Congruent band structures in 154Gd : Configuration-dependent pairing, a double vacuum and lack of \beta -vibrations

We have used the (\( \alpha\), 2n) and (\( \alpha\), 4n) reactions and the AFRODITE \( \gamma\) -ray spectrometer to make a comprehensive study of the nucleus 154Gd below \(\ensuremath 20\hbar\) . While the first excited 02 + state at 681 keV is usually considered to be the head of a \(\ensuremath K^{\pi}=0^{+}\) \( \beta\) -vibrational band, we propose that the data are best described as two separate vacuum states, the ground state and the 681 keV level, each with its own \( \gamma\) and octupole vibrations, pairing and alignments. The implications of this finding, for understanding the structure of transitional rare-earth nuclei, are discussed.