A systematic study of neutron-rich even-even Fe isotopes with a neutron number from 32 to 42 is carried out by using the projected shell model. Calculations are performed up to the spin I = 20 state. Irregularities found in the yrast spectra and in B (E2) values are discussed in terms of neutron excitations to the high-j orbital g 9/2 . Furthermore, the neutron two-quasiparticle structure of a low-K negative-parity band and the proton two-quasiparticle structure of a high-K positive-parity band are predicted to exist near the yrast region. Our study reveals a soft nature for the ground state of N ≈ 40 isotopes and emphasizes the important role of the neutron g 9/2 orbital in determining the structure properties for both low-and high-spin states in these nuclei.

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Recent experiments have reached the neutron-rich Cr isotope with N = 40 and confirmed enhanced collectivity near this sub-shell. The current data focus on low-spin spectroscopy only, with little information on the states where high-j particles align their spins with the system rotation. By applying the Projected Shell Model, we show that rotation alignment occurs in neutron-rich eveneven Cr nuclei as early as spin 8h and, due to shell filling, the aligning particles differ in different isotopes. It is suggested that observation of irregularities in moments of inertia is a direct probe of the deformed single-particle scheme in this exotic mass region.PACS numbers: 21.10. Pc, 21.10.Re, 27.40.+z, 27.50.+e Current nuclear structure studies are devoted to the discussion of enhanced collectivity in the neutron-rich pf -shell nuclei with neutron-number N ≈ 40. One has found strong evidence for compressed first 2 + energy levels and large E2 transitions linking these and the ground states for several isotopic chains around the proton magic number Z = 28, for example, in the Cr (Z = 24) [1,2], Fe (Z = 26) [3][4][5], and Zn (Z = 30) [6] isotopic chains. These experimental results support the early suggestions that near N = 40, pronounced collectivity develops corresponding to the formation of a region of deformation [7][8][9].In the study of neutron-rich nuclei, an important issue is to understand emerging sub-shell gaps which cause substantial modifications of the intrinsic shell structure in nuclei with a neutron excess [10]. While information on collective excitations in low-spin states is useful, a comprehensive knowledge for these exotic nuclei requires the study of higher-spin states in which, due to rotation alignment, quasiparticle configurations are dominant. For an yrast band consisting of the lowest states for each angular momentum, the aligning particles carry valuable information on the deformed singleparticle states. Therefore, investigations of high-spin spectra can yield knowledge on the intrinsic shell structure of single-particle levels.Microscopic calculations have shown that beginning from N ≈ 30, energy minima with sizable prolate deformations show up for the neutron-rich Cr isotopes [11]. In these deformed Cr isotopes, protons occupy up to the πf 7/2 orbit whilst neutrons of the N > 28 isotopes fill in the rest of the pf -shell. With the splitting of singleparticle orbits due to deformation, the proton Fermi level lies between the f 7/2 orbitals π can easily occupy these orbitals. Thus when nuclei rotate, these high-j particles (here, f 7/2 for protons and g 9/2 for neutrons) are among the first to align their rotation along with the rotation-axis of the system, resulting in observable effects in the moment of inertia, which correspond to the phenomenon known as rotation alignment [12]. Thus with increasing neutron number from N = 30 towards 40 and beyond, these high-j orbits dominate the high-spin behavior of these nuclei. This qualitative picture is valid also for nuclei with a soft ground state...

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