High-Spin Structures In The Pr-139 Nucleus

Background: Pr-139 is located in a transitional region of neutron number close to the N = 82 shell. The study of its high-spin states and collective bands is important for systematically understanding the nuclear structural characteristics in this region.Purpose: To investigate the high-spin levels and to search for oblate bands in Pr-139.Methods: The high-spin states of Pr-139 have been studied via the reaction Sn-124(F-19,4n) at a beam energy of 80 MeV. The experiment was carried out at the HI-13 Tandem Accelerator at the China Institute of Atomic Energy (CIAE). The data analysis was done by using the gamma-gamma coincidence method.Results: The level scheme of Pr-139 has been expanded with spin up to 45/2 (h) over bar. A total of 39 new levels and 45 new transitions are identified. Four collective band structures at high-spin states have been newly established. From systematic analysis, one of the bands is proposed as a double decoupled band; two bands are proposed as oblate bands with gamma similar to -60 degrees; another band is suggested as an oblate-triaxial band with gamma similar to -90 degrees. The other characteristics for these bands are discussed.Conclusions: A new level scheme in Pr-139 has been established and the collective bands at high spin have been identified. The result shows that the strong oblate shape-driving effect is caused by neutrons at the high-spin states in Pr-139.