Shell evolution above Z,N=50 within Skyrme density functional theory: The impact of deformation and tensor interactions

Abstract

Background: Recent years have seen considerable effort in associating the shell evolution (SE) for a chain of isotones or isotopes with the underlying nuclear interactions. In particular, it has been fairly well established that the tensor part of the Skyrme interaction is indispensable for understanding certain SE above Z,N = 50 shell closures, as a function of nucleon numbers. Purpose: The purpose of the present work is twofold: (1) to study the effect of deformation due to blocking on the SE above Z,N = 50 shell closures and (2) to examine the optimal parametrizations in the tensor part which gives a proper description of the SE above Z,N = 50 shell closures. Methods: I use the Skyrme-Hartree-Fock-Bogoliubov (SHFB) method to compute the even-even vacua of the Z = 50 isotopes and N = 50 isotones. For Sb and odd-A Sn isotopes, I perform calculations with a blocking procedure which accounts for the polarization effects, including deformations. Results: The blocking SHFB calculations show that the light odd-A Sb isotopes, with only one valence proton occupying down-sloping = 11/2− and = 7/2+ Nilsson orbits, assume finite oblate deformations. This reduces the energy differences between 11/2− and 7/2+ states by about 500 keV for 51Sb56−66, bringing the energy-difference curve closer to the experimental one. With UNE2T1 energy density functional (EDF), which differs from UNEDF2 parametrization by tensor terms, a better description of the slope of e(π1h11/2 − π1g7/2) as a function of neutron number has been obtained. However, the trend of e(π1g7/2 − π2d5/2) curve is worse using UNE2T1 EDF. e(ν3s1/2 − ν2d5/2) and e(ν1g7/2 − ν2d5/2) curve for N = 50 isotones using UNE2T1 seems to be consistent with experimental data. The neutron SE of e(ν1h11/2 − ν1g7/2) and e(ν1g7/2 − ν2d5/2) for Sn isotopes are shown to be sensive to αT tensor parameter. Conclusions: Within the Skyrme self-consistent mean-field model, the deformation degree of freedom has to be taken into account for Sb isotopes, N = 51 isotones, and odd-A Sn isotopes when discussing variation of quantities like shell gap etc. The tensor terms are important for describing the strong variation of E(π = 11/2− − 7/2+) in Sb isotopes. The SE of 1/2+ and 7/2+ states in N = 51 isotones may show signature for the existence of tensor interaction. The experimental excitation energies of 11/2− and 7/2+ states in odd-A Sn isotopes close to 132Sn give prospects for constraining the αT parameter.