Structure Effects and Rotation Alignment in ¹⁵⁹Tm

Abstract

<p>The level structure of ¹⁵⁹Tm has been determined for two different odd-proton bands. The 7/2 [523) band has been constructed up to a spin of I=49/2ˉ (subsequent experiments extending this band up to I=61/2ˉ are referenced and the 7/2 [404] band up to spin I=43/2⁺. In both bands the backbend associated with the i₁₃/₂ neutron alignment is observed. Gamma-ray intensities and angular distribution results are also presented and stretched B(M1)/stretched B(E2) values for transitions in both bands have been calculated.</p> <p>The nature of the second backbend observed in ¹⁵⁸Er and ¹⁶⁰Yb is examined by considering the blocking effect of the h₁₁/₂ proton in ¹⁵⁹Tm. The measured alignments of the h₁₁/₂ proton band, along with the absence of an observed second crossing in ¹⁵⁹Tm, lead to the conclusion that the second backbend results from h₁₁/₂ proton alignment, in agreement with the theoretical calculations. Band-crossing frequencies in ¹⁵⁹Tm are determined from the experimental Routhians and compared to the crossings predicted in Cranked Shell Model calculations.</p> <p>Structure effects in the N=90 region are also examined and the influence of the quasiparticle configurations on the core deformation is considered. Before the backbend, the 7/2 [523] band exhibits large signature splitting, the 7/2 [404] band none, which is interpreted as resulting from the very different driving influences of the K=7/2 h₁₁/₂ and g₇/₂ orbits, respectively, on the core triaxiality. The i₁₃/₂ neutron alignment occurring at the first backbend changes the signature splitting in each band as a result of its dominant driving force to positive values of Υ. The measured B(M1;I→-1)/B(E2;I→I-2) values increase dramatically after the backbend and this is explained in terms of calculated increases in the M1 transition rates and decreases in the E2 rates.</p>