Existence of intrinsic octupole shape in Pm151 : The reflection asymmetric triaxial particle rotor model

The possibility of the existence of octupole deformation in ¹⁵¹Pm has caused interest for about thirty years. In this paper, the observed low-lying positive- and negative-parity structures in ¹⁵¹Pm are investigated by the reflection-asymmetric triaxial particle rotor model. The experimental energy spectra, energy staggering, and parity splitting parameters are well reproduced by the theoretical calculations. It is found that, although the energy spectra can be reproduced without the octupole degree of freedom, the calculated $B(E1)$ values depend sensitively on the octupole deformation parameter β₃₀, and the observed enhanced $E1$ transitions can be reproduced with a notable β₃₀=0.07. The detailed analysis of the intrinsic wave functions shows that these nearly degenerate positive- and negative-parity bands are built on two individual proton configurations, i.e., dominated by πg7/2[Ω=5/2] and πh11/2[Ω=5/2], respectively, which differs from the parity doublet bands built on a single parity-mixed configuration.