Large-scale shell-model calculations for unnatural-parity high-spin states in neutron-rich Cr and Fe isotopes

Abstract

We investigate unnatural-parity high-spin states in neutron-rich Cr and Fe isotopes using large-scale shell-model calculations. These shell-model calculations are carried out within the model space of fp-shell + 0g 9/2 + 1d 5/2 orbits with the truncation allowing 1 ω excitation of a neutron. The effective Hamiltonian consists of GXPF1Br for fp-shell orbits and V MU with a modification for the other parts. The present shell-model calculations can describe and predict the energy levels of both natural-and unnatural-parity states up to the high-spin states in Cr and Fe isotopes with N 35. The total energy surfaces present prolate deformations on the whole and indicate that the excitation of one neutron into the 0g 9/2 orbit plays the role of enhancing prolate deformation. For positive (unnatural)-parity states in odd-mass Cr and Fe isotopes, their energy levels and prolate deformations indicate the decoupling limit of the particle-plus-rotor model. The sharp drop of the 9/2 + 1 levels in going from N = 29 to N = 35 in odd-mass Cr, Fe, and Ni isotopes is explained by the Fermi surface approaching the ν0g 9/2 orbit.

Occupation numbers of spherical orbits in self-consistent beyond-mean-field methods

“…They find that the ordering of levels in 61 Ca is inverted with respect to the standard shell model filling with a sequence 1/2 + , 5/2 + , 9/2 + , closer to the SM-CI ESPEs. However, they propose a configuration (3s 1/2 ) 2ν for the ground state of 62 Ca, which implies a certain resilience of the N = 40 gap, as suggested also by the SCCM description. Indeed, only spectroscopic data on 62 Ti settle these discrepancies.…”

“…However, they propose a configuration (3s 1/2 ) 2ν for the ground state of 62 Ca, which implies a certain resilience of the N = 40 gap, as suggested also by the SCCM description. Indeed, only spectroscopic data on 62 Ti settle these discrepancies. The occupation numbers of the 1g 9/2 and 2d 5/2 spherical neutron orbits are plotted in Fig.…”

“…Recently, this region has been widely studied both experimental [34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50] and theoretically [51][52][53][54][55][56][57][58][59][60][61][62][63] because of its interest as a new island of inversion analogous to the one found at N = 20. Here, a detailed analysis performed in the 64 Cr nucleus is presented first, and then such a study is extended to the neutron-rich N = 40 even-even isotones, namely, 60 Ca, 62 Ti, 64 Cr, 66 Fe, and 68 Ni. Calculations are performed with the Gogny D1S parametrization [8] using a working basis made of eleven major spherical harmonic oscillator shells.…”

“…The LNPS interaction predicts that the orbits 1f 5/2 , 1g 9/2 , and 2d 5/2 become degenerate in the neutron rich part ( 60 Ca), while with the Gogny SSPEs the N = 40 gap remains constant. This makes the two approaches diverge in their predictions of the structure of 62 Ti and 60 Ca, as we shall discuss below. The degeneracy of the ESPEs favors the persistence of quadrupole correlations, responsible for the onset of deformation.…”

“…The agreement that we have found in the case of 64 Cr extends to 66 Fe and to a lesser extent to 68 Ni. Notice that the 2d 5/2 orbit has a non-negligible occupancy only when the nucleus is deformed, hence the large differences between SCCM and SM-CI for 62 Ti and 60 Ca. In the SCCM approach, the 2d 5/2 and 3s 1/2 (not shown) orbits present a similar behavior, i.e., their occupation is maximum in the middle of the chain (Z = 24) and negligible at the shell closures.…”