New low-energy levels calculation for 155Eu

Abstract

We have revisited the low-energy calculation of odd Z 155 Eu in the frame of a semi-microscopic formalism as a support for the interpretation of the experimental results for the multipole mixing ratios of some electromagnetic transitions. The deformation parameters were obtained through a macroscopic-microscopic method, and the proton single particle levels, calculated with realistic Woods-Saxon potential were used as input in a quasiparticle calculation of the first few rotational band heads in the Lipkin-Nogami BCS aproximation. A better agreement is found between the experimental and calculated band heads if compared with previous evaluations and RIPL recommended values.This work is a theoretical support for the interpretation of the experimental results for the multipole mixing ratios of observed electromagnetic transitions between low-energy levels in 155 Eu [1]. In order to explain the experimental results, it was necessary to calculate the energy, angular momentum and parity of the first excited states (E≤ 1MeV). Due to the fact that the parameters in the potential energy are determined for several nuclei and not for a specific one, the previous calculation was not successful in the description of the first excited states. For example, in the work of Nazarewicz et al. In this work, a new calculation of the ground-state and the low-energy levels in 155 Eu is proposed, using the macroscopic -microscopic method [6]. In this sense, the odd-proton single particle levels in a deformed potential plus residual pairing interaction were calculated in order to describe the 155 Eu lowenergy rotational band heads (with E≤ 1 MeV). The groundstate deformation parameters were obtained by minimizing the total energy [6]; the single particle energy spectra and wave functions for protons and neutrons were calculated in a deformed Woods-Saxon potential [7]. The parameters of the potential for neutrons were obtained from Ref. [8]. For protons, these parameters were adjusted in order to adequately describe the main sequence of angular momentum and parity of the low energy excited levels (band heads), as well as the proton binding energy. The residual pairing interaction was considered in the BCS prescription using the Lipkin-Nogami approximation [9,10]. I. NUCLEAR DEFORMATIONWithin the macroscopic-microscopic method in the Strutinskys formalism, the total energy of the nuclear system as a function of deformation can be expressed as [6]:where ε and α are the set of deformation parameters. The bulk contribution to the total energy comes from the liquid drop model. The shell effects represent smaller variations added to the liquid drop energy E macr . The microscopic portion E micr can be divided into two components: the contribution associated with the shell correction energy and the pairing contribution. In order to obtain the equilibrium deformation parameters (ground-state deformation) the total energy is minimized. As the Cassini ovaloid shape parameterization was used, the adopted deformation parameters were the quadrupole ...