic number in the so-called island of inversion around 32 Mg is well established. Recently developed large-scale shell-model calculations suggest a transitional region between normal-and intruder-dominated nuclear ground states, thus modifying the boundary of the island of inversion. In particular, a dramatic change in single-particle structure is predicted between the ground states of 30 Mg and 32 Mg, with the latter consisting nearly purely of 2p-2h N = 20 cross-shell configurations. Single-neutron knockout experiments on 30,32 Mg projectiles have been performed. We report on a first direct observation of intruder configurations in the ground states of these very neutron-rich nuclei. Spectroscopic factors to low-lying negative-parity states in the knockout residues are deduced and compare well with shell-model predictions.

The N = 28 shell closure has been investigated via the 46Ar(d,p)47Ar transfer reaction in inverse kinematics. Energies and spectroscopic factors of the neutron p(3/2), p(1/2), and f(5/2) states in 47Ar were determined and compared to those of the 49Ca isotone. We deduced a reduction of the N = 28 gap by 330(90) keV and spin-orbit weakenings of approximately 10(2) and 45(10)% for the f and p states, respectively. Such large variations for the f and p spin-orbit splittings could be accounted for by the proton-neutron tensor force and by the density dependence of the spin-orbit interaction, respectively. This contrasts with the picture of the spin-orbit interaction as a surface term only.