High-spin states and band structures in the odd-odd ¹⁴⁰Pm nucleus

The light rare earth nuclei with A∼140 are located at a transitional region with the neutron numbers approaching the N = 82 closed shell. At low spins, the single-particle motion will be prominent. At high spins, the proton Fermi surface lies near the lower h_11/2 midshell and the neutron Fermi surface lies near the h_11/2 upper midshell. The alignment of a pair of protons tends to drive the nucleus toward to a prolate shape (γ∼0°) and the alignment of a pair of neutrons tends to drive the nucleus to oblate shape (γ∼-60°). In the N = 79 isotones, the high spin states have been investigated extensively in ¹³⁶La, ¹³⁷Ce, ¹³⁸Pr and ¹³⁹Nd . Many interesting collective bands were identified. However, contrasting to the neighboring nuclei, the information of high spin states in the Z = 61 isotone ¹⁴⁰Pm is still lacking, and needs to be further investigated. Here we report on the re investigation of the high-spin states of ¹⁴⁰Pm. In the present work, the high-spin states of ¹⁴⁰Pm have been investigated through the reaction ¹²⁶Te (¹⁹F, 5n) at a beam energy of 90 MeV. The experiment was carried out by the HI-13 tandem accelerator at the China Institute of Atomic Energy (CIAE). from the γ-γ coincidence relationships, the relative transition intensities and DCO ratio analysis, previous level scheme based on the 8^- isomer has been updated with spin up to 23 ħ. Many new levels and transitions were identified. Six collective bands were observed. Five of them were expanded or re-constructed, and one of them was newly identified. The systematic signature splitting and inversion of yrast πh_11/2⊗νh_11/2 band in Pr and Pm odd-odd isotopes have been discussed. Based on the systematic comparison, two ΔI = 2 bands were proposed as double decoupled bands. Other two bands with strong ΔI = 1 M1 transitions inside the bands were suggested as oblate bands with γ∼-60°. Another band with large signature splitting has been proposed with oblate-triaxial deformation with γ∼-90°. The other characteristics for these bands have been discussed. (authors)