The effective interaction GXPF1 for shell-model calculations in the full pf shell is tested in detail from various viewpoints such as binding energies, electro-magnetic moments and transitions, and excitation spectra. The semi-magic structure is successfully described for N or Z = 28 nuclei, 53 Mn, 54 Fe, 55 Co and 56,57,58,59 Ni, suggesting the existence of significant core-excitations in low-lying nonyrast states as well as in high-spin yrast states. The results of N = Z odd-odd nuclei, 54 Co and 58 Cu, also confirm the reliability of GXPF1 interaction in the isospin dependent properties. Studies of shape coexistence suggest an advantage of Monte Carlo Shell Model over conventional calculations in cases where full-space calculations still remain too large to be practical.

An effective interaction is derived for use in the full p f basis. Starting from a realistic G-matrix interaction, 195 two-body matrix elements and four single-particle energies are determined by fitting to 699 energy data in the mass range 47-66. The derived interaction successfully describes various structures of p f -shell nuclei. As examples, systematics of the energies of the first 2 ϩ states in the Ca, Ti, Cr, Fe, and Ni isotope chains and energy levels of 56,57,58 Ni are presented. The appearance of a new magic number 34 is seen.

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.

Abstract. The shell-model effective interaction GXPF1 is tested for the description of unstable pf -shell nuclei. The GXPF1 successfully describes the N = 32 shell gap in Ca, Ti and Cr isotopes, while the deviation of predicted Ex(2Ti from the recent experimental data requires the modification of the Hamiltonian especially in the T = 1 matrix elements related to the p 1/2 and f 5/2 orbits. The modified interaction gives improved description simultaneously for all these isotope chains. PACS

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We show how shape transitions in the neutron-rich exotic Si and S isotopes occur in terms of shell-model calculations with a newly constructed Hamiltonian based on VMU interaction. We first compare the calculated spectroscopic-strength distributions for the proton 0d 5/2,3/2 and 1s 1/2 orbitals with results extracted from a 48 Ca(e,e'p) experiment to show the importance of the tensorforce component of the Hamiltonian. Detailed calculations for the excitation energies, B(E2) and two-neutron separation energies for the Si and S isotopes show excellent agreement with experimental data. The potential energy surface exhibits rapid shape transitions along the isotopic chains towards N =28 that are different for Si and S. We explain the results in terms of an intuitive picture involving a Jahn-Teller-type effect that is sensitive to the tensor-force-driven shell evolution. The closed sub-shell nucleus 42 Si is a particularly good example of how the tensor-force-driven Jahn-Teller mechanism leads to a strong oblate rather than spherical shape.

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