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

Togashi, Tomoaki*; Shimizu, Noritaka*; Utsuno, Yutaka   ; Otsuka, Takaharu*; Homma, Michio*

We perform large-scale shell-model calculations for unnatural-parity high-spin states in neutron-rich Cr and Fe isotopes with = 31-35. The excitation energies of the states for those odd-mass nuclei sharply decrease with the neutron number, which attracts much interest. In this study, we calculate those unnatural-parity states systematically using an effective interaction comprised of existing ones plus minimal modifications. The calculated energy levels excellently agree with experimental data. The deformations of those isotopes are analysed by using the wave functions obtained by the shell-model calculations. We show that prolate deformation dominates this region, contrary to the previous suggestion based on the isomeric level. It is demonstrated that the sharp decrease of the levels occurs due to the rising of the Fermi surface, but not due to the evolution of the single-particle level as suggested previously.