Signature inversion in weakly-deformed doubly-odd ¹⁵⁸Tm

High-spin states in ¹⁵⁸Tm were populated via the ¹⁴⁴Nd(¹⁹F, 5n) fusion-evaporation reaction at beam energies of 108 and 112MeV. The previously known level structures have been extended and a new band structure has been identified. In addition, a previously identified four-quasiparticle band has been firmly connected to lower known states. This four-quasiparticle band is assigned to the πh_11/2 ⊗νh_9/2(α = -l/2)⊗(νi_13/2)² configuration, thus forming a pair of signature partners with another four-quasiparticle band known as being built on the πh_11/2 ⊗ νh_9/2(α = +l/2)⊗(νi_13/2)² configuration. The band based on the πh_11/2⊗νi_13/2 configuration is observed to show anomalous signature splitting persistently, across the spin where a restoration to normal signature splitting is expected from systematics. Signature inversion is also established in the doublet πh_11/2⊗νh_9/2⊗ (νi_13/2)² bands. Positive triaxial deformation is supposed to play an important role in the occurrence of the unusual signature inversions in the weakly-deformed doubly-odd nucleus ¹⁵⁸Tm.