Several rotational-band structures have recently been observed in mass $A\simeq 130$ nuclei, built on multiquasiparticle states of enhanced quadrupole deformatin, the so-called superdeformed states $\beta \simeq 0.4$. In ${}_{}{}^{137}{}_{}{}^{}\mathrm{Sm}$, a decoupled band built on the ${\mathrm{vi}}_{\frac{13}{2}}\mathrm{}\left[660\right]\mathrm{}{\frac{1}{2}}^{+}$ single-quasineutron orbital, intruding from two oscillator shells above with a predicted $\beta \simeq 0.35$, was observed to low spin and excitation energy. The role of this $\beta$-driving ${\mathrm{vi}}_{\frac{13}{2}}$ orbital, which showed a 50% enhanced moment of inertia, is discussed in terms of the superdeformation in this mass region.

• Received 8 December 1987

DOI:https://doi.org/10.1103/PhysRevLett.61.42